RESEARCH STATIONFO RFRUI T GROWING WILHELMINADORP (GOES),TH ENETHERLAND S

ANNUAL REPORT 197S

Information contained in this report may not be reproduced in total or inpar twithou t permission Price Df.5, - CONTENTS

REMARKS ON THE BRANDSO F PESTICIDES AND HERBICIDES 5

DONORMEMBER S 6

BOARD 7

STAFF 8

POMOLOGY SECTION: 10 Research on top fruit (mechanicalprunin g - planting systems and light measurements) Regulation of fruit setan d vegetative growth (plantmateria l -Jun e drop of Doyenné du Comice -han d thinning of Benoni - chemical thinning-Ethrel A - carbaryl and Amid thin-new compounds - pollination.) Virus research and clonal selection in fruit trees Rootstocks and interstocks of pit and stone fruits (apple-pear-plum-propagation) Fruit variety trials (apple -pea r -plu m - second screening in experimental orchards)

SMALLFRUI T SECTION: 26 Cultural experiments with strawberries (glasshouse strawberries) Variety testing of strawberries (glasshouse strawberries - strawberries in the open) Cultural experiments with raspberries,currants ,an d blackberries Variety testing of raspberries,currants ,an d blackberries Cultural experiments with blueberries

SOILFERTILIT Y SECTION: 29 Soilmanagemen t experiments Influence ofnitroge n on yield and quality of fruit crops Relationship between potassium levels inappl e leaves and soil Bitter pit in apple Weed control

PHYSIOLOGY SECTION: 37 Investigations into factors underlying therelationshi p between the vegetative and generative development of fruit trees (nitrogen analyses - the effect of root temperature on thebehaviou r ofbudde d apple rootstocks-th e effect of temperature on flower-bud formation of Cox's Orange Pippin.) Pre-harvest physiology of fruits as related to fruit quality (the effect of air temperature,roo t temperature,an d airhumidit y on shoot and fruit growth and on theminera l composition of leaves and fruits of apple - fruit quality, time of picking, and storage condition) PHYTOPATHOLOGY SECTION: 40 Apple (canker- frui tro t- phytophthova syringae - powderymildew ) Applean dpea r (silverleaf ) Blackberry (caneblight ) Strawberry (redstel e- Verticillium wilt)

ENTOMOLOGYSECTION : 43 Tortricidsi norchard s (integratedcontro li norchard s- biologica lcontro l onstrawberrie si nglasshouses )

SOILFERTILIT YAN DACAROLOG YSECTIONS : 45 Influenceo fnitroge nleve li nappl etree so nth edevelopmen to fth efrui t treere dspide rmit e

ECONOMICSSECTION : 47 Researcho nth eeconomi csignificanc eo fne wplantin gsystem san dtre eshape s into pfrui tgrowin g Farmcomparativ eresearc hint oth edifference si nfinancia lresult so ffrui t holdings

LISTO FCHEMICAL S 48

PUBLICATIONS 49 REMARKSO NTH E BRANDSO FPESTICIDE SAN DHERBICIDE S

The dosages mentioned for pesticides and herbicides in this report refer to the commercial products.Trad e names ofproduct s have been used for the sake of convenience,bu t thismake s itunavoidabl e that in some cases similar products on themarke t under other tradename s areno t mentioned. No endorsement of named products is intended.A list ofmos t of the commercial products mentioned in this report and their active ingredients is given on page48 . Some of the experiments described in this report arebein g carried out with chemicals and/or concentrations not yet legally approved. DONOR MEMBERS

Those who are interested in thewor k of the Experimental Station andwis h to keep in active touchwit h and support its research, are invited tobecom e donor members. Donor members will receive theAnnua l Report and other publications of the Experimental Stationwithou t charge. The minimum subscription for donor member­ ship is 25 guilders for private individuals inTh e Netherlands, 25 guilders for foreign private individuals, and 50 guilders for institutions, annually. Checks of international postal money orders should be made payable to Proefstation voor de Fruitteelt at Wilhelminadorp,wit h the indication 'Donor Subscription'. BOARD (31thDecembe r1975 )

Board ofth eFoundatio n Research Stationfo rFrui t Growing

Executive committee:

Ir.J .Minderhoud , chairman E.H.M. Jongerius,vice-chairma n J.Kakebeeke , secretary J.L. Nijsten, treasurer Ing.J.S.J.M .Mulder s

Remaining members:

Ir.Joh . Bos J.Burge r N.J.J. Costongs Z. Houter W.d eJage r CR.T. Leenders B.Leij s L.J.M.E.va nWagenber g

Advisor:

Ir.H .Sijtstr a STAFF (31thDecembe r1975 )

Research Stationfo r Fruit Growing

Director: Ir.R.K .Elem a Deputy director: Dr.Ir.S.J .Werthei m

Pomology section: Dr.Ir. S.J.Werthei m Dr.Ir.H.J .va nOoste n (virology, clones,rootstocks , interstocks) Ing. P.D.Goddri e (varieties) J.L. Baarends (guide field plots) M.L. Joosse F. Nijsse

Soil fertility section: Dr.Ir.P .Delve r (stationed byth eInstitut e forSoi l Fertility(IB) ) Ing. P.J.Boldin g (chemical weed control) J.d eSchippe r J.Oel e (laboratory) Mrs CA. deBliek-va n Kleef

Entomology section: Drs. D.J.d eJon g (stationedb yth eInstitut e ofPhytopathologica l Research (IPO)) H. Beeke W. Remijnse

Phytopathology section: Drs. H.A.Th,va nde rSchee r J.P.A.va nDiere n (statistics) H.J. Wondergem (stationed byth eNationa l Board ofAgricultura l ResearchTNO )

Physiology section: Dr. J.Trom p C.A.R. Römer MissJ.C .Ova a (laboratory)

Small fruit section: Ir. J.Dijkstr a A.A.va nOoste n

Economics section: J.Goedegebur e (stationedb yth eAgricultura l Economic Research Institute (LEI)) Administrative and clerical staff: Miss J. Hamelink,hea d Mrs C.C. Geelhoed-Meulenberg Miss K.A. Korshuize Miss J.C. Versluis Mrs J.G. de Schipper-van Dalen (housekeeping) Miss P.C. de Smidt (housekeeping) Editor/librarian : Ing. Tj. van der Kooi

Technician: J. van Dalen

Experimental garden Wilhelminadorp (top fruit): J.J. Lemmens,far mmanage r P. Proost,deput y farm manager A.A. Janse M.C. van Maris A. de Witte Experimental garden Kapelle (small fruits): L.Nijsse , farm manager J.M. de Groene A.M. Verhulst

National Fruit Advisory Service

Ir. R.K. Elema,nationa l fruit adviser Ir. J. Dijkstra,co-ordinato r regional experiments Ir. T.H. Edens,economis t J. Blommers (small fruits) Ing. J.H. Bootsma (stationed at the experimental garden at Kraggenburg) Ing. A. Pouwer (stationed at the SoilAdvisor y Service at Wageningen), soil scientist A. Scholtens (stationed at the experimental garden at Geldermalsen) L.Uitterlinde n (top fruit) D.L. Verwijs (stationed at the experimental garden at Zaltbommel) POMOLOGY SECTION

S.J. Wertheim, H.J. van Oosten,P.D . Goddrie and J.J. Lemmens

Researcho nto p fruit

S.J. Wertheim and J.J. Lemmens

Pruning

MECHANICAL PRUNING

In 1975,th e pruning machine was used on the same trees of Golden Delicious onM. 9 as in 1974.Thre e treatments are compared in two holdings (onewit h spindles, the otherwit h slender spindles) 1.han d pruning, 2. mechanical pruning,an d 3.mechanica l pruning with additional hand pruning. Mechanical pruning induced more vigourous growth.Thi s is understandable, sincemor e twigs and branches were cut thanwit h hand pruning. Mechanically pruned trees yielded morek g (spindles) or equal amounts (slender spindle) compared with hand-pruned trees.Thi s gain inyiel d was,however , almost entirely attributable to fruitswit h a diameter of less than 70mm .Fo r the slender spindles,yield swer emoderat e inbot h years,whic hma y have contri­ buted to the differences in reaction between the two tree shapes. Picking ofmechanicall y pruned trees gavemor e difficulties than picking ofhand-prune d trees,du e to theman y branch and twigstubs . The trials arebein g continued.

PLANTING SYSTEMSAN D LIGHT MEASUREMENTS

In 1975,chemica l light measurements weremad ewithin ,between ,an d under Winston apple trees onM. 9 planted in single and double rows ina nort h to south orientation. The treesha d beenplante d in the spring of 1968.Th e single rowswer e planted 3.25 x 1.25 m, the double rows 3.50 + 1.25 x 1.25 ina trian­ gular design. As can be seen fromFig . 1, 50%o f the incident lightwa s measured under the single rows as against 45%unde r the double rows. In the heart of the trees,th e percentage of the incident lightwa s 60 forbot h systems.Hence ,ther e were only minor differences in the results of themeasurement s in 1975. Picking was carried out inhorizonta l layers of 50x 50 cm through the tree rows. The greatest amount of fruit in the single rows occurred in layers 4, 5, 6, and 8 (Fig. 1).I n the double rows the samehel d for layers 7-11 and 14-16. Thus,mos t of the fruits occur in theheart s of thehedges . In the single rows, fruit weightwa s fairly uniform throughout the tree canopy. In the double rows, the fruits of the lowest tree parts appeared abi t smaller than the average (Fig.1) . Fig. 1indicate s thatmos t of the fruitswit h good colour hang in the upper and outer parts of the trees.Withi n the double rows fruit colourwa s poorer than in the single rows, in spite of themino r differences in available light. Table 1shows ,however , that the colour grade of all fruits of the single and double rows only differed slightly. The results of the light measurements between the rowswer e also roughly the same forbot hplantin g systems.

10 Fig. 1. Cross-section of a single row (below)an d a double row (above)o f Winston trees onM.9 ,plante d north-south, in the eight growing season.Th e light measurements were carried out at the places indicated by thevertica l bars. The values next to these bars are the percentages received light (calculated with the amount of light at aheigh t of 3m taken as 100%). The picking data in each rectangle (representing divisions of the 50x 50 cm layers) show, startingwit h the top value: a. percentage of total fruit number per tree b. percentage nicely coloured fruits (more than 3/4 of each fruit red) in total number of fruits in each layer c.mea n fruit weight(g) .

11 Table 1.Colou r grade of Winston fruits harvested in 1975 from two planting systems.

Planting system % fruits Fully ± 1/4 ± 1/2 ± 3/4 Fully green red red

Single rows 0.1 11.3 53.6 34.8 0.2 Double rows 0.2 18.0 45.9 35.4 0.5

Table 2 shows the yield over all growing seasons. It iseviden t that the yield per treewa s slightly higher for the double rows.

Table 2.Yiel d ofWinsto n onM. 9 in two planting systems from 1969 to 1975. Trees planted in the spring of 1968.

Planting Trees/ Kg/tree Tons/0. 9h a system 0.9 ha 1969-1975 1969 1970 1971 1972 1973 1974 1975 Total (%)

Single rows 2,215 1.6 5.5 7.5 4.8 9.5 9.6 14.5 53.0 117 (100) Double rows 3,032 1.9 5.8 7.2 6.5 9.0 11.9 16.5 58.6 178 (152)

Perha ,th e double rows greatly out-yielded the single rows,becaus e of the highernumbe r of trees perha .Thus , the yield canb e augmented considerably by increasing thenumbe r of trees perha . The accessibility of the trees in the double rows for pruning, thinning,an d picking was rather poor compared with that of the single rows,becaus e the former were planted too close together in the row. However, if adequate space per tree had been given,th e treenumbe r would not have been sufficiently higher than that of the single rows.

Regulation offrui t set and vegetative growth

S.J. Wertheim

Plant material

In a fruit tree nursery the effects of four pinching agentswer e compared on growing buds of Belle de Boskoop and Winston onM.9 .Th e chemical compounds (Off-Shoot-0,P P 528,MS B 25,105,an d NC 9634)wer e applied at abu d height of 55 cm above the union.Untreate d and hand-pinched trees served ascontrols . BothM &B 25,105 (0.15%)an d NC 9634 (0.8%)gav ever y promising results. NC 9634wa s sprayed together with 0.72%Lissapol .a surfactant. On Belle de Boskoop 4.8 (MSB 25,105)an d 4.1 (NC 9634), good featherswer e formed per tree asagains t none on theuntreate d trees.Unde r good feathers is understood side shoots at aprope r height (>5 0 cm above the union), of sufficient length,an d with awid ebranc hangle . For Winston, these figureswer e 3.9, 3.6,an d 0.1 respectively. Off-Shoot-0 gave reasonably good results butwa s less satisfactory than either M &B 25,105

12 orN C 9634. PP 528wa sno t satisfactory: side shootswer e formed too close to each other and with a smallbranc h angle.Th e impressionwa s obtained that chemicals that kill the growing point areno t good feathering agents,wherea s those that only damage itma y givebette r results.

June drop of Doyenné du Cornice

The chemical compound Curbiset,use d to increase fruit set of gherkins,wa s applied to trees of thepea r cultivar Doyenné du Comice toreduc e June drop. Since Curbiset is thought to take effect byblockin g auxin transport from the ovaries, it seemed possible that such a chemicalmigh t counteract June drop of young pear fruits. Fruit set of the experimental treeswa s first promoted by a spray of 15pp m GAi,7 at the early flowering stage,an d Curbiset was applied at the end of the flowering period ina dose of 0.08% or 0.16%. The treeswer e badly damaged and a heavy leaf drop followed. The experiment was terminated prematurely.

Hand thinning of Benoni

The fertile apple cultivar Benoni is oftenbiennia l and the fruit size inade­ quate. An experiment with hand thinning was started to find outwhethe r there is an optimal degree of and time forhan d thinning. On four-year-old trees onM.9 ,fiv e thinning treatments were carried out, i.e.,n o thinning; thinning to one fruit per cluster,pe r two clusters,o r per four clusters;an d complete thinning. The thinningwa s done at three times, i.e., at pink bud or 15o r 30day s after full bloom (2,27-2 8May , and 9June , respectively). One-year-old twigswer e deblossomed completely. At harvest itwa s found that the thinning standards were not fully reached, because of an unusual June drop in treatments leaving many fruits (unthinned and thinned to one fruit per cluster). (See Table 3,unde r fruits per 100flowe r clusters.)A n increase in the degree of thinning led to adecreas e inyiel d and and increase in fruit size.Thinnin g to one fruit per two clusters gave an opti­ mal grade. Concerning the time of thinning, it canb e seen from Table 3 that every delay decreased fruit size further. The results of theblossom-cluste r counts must be awaited before definitive conclusions canb e drawn.

Table 3.Result s of ahand-thinnin g trial for Benoni onM.9 .

Treatment Number Fruits per Kg Fruit Kg per tree of of per weight fruits tree 100flowe r trees tree clusters (g) > 65 > 70m m

Unthinned 8 196 146 15.4 78 1.1 0.2 Hand thinned to1 fruit per 1cluste r 24 114 79 12.6 111 7.2 3.4 2 clusters 24 69 48 9.6 139 8.3 5.7 4 clusters 24 37 26 6.1 164 5.8 5.0 Hand thinned at Pinkbu d stage 24 72 50 9.7 136 8.0 5.7 FB* + 15day s 24 73 51 9.4 129 6.9 4.8 FB* + 30day s 24 75 53 9.2 122 6.3 3.6

* FB =Ful l Bloom

13 Chemical thinning - Ethrel A

BENONI

For five-year-old Benoni trees onM.9 , 0.3% Ethrel-A (1,440pp m a.i.) was applied at theballoo n stage of the flowers on the oldwood . This treatment induced satisfactory thinning. On unsprayed trees,94 6 fruitlets per treeha d tob e hand thinned,o n sprayed trees 445.Pe r 100flowe r clusters,thes e values were 556 and 168,respectively . Shoot and leaf growthwer e slightly suppressed by the treatment,bu t this effect was temporary. The thinning treatment with Ethrel-A slightly increased preharvest drop, i.e., from 0.6 to 2.9 fruits per tree,bu t all trees were sprayed with 20pp m 2,4,5-TP 10day s prior to picking. Mean fruit weight of the treated treeswa s lower than for untreated trees, viz. 156 against 176g . Thenumbe r ofk g of fruits smaller than 70m m diameter was, however,abou t equal forbot h treatments,viz . 14.0 and 13.4k g per tree, respectively. Fruit colour on chemically thinned treeswa s slightly improved. The trialwil l be continued with the sametrees .

BELLE DE BOSKOOP

For the third successive year,Ethrel- A was sprayed on the same trees.Th e chemical was sprayed once (0.05% or 0.075%)whe n 10%o f the flowers had opened. Fruit setwa s light in 1975.Ethrel- A thinned at 0.075% but not at 0.05%. Since thinningwa s not necessary, the thinning action of 0.075% Ethrel-Awa s excessive. In 1974,th e same held forbot h concentrations. On this basis it seems dangerous to use Ethrel-A on this cultivar before fruit set canb e judged properly. Unthinned trees yielded 87.6k g per tree in the 3-year period, hand-thinned trees gave 86.4kg ,Ethrel-A-treate d trees 77.2k g (0.05%)o r 64.3k g (0.075%). Hence,Ethrel- A caused a considerable reduction inyield . The sole advantage offered by Ethrel-Awa s the smaller variation innumber s of blossom clusters per tree.Fo r example,hand-thinne d trees had 572,336 ,an d 895 flower clusters per tree in 1973, 1974,an d 1975,wherea s trees sprayed annually with 0.05% Ethrel bore 504,515 ,an d 889 flower clusters, respectively.

LAXTON'S SUPERB

Ethrel-Awa s sprayed in 1975 for flower thinning on the same (now five-year- old) trees as in 1974. In 1974,0.25 %wa s used and thinning was slight. In 1975, 0.30%wa s applied. The treated trees flowered slightly more in 1975.Treate d trees had 150,untreate d 103flowe r clusters per tree.Th e thinning effect of Ethrel-A was again light.O n untreated trees, 173fruit s per tree had tob e removed, on treated trees 123. Total yield over the two-year period was 15.6k g per tree for hand-thinned trees, and 17.2k g for the sprayed trees thinned additionally by hand. In an orchard at Krabbendijke, countswer emad e on five-year-old Laxton's Superb trees onM.9 ,som e ofwhic h had been treated with 3 litres Ethrel-A per ha in the previous year.Tabl e 4 shows the results.Ethrel- A clearly broke the biennial rhythm. Off-year-trees did not flower at all,on-year-tree s flowered normally.

14 Table 4.Numbe r and %off-yea r trees of Laxton's Superb onM. 9 in 1975 (the fifth growing season)afte r Ethrel-A treatment in 1974.

Treatment Parcel1 Parcel 2 1974 Number Number of Number Number of of trees off-year trees (%) of trees off-year trees (%)

Untreated 206 65 (31.6) 206 92 (44.6) Ethrel-A 206 7 ( 3.4) 206 6 ( 3.0)

Table 5. Results of chemical flower thinningwit h Ethrel-A on Winston in 1974 and 1975.Value s are means of ten trees per treatment.

Treatment Flower Fruits/100 Kg/tree Kg/tree of clusters/ flower fruits >6 5 mm clusters m tree atharves t 1974 1975 1974 1975 1974 1975 Total 1974 1975 Total Unthinned 245 227 77a 102a 23.2 30.5 53.7 17.7 25.4 43.1 0.25%Ethrel - -A 244 293 38b 61 c 13.-9" 25.6 39.5 12.5 23.0 35.5 0.30%Ethrel - -A 241 309 33b 49c 12.4 23.5 35.9 11.6 21.8 33.4 Hand thinned 1) 249 269 75a 77b 24.0 27.3 51.3 18.9 24.2 43.1

Values followed by different letters differ significantly (P= 0.05). 1)I n 1974,4 6 fruits and in 1975,8 3 fruits per treehan d thinned, or 19an d 31 per 100flowe r clusters, respectively.

WINSTON

On ten-year-old trees onM.9 ,Ethrel- A was investigated for the second year on the same trees.Tw o concentrationswer e applied, 0.25% and 0.30%. Spraying was done at 15%ope n flowers.A s in 1974,bot h concentrations thinned too strongly. The thinning led to larger fruits,bu t this could not compensate for the decreased fruit number (Table 5). Therefore,flowe r thinning onWinsto n with the above-mentioned concentrations cannot be recommended.

Carbaryl and Amid thin

COX'S ORANGE PIPPIN

Carbaryl and Amid thinwer e compared in a trial on six-year-old trees onM.9 . Carbaryl (0.15% of a 50%commercia l product)wa s sprayed 15,20 ,27 ,o r 35 days after full bloom. Fruit diameter on these spraying dates was 6.5, 9.8, 14.4,an d 22.6, respectively. At full bloom, the ovaries averaged 2.8 mm. Amid thin (0.072%) was applied 8, 15,20 ,o r 27 days after fullbloom . Eight days after full bloom, themea n fruit diameterwa s 3.5 mm. Fruit setwa s moderate: 116 fruits per 100flowe r clusters.Jun e drop greatly reduced thenecessit y for hand thinning. Onhand-thinne d control trees only 20 fruits per tree had tob e removed, or 6pe r 100flowe r clusters. None of the treatments thinned significantly. The last of the series ofAmi d thin treatments increased fruit set significantly to 79 fruits per 100flowe r clusters as compared to 50 for untreated trees.Thi s isa know n effect of a late treatment with an auxin-type chemical thinner.Th e trial isbein g continued.

15 GOLDEN DELICIOUS

On eight-year-old trees onM.9 , Amid thinwa s applied atvariou s intervals to find thebes t stage for fruit thinning with this compound. Amid thin (0.072%) was sprayed 7, 14,21 ,o r 27 days after fullbloom .Frui t diameter onwoo d older than oneyea rwa s 2.9, 4.5, 7.3,an d 10.3mm , respectively. At flowering, this diameter was 2.8 mm. Fruit setwa s good: 309 fruits per 100flowe r clusters before June drop on untreated control trees.Pe r tree,6 2 fruits had tob e thinned from thehand - thinned controls; this corresponded with 20 fruits per 100flowe r clusters. None of theAmi d thin treatments thinned significantly. Unthinned treesha d 80 fruits per 100cluster s atharvest ,an d the four successive chemical treatments 76, 78,77 ,an d 83.Ami d thin therefore did not appear tob e aver y effective in 1975.

New compounds

GOLDEN DELICIOUS

On five-year-old trees onM.9 ,Rhodofi x (anNAA-product )wa s compared with carbaryl and Amid thin.Ami d thin (0.072%)wa s applied first at 7.9 mm mean fruit diameter. Rhodofix (0.12%)an d carbaryl (50% commercial product)wer e used when the fruits on the old wood averaged 12.2m m diameter. Fruit setwa s good, 239 fruits per 100flowe r clusters before June drop on unthinned control trees.O nhand-thinne d controls,3 4 fruitletswer e removed per tree or 20 per 100flowe r clusters.Ami d thin did not thin significantly,bu t both of the other chemicals did. Thenumber s of fruits per 100flowe r clusters atharves t were 45 (untreated),4 2 (Amid thin), 37 (Rhodofix), 31 (carbaryl), and 31 (hand thinned). The difference between Rhodofix, carbaryl and hand thinning was not significant.

Pollination

NEW"VARIETIE S

Various crosseswer emad e between new and oldervarietie s in 1975,a s in pre­ vious years,t o seewhethe r new varieties can be planted together with older ones without causing cross-pollination problems.Th e list givenbelo w summarizes the results, compared to freepollinatio n of the samenumbe r (100)o f flowers. According to this grading of the results,goo d (+)mean s that fruit set after cross pollination was higher than after openpollination ,moderat e (±)tha t the setwa s roughly equal,an d bad (-) that fruit setwa s better after open pollina­ tion. Pollination was carried out in the balloon stage,withou t damaging the flower.Afte r pollination, the stigmas were covered withvaseline .

16 TOH«

r- CD eu S H (D € m M' O O Z O M •o 2 Z 55' o. O 3" w o a X w X en 3 s; h-' S" (D n O «r cri c c (0 w a. (0 3 3 (0 < m •n Tl "j o O S" S s 1 m rt 1—» H h-* i— o -i X (0 01 vO < M- O O O X3 tu ë 3 r S en S S s V a (B (D (0 m M* m 3 3 a. h«* s; l-l 3 n 3 i (0 lu S' 03 O m H* ?r H* H* 3 (V (V £0 D. 3 n> 3 01 3 3" -t O 3 rt O i-i 3 0Q 31 (A 3" i-l TO TO (U ?r ((UV rt>to •o O m O O o •5 •6' hti l-h l-l l-h 3 n S" 3 Q. • • o (D •o 3 O 3(0 U CO S S 3- S 3 T3 It w 01 R 5' 5' 5T IB i-i ft^-i ftç-> n r^ 01 R K D. S 2. 3 O 05 CO Co (A •—* hn o 01 •o o •a rr »—s ^-» 00 X~ -• o«f 1 (T> CJ1 O m< t~>» H' W I—> 5~a n a h-1 0 D. 3 CT (t> h-» I-! rt> <-• tu o T3 3 (-<• 01 3 o •o n> rr ni en 3" fl>3 o a> l-l 01 i-( en 01 3 3 H- 3 K* t-1 3 o m -a ni (t> i-ti a 3 a 3 S) h-1 en en O i-ti H* TOi-i H« "ai S i-i en 01 en o •a l-h -a roi 3 rt en TO en H' i-l O 01 01 3 s~a hh 3 rt o o e/i rt •a n 01 n> 01 0 3 X O -co 3 O l-h CL en 3 i—j O •a O O O 01 3 X i-l i-i am -a 0i 0) i-i en 3 oro. •-«a 01 TO O CD S n i-f H' o Oi rt 3 H' 3* •o O*— > TO(D •3 ro o M- n>te) 5~a 3 1-" h-* O* 13 rt h-» T3 i-l ro o H- 3 ru a 01 m -KJ rt Cd l-h i-l H* o h-1 (0 3 01 O (0 ?r S 01 0> o 3 m 3 o i-( t-"- •o 01 3 O i-< rt n H S 3" n> 01 m 0 l-i 01 fi a. O o 3" 01 3 01 rt i-l

17 Mother variety Pollinator Result Cox's Orange Pippin Discovery Karmijn de Sonnaville Discovery Discovery Cox's Orange Pippin Laxton's Superb Benoni Lombarts Calville Benoni Melrose Gloster Jonagold Winston Jonagold Cox's Orange Pippin Jonagold Lombarts Calville Gloster Golden Delicious Gloster Cox's Orange Pippin

Self-pollination of flowers of Karmijn de Sonnaville, Septer,an d Gloster invariably gave a smallnumbe r of fruits,indicatin g that these cultivars are not completely self-incompatible.

ORNAMENTAL MALUSI N POTS

For the second year, Malus 'Aldenhamensis' trees inpot swer e placed between trees of Belle de Boskoop onM. 9 in anine-ro w block. Partitions were made in the fivemiddl e rows. Each partition being six trees separated by three guard trees. In these partitions the pollinator density was 0, 10,o r 33%.Th e treatments were applied in triplicate.A s in 1974,yiel d was the same for all treatments in 1975.Th e flowering periods of Boskoop and 'Aldenhamensis' overlapped adequately (Fig. 2).Sinc e fruit setwa s fairly light,a n increase should have beenpossible .

ORNAMENTAL MALUSA S GRAFTS

InMarc h of 1974,o n twoholding s with Cox's Orange Pippin onM.9 , Malus grafts were applied to the top of thevertica l leader. In the orchard at Wissenkerke, Malus 'Hillieri'wa s used, at Lewedorp Malus 'Aldenhamensis'. The grafts flowered in 1975,bu t too late for themai n variety (Fig. 2).Th e same was found in earlier trials. Inbot h orchards three treatments were used, each in quadruplicate,viz .n o pollinators, one graft to six trees,an d one graft to three trees. The yield of the Cox treeswa s not significantly affected by the treatments. The numbers of fruits per labelled branch per treewer e 28 (untreated), 25 (one graft to six trees), and 25 (one graft to three trees). The late flowering of the Malus cultivars was undoubtely responsible for these disappointing results.

FLOWERING TIMES OF MALUSAN D SOMEAPPL E CULTIVARS

Fig. 3 shows the flowering periods of several Malus and apple cultivars in 1974an d 1975,fo r trees planted in the Wilhelminadorp experimental garden. Both groups showed the samevariatio n between 1974an d 1975.Mor e experience must be accumulated before any Malus cultivar can be recommended for practical use. Comparison of Figs. 2an d 3 indicates that the flowering periods of Malus 'Hillieri' and 'Aldenhamensis'var y relative to that of Cox.Graft s flowered later than trees.

18 •*j H' - CM G) C/î > G> m i £ $ W <"> o —• w LO n u> ~ o o »*• (0 *<_ n> 3 s3 (BD I E ra ~v #-*v ca •o •«1 3 3 X 3" 3 f-J O H ta rt> O 0) 1 O o o 3 n» H- «S. o •x. o n«> 3 Q. a> 3 O a i (V H> T Hi O 3 3 3 AI i-i OQ C OTW rr o V) 3 •u N) rt>i-l O H- 8^ O o. O co s9&&&stt*S «*a»Sä«:» •O n> o 3 Hl tn Hl a h-* a> O t-~» i t s T3 n> co Ni ai < o 0) o S-3 «CO er U 3 i—• 3 a> 0 CL •< § CU (O Hl T•3a t—' h-1 fc* O CD slt> O 1-1 c en H» a» rt * H' < C 3 o Cu Hl o. o 00 i-C o-a s~a • o ta •o eu a i-l 3 en Hl o t-1 o O t«l H H

19 Virus researchan dclona l selection infrui t trees

H.J. van Oosten

Different sources of the importantvariet y Cox's Orange Pippin are being studied in several trials at the Station. The oldest trial (planted in 1973)concern s 18source s ofDutc h origin, selected in commercial orchards on the basis of growth and productivity. Nothing isknown ,however ,abou t the presence ofviruse s in these trees. Interesting differences were seen in 1975wit h respect tohabitus ,arrangemen t of leaves, and yield. The differences in fruit set (Table6)i n particular were remarkable. The second trial (planted in 1974)concern s several virus-free sources from the Netherlands,England , and Switzerland are compared. A small yield was obtained in 1975.Afte r picking, the appleswer e compared with respect to skin characteristics (colour,russeting )b y anumbe r of observers. The English LA 62E and theDutc h T12wer e evaluated as thebes t 'normal' Cox's Orange Pippin. The apples of some sources had more colour than normal. In this respect those of the Swiss source 'Kummer'wer e remarkable: abrillian t orange-red, sometimes with very small sectorial chimaeras. In a third trial (planted in 1974)som e Dutch and English sources fromcommer ­ cial orchards were compared. The apples of the English sources usually had more colour andwer e more striped than theDutc h ones. The important virus-free clones A and B of Golden Delicious arebein g compared in several experimental orchards. Earlier resultswer e contradictory with respect todifference s in russeting of the fruit skin of apples of the two clones. It isno wmor e clearly evident that the fruits of clone B are smoother than those of clone A. The trees of clone B are also slightly more vigorous and productive,a t least in the earlyyears . Thenecessit y for testing of heat-treated plantmateria l has already become evident in the last two years.Thi s year, another heat-treated tip graft of Cox's Orange Pippin produced red fruits. Some spur types of Golden Delicious (Testerspur and Starkspur)wer e variable in growth orwer e not spur at all.Onl y trees of Golden Yellospur showed distinct spur growth.Fiv e heat-treated sources of Red Jonathanwer e evaluated as to fruit colour. Two sources had dark red apples but the other three did not. It should bementioned ,however , that although several deviations have been found, themajorit y of theheat-treate d plant material examined was normal in growth and fruiting. The results of trials comparing virus-free and virus-infected trees confirmed the conclusions drawn last year.Th e growth of virus-free trees of vigorously growing varieties planted on fresh soil remains critical. It isexpecte d on the basis of current knowledge that the trees of these varieties must be planted further apart in the row thannormal .

20 Rootstocksan dinterstock so fpi tan dston e fruits

H.J. van Oosten

Apple

Two trials have been planted so far at the 'Wilhelminadorp' station for a study on the practicalvalu e of thene wM.2 7rootstock . The first was planted in 1974wit h Cox's Orange Pippin,Re d Boskoop,an d Winston onM. 9 and M.27.Al l trees are free of known viruses. The growth of the trees of all three varieties was more vigorous onM. 9 than onM.27 .A small yield was obtained in 1975.Th e highest yield per treewa s produced onM.9 .A t the end of the season the leaves of Cox's Orange Pippin onM.2 7 were redder than those on M.9. Such differences were not seen in the othervarieties . A second trial was planted in 1975wit h the varieties Cox's Orange Pippin,an d Red Boskoop on the following rootstocks:M.9 ,M.27 ,3426 ,Budagovsk i 9 (B.9), and M.13-chimaera. In thenurser y theweakes t growth was seen on 3426 and the most vigorous onM.13-chimaera . The trees onM.9 ,M.27 ,an d B.9 were comparable as to growth at this stage.

Pear

Itha s been shown in a trial at Wilhelminadorp that trees of Beurré Hardy and Doyenné du Comice flower more abundantly on quince 'selection Adams'tha n on quince MA and MC. As a result,th ehighes t yields per treewer e obtained on quince 'selection Adams'.O n quinceMC ,however , thehighes t fruit set (number of fruits per 100 flower clusters)wa s obtained over aperio d of 8years .Thi s and the weaker growth of the trees on quince MC are very interesting aspects of this rootstock. In another trial the rootstocks quince MA and MC are being compared for the varieties Conference and Doyenné du Comice. No differences were seen in growth of the trees on quinceM A and MC. In 1975 the trees flowered for the first time. Those on quince MC produced the most flowers (Table6 ) and gave ahighe r yield. The growth of theDoyenn é du Comicewa s reduced by higher budding on theroot - stock, but the number of flowerswa s increased (Table 7).Wit h Conference, such differences were not seen.

Plum

In a small trial with the variety Victoria on the rootstocks Brompton, Prunus pumila, and P.spinosa, large differences were seen in the first two years.O n P.pumila growth was weak but on Brompton itwa s vigorous. On both of these root- stocks growth was steady,bu t on P.spinosa itwa s highly variable,mos t likely due to the use of seedling rootstocks. The first yield was obtained in the second year after planting (1975). The lowest yield and the smallest fruits occurred on Brompton.

Propagation

Unrooted but well-etiolated rootstocks ofM.2 7 from the stoolbed form hardly any roots after planting. Dipping of rootstocks (diameter less than 9mm ) ina n IBA solution stimulated rooting. The percentage of rooted layers was 28 for the untreated rootstocks and 83 after a 5-second dip in a solution of 2500 dpm IBA.

21 Rootstocks thicker than 9m m did not react to this treatment. The treated root- stockswer eno t given bottom heat for someweek s butwer e boxed up before planting. Treatment of unrooted well-etiolated M.9 layerswit h IBAwa s also successful, although the differences between the treated material (2500dp m IBA)an d the controlswer e much smaller.

Table6 . Number of flower clusters,frui t set,an d yield of six sources of Cox's Orange Pippin in 1974an d 1975 combined.

Source Flower clus- Fruits/100 Kg/ Mean fruit ters/tree flower clusters tree weight (g) a 190 75 15 157 b 177 75 15 158 c 164 75 14 152 d 219 45 14 188 e 261 42 15 181 f 155 39 9 191

Table 7.Th e influence of height of budding on flowering and yield ofDoyenn é du Comice trees on quinceM A and MC in 1975.

Rootstock Height of Flower clusters/ Tota 1 yield (kg) budding (cm) tree (30 trees)

MC 10 15.1 1.7 20 19.5 4.4 30 33.7 8.5

MA. 10 1.1 0.2 20 1.8 1.2 30 17.8 3.6

22 Fruit variety trials

P.D. Goddrie

Apple

In the spring of 1975 sevenne w varieties were set out in the first screening trials atWilhelminadorp . In general,frui t set and production of almost all of the apple varietieswa s good in 1975.Discovery ,whic h isno t aver y productive variety, also yielded well in197 5 (17.9k g per tree). Since it started tobea r and including 1975,thi svariet y (planted in the spring of 1967o nM.9 )ha s yielded 77.8k g per tree. The yield of Karmijn de Sonnaville,plante d onM. 9 in the spring of 1966, remains very constant. In 1975,productio n amounted to 36.8k g per tree,bu t because of very heavy russeting the fruitswer e less bright than in previous years. Jonagold, inwhic h many fruit growers have become interested, also yields constantly. The trees of thisvariety ,plante d in the spring of 1971 onM.9 , produced 45.4k g per tree over the 1972-1975period . A small storage trial showed that thisvariet y canb e stored well until thebeginnin g of June.A numbe r of people qualified the taste of Jonagold inJun e asbette r than that of Golden Delicious at that time.However ,althoug h the fruits arevariabl y blushed, Jonagold must not be considered a red-coloured storage variety. In the spring of 1971 thevariet y Summerredwa s planted onM. 9 in the first screening trials at Wilhelminadorp. At the end of 1975 thisweak-growin g variety had yielded 41.8k g per tree,givin gwell-coloured , rather large fruitswit h a fairly good taste. Since the ripening period is comparable with that of James Grieve, Summerred isno t tob e considered a storage variety. The skin colour of the IVT-variety Elstar,plante d onM. 9 in the spring of 1972,wa s very bright in 1975. The ripening period of thisvariet y is about the same of that of Cox's Orange Pippin. It is a good cropper,havin g given an average yield of 27.3 kg per tree in the 1973-1975 period. Because 1975wa s ayea rwit h a lot of canker,a good impression of the suscep­ tibility of the varieties for this disease could be obtained. Wellspur appeared tob e very susceptible,s omuc h that the trees had tob e grubbed at the end of the 1975 growing season.MacSpu r too showed a lot of canker; after removal of the infected parts hardly a single viable tree remained. In the trialwit h Winston and somemutation s of thisvariet y Dubbele Winston seemed tob e giving somewhat higher yields after three years ofproduction ,du e to lessbar e shoots,bu t the average fruitweigh t of thismutatio nwa s only 6g higher than that of standard Winston.Gradin g of the yields showed that 87.3%o f the fruits of Dubbele Winstonwer e bigger than 70mm ; for standard Winston this percentage was 84.9. The Japanese variety Akane,whic h is aweak-growin g summer variety with bright red fruits,wa s planted in the spring of 1973 onM.9 .Th eyiel d at the end of 1975 amounted to 6.2 kgpe r tree.Frui t colour can be improved by leaving the fruits on the tree,whic h canb e done very satisfactorily with this variety. In the spring of 1972 anumbe r ofFrenc h clones of Golden Delicious wer« planted in the variety trials atWilhelminadorp . Starting with the first yields in 1973,almos t all of them showed extremely smooth fruits,usuall y somewhat more blushed thannorma l standard GoldenDelicious . In 1975 several taste evaluations were carried outwit nne wvarieties . In two assessments Elstar was considered the best-tasting variety,bu t the taste of Jonagold and of Karmijn de Sonnaville was almost as good as that of Elstar.

23 Fig.4. Elstar,a promisin g variety in the fruit trials.

24 Several taste evaluations made in 1974an d 1975wit h the yellowish-green Schone van Boskoop and its red mutations Lambrechts and Schmitz Hübsch showed that there canhardl y be said tob e differences,althoug h therewa s a tendency to evaluate Rode Boskoop Schmitz Hübsch ashavin g apoore r taste. On the basis of the results obtained in the last fewyears ,twent y varieties were grubbed at the end of 1975.

Pear

In the spring of 1975 fourne w varieties were set out in the first screening trials at Wilhelminadorp, three of them originating from the pearbreedin g program of the IVT atWageningen . Although all varieties flowered very well, fruit setwa s in general verypoor , except for Gieser Wildeman and Winterrietpeer,whic h gave an adequate yield.

Plum

In the spring of 1975 one newvariet y was included in the first screening trials. With plums too,frui t setwa s extremely poor, and as a result information on thevarietie s under study canhardl y be given.

Second screening trials in experimental orchards

In apple trial nr. 71.0.3.,plante d in the spring of 1971,Karmij n de Sonna- ville is still giving thehighes t yields.Fro m 1972 to 1975 the production of this variety amounted to 41.8k g per tree (average of five experimental orchards). In the same period of the same trial the standard variety Cox's Orange Pippin yielded 36.9k g per tree,Oranj e de Sonnaville and Holstein respectively 27.4 and 27.5k g per tree.Alkmen e gave the lowest yield, 21.2k g per tree. In the apple trial nr. 73.0.1.,plante d in the spring of 1973,Karmij n de Sonnaville (virus-free)ha s again given the highest yields after three growing seasons. The production of thisvariet y in 1974+ 1975 amounted to 22.4k g per tree (average of five experimental orchards). But virus-free Elstar andvirus - free Cox's Orange Pippin (T61)ar e also doing well,wit h respectively 17.3an d 16.4k g per tree in the same period. Standard Cox's Orange Pippinwa s less pro­ ductive with 10.2k g per tree;Alofs ' Cox Spur produced the least,i.e. , 8.6 kg per tree in the same period. Three summer varieties from the IVT at Wageningen are producing small yields, in contrast with IVT5544-22 0whic h yields rather well. In the spring of 1975 apple trialnr . 75.0.13.wa s planted in five experimental orchards, including thevarietie s Jamba 69,Gloste r 69,Jonagold , and Septer. These varieties will be compared with the standard varieties James Grieve and Golden Delicious.

25 SMALL FRUIT SECTION

J. Dijkstra,J . Blommers,L . Nijsse and A.A. van Oosten

Cultural experiments with strawberries

Glasshouse strawberries

Cold-stored propagation-field plants produce many more runners than normal propagation-field plants,an d therefore the removal of runners from cold-stored propagation-field plants gives muchmor ework . The possibility of slowing down runner production isbein g investigated, with use of the I.C.I, chemicalR38807 . Waiting-field plants of Gorella and Redgauntlet were sprayed with 0.5, 1.0,an d 2.0 kg R38807 perh a onAugus t 15th or 30th.Th ehighe r the concentration of R38807, the fewer runnerswer e produced. Unfortunately, also plant growth was strongly retarded, resulting in amuc h lower fruit production. Research on the influence of the temperature inNovembe r on flower development was continued. Cold-stored propagation-field plants of Glasa,Gorella ,an d Redgauntlet were planted on June 21st and July 8th in 1974.Fres h propagation- field plantswer e planted at the beginning ofAugust . Before planting in the glasshouse inmid-December , the plants were divided into groups that were held in cold storage for 0, 3an d 6weeks ,respectively . Cold storageha d apositiv e effect on the yield of Glasa,bu t ripening was delayed 3 to5 days. Gorella and Redgauntlet showed no influence of cooling on production. The fresh propagation-field plants invariably gave a lower yield than the cold-stored propagation-field plants,du e to thewe t autumn inwhic h the former did not growmuch . Redgauntlet gave a second crop inJuly . Production varied between 2.4 and 3.8 kg/m2. The second crop of the freshpropagation-fiel d plants was also smaller than that of the cold-stored material. "Tw o spra' S with calciumnitrate during blossoming timeha d no influence on the quality of either the flowers or fruits of Redgauntlet.

Variety testing ofstrawberrie s

Glasshouse strawberries

The suitability of Glasa,Primella , Senga Gigana,an d Karina (IVT6808 )fo r early forcing purposes was compared. Glasawa s again the earliest variety,bu t production wasver y low.Primell a gave many small fruits and a lowpercentag e of first quality. Karina was amoderatel y good variety. Despite the mediocre quality of the plants, Senga Gigana gave the highest yield. In a lightly heated glasshouse Redgauntlet, Gorella,Mida , Sivetta,an d IVT 67100wer e compared. Redgauntlet gave a lowyiel d and a lowpercentag e of first quality fruits.Mid a performed badly too.Gorella , Sivetta,an d IVT6710 0 were roughly the same. Sivetta had the highest percentage first quality fruits.

26 Strawberries in the open

Yieldswer e disappointing in general.I n avariet y trial,Z b 6855 gaveb y far thehighes t yield,bu t fruit quality isba d and taste only moderate; since thisvariet y is also susceptible to Vevtioilliym, it cannotb e seen as ausefu l accession.Gorell a gave a lowyield . Jescoprove d very susceptible tore d core, andMek o and IVT 70060ha d good-tasting but small fruits.Sivett a showed a relatively good production with fruits of excellent quality.

Cultural experiments with raspberries, currants,an dblackberrie s

For the third successive year the soil-improvement experiment with Mailing Promise gaven o differences inyield . The average yields for three years varied from 142 to 147kg/are .A small trialwit hwindsheite r for raspberries also gave no differences. In the pruning trialwit h red currants,shor t pruning gave a lower yield than longprunin g and mechanical pruning (averaged over five years). Jonkheer van Tets yielded better than Stanza and Rondom. Berryweigh t and truss length of Jonkheer van Tetswer e also thebest . In a study on the effect of pruning date on red currants onlyver y late pruning i.e., at thebeginnin g of blossoming, gave lower yield and quality. In theplant-distanc e trial forblac k currants,onl y a slight difference in yield was seen after 6year s between the cuttings planted at 15,30 ,an d 60 cm and the bushes.Yiel d fell after the original distance of the cuttings on 15c m was increased to 30o r 60 cm. In the pruning trial forblackberrie s therewa s no difference between autumn and spring pruning on either Himalaya or Thornless Evergreen.Average d over four years,autum n pruning on Himalaya gave 11%les syield .Thornles s Evergreen showed no difference. In a trial concerning the effect of pruning dates on Himalaya,ver y late pruning (mid-April)gav e a loweryield .

Variety testingo fraspberries , currants,an dblackberrie s

In the variety trial for raspberries,Gle n Clovawa s themos t productive. This variety ripens at the same time or abi t later thanMailin g Promise.Th e late-ripening variety Schönemann also gave good results.Yiel d of the late- ripening Sirius was low.Yiel d ofMailin g Promise and Spica,whic h ripen at the same time,wa smoderate . In an initial screening of thevarietie s Orion,Admiral ,Veten ,an dMultiraspa , yieldswer e again disappointing. So far,thes evarietie s dono t seem tob e an improvement on the oldervarieties . In thehedge-heigh t experiment for red currants,whic hwa s terminated in 1974, Stanza gave thehighes t yield over the five-year period.A s a result ofman y unfruitful bushes,yiel d of Rondomwa s low. In 1975Maarse s Prominent and Redlake gave very good yields; in this trial,average d over five years,th e yield of these twovarietie s almost equalled that of Jonkheer van Tets. In theblack-curran t trial yield of Black Reward was for the first time disappointing. This late-floweringvariet y was affected by theba d weather during blossoming time in 1975.Average d over sixyears ,however , theyiel d of

27 Black Reward was higher than that of Wellington XXX or Baldwin Hilltop. In thevariet y trial onblackberrie s Himalaya gave thehighes t yield for the fifthyea r in succession (3.64 kg/m2); Thornless Evergreen yielded 2.64 kg/m2, Smoothstem 1.94 kg/m2, and Thornfree 1.62 kg/m2. Smoothstem ripens some days later than the late-ripening variety Thornfree, is susceptible to fruitrot, and difficult topick . The lowyiel d of Thornfree wasmainl y due to cane diseases; in other places resultswit h Thornfree have been good.

Cultural experiments with blueberries

In the plant-distance trial at Horst the highest plant density (6,000plants / ha)gav e thehighes t yield. Production per plant was not influenced byplant - distance. Thebes t varieties in this trial are Bluecrop and G 71.Berkeley , Collins, Coville and Ivanhoe were susceptible to stem canker caused by Godronia oassandrae. The I.S.H.S. variety trial was terminated. The European Goldtraube selections H Ian d G 71 gave the best results.Th e growth of H Iwa s very good and the yield high. Growth of G 71 is slightly lower,bu t at ahighe r plant density yield equals that of H I. Colour and taste of the berries of G 71 are somewhat better. Yield of theAmerica n varieties was again low,becaus e many bushes died. The percentage of dead bushes varied from 25 to 75.Th e American varieties possibly have higher soil requirements.

28 SOIL FERTILITY SECTION

P. Delver Soil management experiments

Four plant densities (beds,single-ro w systemswit h trees planted at 300 x 102, 355 x 136,an d 395 x 205 cm distance), and fivenitroge n dressings (0,70 , 140, 210,an d 280k g Npe r ha)ar e compared in an experiment with Schone van Boskoop M.9 apple trees,plante d in 1972.Th e single-row systems also comprise twowidth s of tree-strip free ofweeds : 150-200 cm (wide strips) and 30-40 cm (narrow strips). With the latter the large area of grass on the alleyways strongly competes with the trees for moisture and nitrogen.Mor e details of this experiment have been given in the 1974Annua l Report (p.25) . The trees in thevariou s planting-systems showed awidel y different response tonitroge n fertilization. This was attributed todifference s inprunin g and in light conditions.T opromot e growth of the lowerbranche s and formation of sturdy frame branches, the centralleader so fwidel y spaced treeswer e pruned after plan­ ting at a lower level above the ground thanwer e those of densely planted trees. The latter, in thebed s and in the 300x 102c mplots ,wer e pruned at greater height topromot e development of slender trees.Als o in later yearswidel y spaced trees were more severely pruned especially by removing young shoots.Thi s treat­ ment resulted in differences in shoot growth. In 1972,fo r instance,tree s whose central leader was cut at 70,80 ,9 0 or 100c m above the ground produced on average 280,260 ,23 0o r 190c m ofne w shoots,respectively . The differences inprunin g are also reflected by the following data.Fro m the total length of shoots produced in 1973, 13, 17 (26),2 4 (25)an d 34 (38)% wa s pruned away in thenex t winter. The numbers betweenbracket s refer to trees in narrow strips,th e others to thebed s and the single-rowswit hwid e tree-strips, in the order stated above. Whether strong pruning, as in the "395x 205"c m rows,wil l soon result in the development ofbigge r trees,wit h more fruit-bearingwoo d than the densely planted trees,depend s on thenatura l vigour and on soilproperties .Th e growth of these trees on shallow clay overlying sand isno t very vigorous,an d 1973 and 1974wer e rather dry.Rando m measurements showed that at end of 1974 trees in the beds had more fruit-bearing wood (as aresul t of limited pruning) than trees planted at 395x 205 cm. The latter however showed more growth,havin g a greater stem girth increase.Apparentl y the extra shoot growth brought about by severe pruning did not compensate for the extra loss of pruned shoots. Nitrogen percentages in the leaves given in Table 8 reflect a distinct influ­ ence of the fertilizer dressings and of competition exerted by grass in theplot s with narrow tree-strips.Unde r conditions ofnitroge n deficiency (lownitroge n dressings,narro w tree-strips), severe pruning in the plotswit hwidel y spaced trees caused higher nitrogen percentages.Th e same effectwa s found in the pre­ ceding years.Prunin g apparently mobilizes nitrogen reserves in the tree or enhances uptake by theroots . Table 9finall y gives the yields.Multiplyin g these by 3430 (beds), 2930, 1860 or 1110 (trees per ha)respectively , gives the production per ha.Dens e planting systems then show a considerable advantage overwid e planting. In acomplicate d way the yields per tree are influenced by nutrition,pruning , growth, exposure and possibly even shelter. Competition forwate r and nitrogen

29 Table 8.Nitroge n percentages in leaves of Schone van BoskoopM. 9 on 6Augus t 1974relate d tonitroge n dressings ink gN per ha,plantin g distance, andwidt h of the tree-strip.

Plant density, soilmanagemen t 0 70 140 210 280

Beds, clean weeding 2.26 2.48 2.52 2.50 2.51 300x 102,wid e tree-strip 2.29 2.41 2.46 2.56 2.54 355x 136,wid e tree-strip 2.37 2.46 2.53 2.55 2.52 395 x 205,wid e tree-strip 2.33 2.47 2.48 2.52 2.51 300x 102,narro w tree-strip 1.81 2.04 2.28 2.40 2.45 355 x 136,narro w tree-strip 1.96 2.25 2.37 2.47 2.51 395 x 205,narro w tree-strip 2.03 2.16 2.39 2.48 2.45

Table 9. Yields in 1975 of Schoneva n Boskoop planted in 1972,i nk g per tree.

Plant density, soilmanagemen t 0 70 140 210 280

Beds, clean weeding 13.6 13.6 13.3 12.9 12.6 300x 102,wid e tree-strip 12.5 13.1 13.5 13.7 12.6 355 x 136,wid e tree-strip 14.6 15.5 16.5 18.2 16.5 395 x 205,wid e tree-strip 15.7 19.9 19.4 20.2 18.6 300x 102,narro w tree-strip 6.9 9.5 10.0 9.9 10.3 355 x 136,narro w tree-strip 8. 1 1 1.0 10.7 13.6 13.2 395 x 205,narro w tree-strip 9.0 11.8 14.8 13.6 12.7

had by far the greatest influence as isdemonstrate d by those trees innarro w strips that received only a little nitrogen. The difference between trees in narrow and wide strips has in the first place tob e attributed to lower fruiting in thenarro w strips (weak buds, low fruit-set or high fruit-drop) and to smaller fruits rather than to a smaller size of the trees.Difference s in length of the fr-uit-bearingbranche s were not distinct except for the 300x 102 cm plant dis­ tance.N o doubt the effect of competition has been intensified by the relative dry conditions in this experiment. The response tonitroge n also showed a relationship with plant density: the maximum relative yield increase for the first four systems (without competition) was 0, 10, 25,an d 29%;fo r the remaining three:49 ,68 ,an d 64%.I n other words, the favourable effect ofnitroge n tended todecreas e themor e densely the treeswer e planted. This result suggests that differences in degree of pruning and exposure may influence the effect of nitrogen. They do influence the produc­ tion per tree:fo r instance at the start of 1975 trees in the beds had more fruit- bearing wood than trees in the "395 x 205 cm" rowswit h wide strips,bu t their production was considerably less,partiall y because the fruitswer e smaller (201 g/fruit as against 228 g in the latter system). In the former, fruit weight response tonitroge n wasver y weak, in the latter,however , itwa s very positive. These provisional results may turn out tob e of paramount importance forprac ­ tical growers. Inmoder n orchards nitrogen dressings have been considerably reduced of late,becaus e of the lownitroge n requirement of the grass strip system. The present tendency to increase plant density and toprun e lessma y motivate a further decrease in the use of nitrogen fertilizer. In the same experiment the incidence of shoot canker was observed. The trees were rather heavily infected after thever y wet summer and autumn of 1974an d the next mild winter.Man y cankered shoots had tob e pruned away in the unfertilized

30 plots of thebed s and on spots known forba d soil structure. In two rows of Cox's Orange PippinM. 9 and one row of each of Schone van Boskoop and WinstonM.9 ,a small experiment on trickle irrigationwa s started. The trees received 8 or 16 litres per tree on 125day s between 29 April and 13 September. In this first year of the experiment therewer e no differences in growth oryiel d related to the treatments.Ta pwate rwit h a pressure reduced to 1 atmwa s used. Even though a fine filterwa s used, thenozzle s gradually became blocked because of very fine rust particles in thewater .

Invariou s soilmanagemen t experiments tree size at planting and, in the following years,yield s per treewer e recorded. Thus the influence of tree quality onproductivit y could be investigated and a favourable effect on various apple varieties was assessed. For instance in 1966 two-year-old Cox's Orange Pippin treeswer e planted at 400x 237 cmdistanc e in anexperimen t at Oosthuizen. At planting the stem diameter ranged between 16an d 25mm . Over a five-year period (1968-1972), trees with diameter exceeding 20m m yielded 13k gmor e than trees with an initial thickness of 17mm .

Influenceo fnitroge n onyiel d andqualit y offrui t crops

Recently nitrogen dressings inmoder n Dutch orchards have been considerably reduced fromwel l over 100k g N per ha to 0-80 kg,mainl y because of the low nitrogen requirement of the grass strip systemwher enitroge n taken upb y the grass is cycled bymulching . Poor financial results,a slightly more vigorous tree growth in a grass-strip culture,an d possibly diminished nitrogen require­ ments in dense lightly-pruned orchards (see preceding chapter)als o have played a role. Invariou s experiments omission of fertilization caused little orn o yield reduction. Sometimes,ther ewa s also a slight improvement in fruit quality. The question remains whether such lowdressing s imply a risk ofyiel d losses due to formation ofwea k flowers and failure of fruit set.Thes ema y occur under dry soil andweathe r conditions or after extreme leaching ofnitrat e by excessive rainfall. The profit from less labourneede d for pruning and mowing, lessferti ­ lizer costs,an d improved fruit quality has tob e balanced against the risk of a too great reduction inyield . Invie w of the above considerations the fertilizer experiment with apples on sea clay atNumansdor pwa s continued with modified treatments.Th e experiment was started in 1970wit h sixGolde n Delicious and five Cox's Orange PippinM. 9 trees per plot in six replicates.Unti l 1974 the treatments were:contro l (no nitrogen)an d 120k gN per ha either as calciumammonium nitrate or as two different qualities of "Gekro",a n organic nitrogen fertilizer of animal origin. In 1970-1974 fertilization resulted in only a smallyiel d increase of Golden Delicious,wherea s Cox's Orange Pippin trees usually showed a slightly lower yield. The effect of fertilization,however ,depende d on the position of the plots. There were tworow s of plots:a wester n wind-protected row alongside an olderhedg e at 8m distance,wit h shadow in the afternoon.Nitroge n percentages in the leaveswer e higher than elsewhere in the orchard and Cox's trees inparti ­ cularwer e also bigger than elsewhere. The other half of the plots in an eastern row, were more exposed and lesswind-protected . In thewester n row, Golden Delicious trees did not respond to fertilization whereas Cox's trees showed a yield reduction. In the eastern row omission of nitrogen resulted in distinctly lower yields of Golden Delicious whereas itha d no effect on Cox's Orange Pippin.

31 These findings demonstrate that fertilization effectsver y much depend on the vigour and growing conditions of the tree. In 1975 the treatmentswer e altered. The control plots remained unfertilized but the plots fertilized sofarwer e evenly distributed over three new treatments: 0, 60o r 120k g N per ha spring dressings of calcium ammoniumnitrate .

Table 10.Yiel d response in the fertilizer experiment at Numansdorp.

Treatments Kgpe r tree Kg per tree Cox's Orange Pippin Golden Delicious

No fertilization since 1970 45.4 52.7 No fertilization since 1975 40.2 48.7 60k gN per ha since 1975 39.5 47.7 120k g N perh a since 1975 40.9 50.3

The above table shows that in 1975wit h bothvarietie s the best results were obtained onplot s thatha d received no nitrogen since 1970.I twa s calculated that the inter-tree variability of theyield s of Cox's Orange Pippin on these plots,wa s somewhat smaller than onplot s that had been previously dressed. This points to anunfavourabl e effect ofnitroge n on fruiting, probably by reduced fruit-set or increased fruit-drop.Unde r the conditions of good natural growth in this experiment, thisnitroge n effect may bebrough t about by enhanced growth.

The taste of two lots of Schone van Boskoop apples,store d until February 1975, was judged by 28persons .Th e apples came fromwell-fertilize d plots or fromnon - fertilized plotswit h distinct nitrogen deficiency, in the experiment mentioned in the previous section. The participants were asked to express their preference for one of the two lotsbu t on average no difference was found. The experiment was repeated on amuc h greater scalewit h apples from the 1975 yield. This time the apples came from plots fertilized with 0, 140o r 280k g N perha ,i n three replicates,picke d on three successive dates,an d stored for three different periods.Th e experiment isno t yet finished but laboratory deter­ minations on apples stored until December have shown distinctly higher percentages of sugar and acid in 0N-apples . Later picking resulted in a slight decrease in sugar and acid contents.Moreove r these contents seemed tob e positively correla­ tedwit h the potassium status in the leaves of the plots involved.

Relationship between potassium levels inappl e leavesan dsoi l

The potassium supply in orchards has lately attracted attention since a positive correlation exists between the K-status of leaves and fruits and the susceptibility of apples tobitte r pit and breakdown. Fertilization aims at supplying the soil with very moderate amounts of potassium, and onmarin e soils average annual dres­ sings forman y years haveno t exceeded some 40k g of K2O per ha.Th e question now arises towha t level theK-statu s of the treema y be reduced without influencing productivity. Investigations also involve the K-status in soil of the grass-strip culture. The pot trialwit h Schone van Boskoop M.9 thatwa s started in 1974wit h two- year-old treeswa s continued and terminated in 1975.Th e potswer e filled with soil sampled from layers of successive depth from the clean-weeded tree-strips

32 of a ten-year-old fertilizer experiment with pears.O nplot swher e the grassed alleyways had received no orheav y nitrogen dressings for several years (totalling 360k g N per ha annually), the tree strips had received only very little ora great amount ofmulche d grass,respectively . The soilwa s sampled at 0-1, 1-3, 3-7, 7-15 and 15-20 cmdept h in the little-mulched strips and at 0-2,2-4 , 4-9, 9-12, 12-20 and 20-30 cmdept h in theheavy-mulche d strips.Wit h depth several soil properties changed: deeper layers had much lower K,N , and organic matter contents andwors e soil structures than layers at the top.A t equal depth, soil inheavy-mulche d strips contained more K,N and organic matter than soil from the little-mulched strips.Th e separate layerswer e transferred intopot s and trees were planted at the start of 1975. The first year, leaf analysis already showed that the uptake of nitrogen and potassiumwa s considerably reduced with increasing depth of origin of the soil layer. Trees growing in soil from 0-4 cmdept h had much bigger and greener leaves and higher nitrogen and potassium contents (1.60 - 1.86%K ) than trees in soil from 20-25 cmdept h (0.54- 0.93%K ); Furthermor e itwa s found that at equal content of HCl-soluble K in the soil, leaf-K contents were higher in theheavy - mulched than in the little-mulched soil. The subsoil deeper than 10c mcontri ­ butes much less to theK-suppl y of the tree than the topsoil because the K-content is lower and K ismor e bound to the clay complex than to thehumu s complex and therefore is less available to the tree.Besides , the subsoilha s apoore r soil structure and is less intensely rooted. These conditions alsonegativel y influence the uptake of potassium. The difference in soil structure was demonstrated at the end of the experiment when the pots were emptied. In contrast to trees in topsoil,tree s in subsoil had not penetrated the dense angular blocky aggregates and the rootswer e easily released from the soil. There was some tree-pit in the fruits picked in 1975.I twa s somewhat related to the fruit load and toa Kdressin g given tohal f of the pots,bu t the soil • depth had little influence.Tree s in subsoil although very low inK status also produced some pitted fruits and here irrigularwate r supply, due to the poorer soil structure,wa s believed tob e amajo r factor.Thi s finding points to the. importance of good moisture conditions for thebitte r pit problem. Potassium deficiency in apple treeswa s observed ina few holdings on sea clay in thewes t of the country. From 300 trees various datawer e collected toinves ­ tigate the relationships between potassium in the soil, %K in the leaves,K deficiency symptoms,an d growth and productivity. K deficiency has a strongnega ­ tive effect on fruit growth, themor e sobecaus e it predominantly occurs on trees with a rather heavy crop load.A decrease in fruit size could already be observed on trees with only slight symptoms of K deficiency. Shoot growth isals o retarded. Cox's Orange Pippin and James Grieve are susceptible varieties, Schoneva n Boskoop is slightly less susceptible and Golden Delicious is the least susceptible variety. Fruit prematurely picked from Schoneva n Boskoop treeswit h K deficiency, at the end of July,develope d low temperature breakdown after cold storage for 12 days only. Irregular bearing Benoni trees showed deficiency symptoms only in heavy cropping trees. Very serious K deficiency was observed in a fruit orchard near Hem (NH).I n this area land was re-allocated in 1964-1965 and on that occasion the soil was deep-plowed and levelled. The soil is amarin e silt ridge,ligh t textured and originally with a shallow topsoil low inhumus . In such soils the structure is susceptible todeterioratio n and indeed the influence of deep-plowing and level­ lingb y heavy implements was still visible after ten years:K deficiency occurred where the topsoil had been partly removed and soil compaction was evident.A low water-holding capacity and strong groundwater fluctuations were further charac­ teristics of the soil and in the profile many dead rootswer e found.

33 In the soilmanagemen t experiment at Oosthuizen the relationship between K in the 0-20 cm topsoil (HCl-extract), and K contents in the leaves of Cox's Orange Pippinwa s found to depend upon the soilmanagemen t and themowin g method. Chemical weed control without tillage on the tree strips showed 0.20% higher K- contents at equal soil-Kvalue s compared with plotswher e the tree strips were rotary tilled. Higher K-percentages at equal soil-Kvalue s in the tree strip were also found onplot swer e themow n grasswa s left on the grass alleyway as compared withmulchin g on the tree strip. In the former case the K-content in the tree strip decreases considerably because of omitted K supplywherea s the tree finds an important K-source under themulche d grass-strip.

Bitter pit inappl e

P. Delver and A. Pouwer

As in earlier years,muc h attentionwa s given to improvement of the prediction of the susceptibility tobitte r pit and breakdown on thebasi s of the level and regularity of cropping and of leaf analysis.Advic e isals o given to individual fruit growers on thebes t time for picking and on the expected keepability of thefruits . So far, the interpretation of leaf composition has been based on the (K+ Mg ) /Ca-ratio calculated from the contents expressed asmilliequivalent s per 100g drymatter .However ,evaluatio n of the orchard factors and analytical data depended onpersona l experience and opinion of afe w specialists only. To permit computer calculation of the chance ofbitte r pit,a ne w interpretation systemwa s developed by A. Pouwer,P . Oud,an d S. Bommeljé. In this systemmor e importance isattache d to the cropping level and toK contents thanwa s the case in the old system. The cropping level in the orchard isno w assessed by a standardized method. The susceptibility of low-temperature breakdown (related tover y lowK contents) isals o taken into consideration. In the testing of thene w system, fruit analysis iscompare d with leaf analysis,bu t the former is too expensive for practical use because of high cost of sampling, transport, storage,an danalysis . A. Pouwer and S. Bommeljé tested thene w system of prediction in a great number of orchards in fruit-growing areas in theprovince s of Limburg and Noord- Holland aswel l as in the Southwestern part of The Netherlands. Initially, leaf and fruit samples of each of twovarieties ,Cox' s Orange Pippin and Schone van Boskoop,wer e taken between 23Jul y and 6Augus t from 60moder n plantations. After evaluation of the leaf-analysis results,3 0 orchards of each variety with widely varying K and Ca contents were selected, and 100-kg lots of average-sized appleswer e picked and stored in the Sprenger Institute atWageningen . After storage, fruit sampleswer e taken for analysis.Th e storage results and their relation topredictio n areno t yet available. From 26 July to 10Augus t theweathe r was extremely hot and dry (daytime temperatures 30-32°C). Therefore, 32 orchards were visited again on 12Augus t and sampleswer e taken to investigate the influence ofver y hot weather and excessive transpiration on leaf and fruit composition. The orchards hadprevious ­ lybee nvisite d for leaf and fruit sampling on 25 and 24July . K contents in the leaves showed a slight increase (0.05-0.10%), as did theN percentages,bu t otherwise no particular or unexpected changes took place in this short period. Some investigators have reported adro p in calcium levels indicating export of calcium from growing fruits in periods of extremely highmoistur e stress exerted on fruitsb y excessive leaf transpiration. Such adecreas e could not be

34 demonstrated in any of the 32orchards ,eve n though theweathe r was very hot. On the contrary, during the three-week period calcium seemed to increase more than usual,i.e. , on average by 1.4 mg Ca per fruit for Cox's Orange Pippin and 2.0 mg for Schone van Boskoop.

A similar investigation,mad e to test the value of bitter-pit prediction,ha d been carried out in 1974 in fifteen Cox's Orange Pippin orchards (see Annual Report for 1974,p . 28).Afte r storage,health y apples and apples showing bitter pit or senescent breakdown (browning of the skin)wer e individually analysed, and the following results were obtained. Healthy fruits,70-7 5 mm indiameter , showed highly variable amounts of calcium, ranging from 2.9 to 6.1 mg (average 4.1 mg)pe r 100g freshweight . Fruits with disorders contained from 2.3 to 4.1 (average 3.2)m g Cape r 100g . Low calcium levels were found inpitte d fruits aswel l as in fruitswit h brown discoloration of the skin.Fruit s with ahig h calcium content did not show any of these disorders,bu t a low calcium content does notnecessaril y mean that the fruit will not remain healthy. Variations in calcium level inhealth y fruits and in the degree of disorders in fruits low in calcium are apparently dependent upon other -no t measurable - factors,suc h as cell size or ripeness at picking. Similar analyses of single fruits were made after storage of Cox's Orange Pippin apples picked in 1974 from the soilmanagemen t experiment at Oosthuizen. In this lot bitter pit occurred toabou t the same degree (2-4%)a s in the fifteen orchards mentioned above (3.8%b.p . on average) and the analysed fruits had the same size.Th e calcium level inpitte d fruits ranged from 2.3 to 4.2 (average 3.1) mg Ca per 100g freshweight . Healthy fruits contained 4.5 mg Ca on average. In this soil management experiment the effect of chemical weed control or rototillage on the tree strips aswel l as ofmulchin g themow n grass either on the tree strip or on the grass alleys isbein g investigated. In 1975,th e yield per tree was 37.8,38.8 ,38.4 ,an d 36.0 for treatments CC,CM ,M C andMM , respec­ tively (CC and CM =herbicid e treatment without tillage on the tree strip,an d with mulching on the tree strip or on the grass strip,respectively . MC andM M = soil tillage on the tree strip andwit h the same mowing methods aswit h CC andCM) . An area of 1.4 hawa s planted in February of 1975wit h Cox's Orange Pippin and Schone van Boskoop trees,bot h varieties free of viruses,fo r an experiment on the effect of four soil and mowing treatments on bitter pit and yield, asmenti ­ oned above (CC,CM ,MC ,MM) .I n addition,potassiu m fertilization on half of the plots and four treatments of water supply by means of trickle irrigation will be applied. Three hundred single apples var.Winston ,picke d in 1974,hav e been analysed for K, Ca,an d Mg. For each fruit,positio n in the tree,ligh t exposure, and size were recorded. The data are tob e analysed by computer. The provisional results suggest that K and Ca contents are strongly affected by the tree of origin and that distinct tree-to-tree variations exist. Fruit diameters were measured for trees ofvariou s apple varieties showing tree pit.N owithin-tre e correlations between bitter pit and fruit size were found. In a largenumbe r of apple varieties from screening trials stored by the Pomology Section, great differences inbitte r pit occurred by the end of November. Healthy and affected fruits of several varieties were analysed to study varietal differences in the limiting composition atwhic h fruits start to develop bitter pit. Provisionally, such differences do indeed seem to exist.Mos t varieties remain healthy if the K/Ca ratio in the dry matter is under 30;som e pitting was found atvalue s between 28an d 35,an d fruitswer e extremely pitted at values above 35.However , somevarietie s of sweet apples remained free of pitted fruits atK/C a values of about40 .

35 Weed control

P.J.Boldin g

Inth eexperimenta lgarde na tNumansdor pa tria lha sbee nstarte dt ostud yth e effecto fdifferen tcrop ssow nunde rappl etree sa sa substitut efo rwee dcontrol . Thetree sar eGolde nDeliciou sM. 9plante d ina doubl ero wi nth esprin go f1973 . Theai mo fth eexperimen ti st oinvestigat ewhethe ra ligh tcove ro flow-growin g weedscoul db emaintaine dwithou tto omuc hcompetitio nfo rth etrees .I fthi swer e thecase ,th eus eo fherbicide scoul db erestricted . Thefollowin gtreatment sar eapplied ,wit hfou rtree spe rplo tan dthre e replications: 1.Natura lvegetatio n (grasses, Polygonum sp., Convolvulus arvensis L., Senecio Vulgaris L.), treatedwit hherbicide si fnecessar ybu tno twit hSimazin) ; 2. Thesam ebu tmow ni fnecessary ; 3.Re dfescu e (Festuea rubra L.), mowni fnecessary ; 4. Annualmeado wgras s (Poa annua L.),mow ni fnecessary ; 5.Mouse-ea r (Cerastium (Dill.)L.); 6.Stonecro p (Sedum acreL.) ; 7. Control (cleancultivation ;applicatio no fSimazin ,Gramoxone ,o rAamitrol) .

Thesecrop swer esow no n2 5Apri l 1975,an ddevelope dwel li nth ecours eo f theseason .Analysi so fleave ssample d 18Jul yha sshow nthat ,s ofar ,annua l meadowgras sha sha dth estronges teffect ,lowerin gth eN percentag et o2.0 3 (control:2.45 %N) . Furthermore,du et ocompetition ,treatment s 1-4ha dsomewha t loweryield stha n5-7 .

36 PHYSIOLOGY SECTION

J.Trom p Investigations into factors underlying the relationship between the vegetative and generative developmento ffrui t trees

In 1974,a po t experiment was started with one-year-old Cox's Orange Pippin trees to study the effect of an "early" (afe wweek s after full bloom)an d a "late" (shortly after shoot growth had ceased)applicatio n of GAit+7 on shoot growth and flower-bud formation ofvertica l and horizontal trees which were unfertilized, fertilized in the spring, or fertilized late in the summer. The early application ofGA l)+7 favoured growth in thehorizonta l trees;th e effect wasmuc h more marked than in thevertica l trees.Th e positive effect of the horizontal position on floweringwa s greatly reduced by GAi,+7; the time of application was of little importance. Therewa s no clear effect of the nitrogen treatments, possibly due to the rather rich soilmixtur e used. The experiment will be continued for another year. In ane w experimentwit h Red "Schone van Boskoop" the effect of some GAi^ and SADH treatments on growth and flower-bud formation isbein g studied. Since the data in the literature indicate that ammonium stimulates flowering, the experiment isbein g done on unfertilized trees and trees given either anitrat e dressing or an ammonium dressing.

Nitrogen research

In the spring of 1975 an experiment was commenced with three-year-old Golden Delicious trees planted innitrogen-poo r soil, toevaluat e the effect of three nitrogen treatments,viz .unfertilized , nitrate fertilized,an d ammoniumferti ­ lized on growth of shoots and fruits and on flowering.N-Serv e (2-chloor-6 (trichloormethyl)pyridine )wa s added to the soil topreven t nitrification of ammonium. Theminera l composition of leaves and fruitswa s followed throughout the season. In addition, the soluble nitrogen fraction isbein g analysed for amides and amino acids.Th e mineral data may indicatewhethe r ammonium really enhances the chance that bitter pitwil l occur. In 1975 therewa sn o difference in shoot and fruit growth between the nitrate and ammonium treatments;th e unfertilized trees lagged slightly.Analytica l data areno t yet available.

The effect of root temperature on the behaviour of budded apple rootstocks

Under controlled-environment conditions shoot growth, leaf development,dry - matter production,wate r consumption, andminera l composition of rootstocksM.9 , M.26, andMM.10 6budde d with Cox's Orange Pippin inAugus t of 1974wa s studied at all combinations of a low (40-50%)an d ahig h (85-90%)humidit y with three root temperatures. Ingeneral ,dry-matte r production increased in the ascending sequenceM.9 ,M.26 ,an dMM.106 . The increase indr ymatte rwa s leastmarke d at 10C and greatest at 17°C,a t least inM. 9 andM.26 .MM.10 6wa smos t productive at aroo t temperature of 26°C.Generall y speaking,dry-matte r production was higher at the high than at the lowhumidity .

37 With respect to theminera l analyses,a few data for K and Ca in thene w growth will be given here. In contrast towha t was found for the unbudded rootstocks (AnnualRepor t for 1974,pp . 31-32), the effect of air temperature and humidity was rather slight. InM.2 6 the K/Ca-ratiodecrease d at increasing root temperature. InM. 9 this ratio was highest at 17°C;i nMM.10 6 therewa s no effect at high humidity but a slightly reduced valuewa s found at 10°Ca t lowhumidity .Fo r all rootstocks the K/Ca ratiowa s highest at the high humidity.

The effect of temperature on flower-bud formation of Cox's Orange Pippin

In the spring of 1975flowerin g datawer e obtained for trees of Cox's Orange Pippin exposed to four temperature treatments from full bloom in September 1974 until January 1975 (for details seenex t section). In the trees held at 17°C during the second and the third periods the higher temperature in the preceding period resulted in a considerably reduced flowering abundancy. This effect was completely nullified when the temperature was kept at 24°C during the third period. Namely, flower-bud formation was reduced in the trees held at 17°C in the first fewweek s of theexperimenta l period but was enhanced when the tempe­ raturewa s high during thatperiod . If thenumbe r ofwell-develope d flowers per cluster is taken as criterion,cluste r quality was very poor in the trees given the higher temperature in the last period.

Pre-harvest physiology offruit s asrelate d tofrui t quality

The effect of air temperature, root temperature, and air humidity on shoot and fruit growth and on the mineral composition of leaves and fruits of apple

In 1974,startin g at full-bloom, four temperature treatments were applied to three-year-old Cox's Orange Pippin trees,asfollows : 15/9- 16/10 16/10- 19/11 19/11 - 7/1 17°C 17°C 17°C 24°C 17°C 17°C 17°C 17°C 24°C 24°C 17°C 24°C Winter restwa s extended until August.Roo t and air temperatures were equal throughout. Thehig h temperature given during the first 4-5 weeks after full-blossom stimulated shoot growthmarkedl y as compared with the low temperature treatment. This differencewa s not levelled when the treeswer e kept at the same temperature (17°C) in the second and third periods. Increasing of the temperature in the third period (about nineweek s after full bloom) stimulated growth considerably in the treeswhic h had been exposed to 17°C in the twoprecedin g periods (17-17- 24 treatment)bu t had no effect in the 24-17-24 treatment. With respect to the growth of the fruits,th e main finding is that the in­ crease in temperature from 17°C to 24 C in the third period enhanced the rate of fruit growth (expressed as freshweight ) inbot h the 17-17-24an d the 24-17-24 treatments,bu t had no effect if considered on a dryweigh t basis.A t any given date throughout the season the amounts of Cape r fruit were about the same in the 24-17-17an d 24-17-24 treatments,bu twhe n the temperature was kept at 17°C during the first period, the elevation in the last period (17-17-24 treatment) raised Ca in the fruits to a level exceeding that of the 17-17-17 treatment.

38 Ina second experiment with three-year-old Cox's Orange Pippin trees the effect of root temperature (five steps:6° , 12°, 17°, 24 ,an d 30°C)an d soil moisture content (three steps:30 ,25 ,an d 20%;soi lmixtur e sand +po t soil) was studied. Air temperature was about 20°C throughout. Shoot growth increased with increasing root temperatures over the entire range.Th e same holds for the growth of the fruits up to 17°C;a furthe r rise in root temperature had no effect. In the range studied, an increase in soil moisture content favoured shoot and fruit growthmarkedly .Analytica l data areno tye t available. Finally, a third experiment was started, againwit h Cox's Orange Pippin,t o evaluate the effect of four air humidity treatments viz.:hig h (85-98%) through­ out; low (40-50%) throughout; low atnight ,hig h during the day,an d high at night, low during the day. This experiment is still inprogress .

Fruit quality, time of picking, and storage conditions

In cooperation with the Extension Service the series of experiments on the effect of picking time and storage conditions on quality of Golden Delicious was continued. The 1974/1975 experiment was similar to the preceding experiment. Fruits were picked indifferen t places in TheNetherland s on three dates (25/9, 10/10 and 18/10)an d stored under three sets of conditions,viz .ordinar y CA- storage (6-7%C0 2, 14-15%0 2)a t 3°C; scrubbed CA-storage (3-5%C0 2, 3%0 2)a t 3°C, and scrubbed CA-storage (3-5%C0 2, 3%0 2)a t 1°C.Th e storage data fall outside the scope of this report. Respiration showed the same level in 1974a s in 1973o rwa s slightly higher. The colour value did not differ from that of theprecedin g year. In the picking period, fruit colour and starch level changed as described in the 1974Annua l Report (p. 33).A remarkable findingwa s that different fruit samples sometimes showed similar colour values but divergent starch levels.Fo r example,th e colour values (estimated by colour chart) for fruits from Geldermalsen and Limburg were 5.4 and 5.1 and the starch values (iodine test,arbitraril y chosen values) 3.0 and 6.9, respectively. The taste of the fruitswa s evaluated by a taste panel early inJune , 1975.Ther ewa s little effect ofpickin g time but,generall y speaking, the fruits picked at the end of September received a slightly lower score. The storage conditions did not affect fruit taste. In addition toGolde n Delicious,th e respiration intensity of a few lots of Cox's Orange Pippin fruitswa s followed around thepickin g period. The data for both varieties confirm our previous experience that the estimation of respiration intensity isno t ofmuc h practical use to indicate the picking time. In contrast towha t was found for C02, the production of ethylene proved to increase sharply in ripening fruits andma y therefore be amor e helpful criterion for the predict­ ion of the picking time.

39 PHYTOPATHOLOGY SECTION

H.A.Th,va n der Scheer

Apple

Canker

Infection by Nectria galligena Bres.wa s investigated inLob o trees.Fo r this purpose,branc h segments bearing cankerswer e hung in the trees during two-month periods from 1Marc h 1974 to 1Marc h 1975.Mos t of the infections occurred in the November/December period. In the September/October and January/February periods thenumbe r of infectionswa s only half,an d in the remaining periods a quarter,o f that in theNovember/Decembe r period. Cankers in the tops of trees are sources of serious infection. Once again, thiswa s clearly shownwhe n branch segments with sporulating cankers were hung over shoots of a stoolbed of apple rootstockM. 2 or on the soilnea r theshoots . In the latter case thenumbe r of infections in the shootswa s only one-fourth of that in the former.Furthermore , some of the shoot infections in plots with branch segments on the soilma y be caused by cankers on thebranc h segments hung inneighbourin g plots.Th e risk of infection caused by spores from cankered prunings left on the orchard floor after cutting isbein g investigated. One year after the start of the experiment,n o increase in the number of infections in the orchard trees could be seen. Ina trial performed in a stoolbed ofM. 2 apple rootstocks inwhic h fungicides were tested for control of canker, two sprayswit h 0.14% a.i. thiophanate methyl in the leaf-fall period in 1974gav e the best results.On e spraywit h 0.14% a.i. thiophanate methyl at the end of the leaf fallwa s somewhat less effective than two sprayswit h this fungicide,an dha d the same effect as two sprayswit h 0.25% a.i. copperoxychloride and gave somewhat better results than two sprays with 0.16% a.i. captafol.Lowerin g of the concentration from 0.14% to0.07 % a.i. thiophanate methyl resulted in adramati c decline of the control rate.Thi smigh t explain the failure of thiophanate methyl in some commercial orchards,becaus e no resistent strains of the fungus have been detected. Sofar ,th e growth of the isolates onnutrien t agarwit h 10an d 25 ppm a.i. thiophanate methyl was inhibited, and at 50pp mn o growth occurred atall .

Fruit rot

Fruits of four apple varieties were tested for susceptibility to spores of Peziaula maliaortiois (H.S.Jacks.)Nannf . and P.alba Guthrie.Decreasin g suscep­ tibility tobot h fungiwa s seen inGolde n Delicious,Cox' s Orange Pippin, Schone van Boskoop,an d Winston, in that order. As in theprecedin g year, thiophanate methyl gave good control of Gloeosporium rot inGolde n Delicious.Tw o sprayswit h 0.07%a.i . thiophanate methyl tended to be somewhat better than one spray before picking. Two sprays gave good control of rot,eve nwhe n the second spraywa s applied fourweek s before picking. Incidental serious attacks onGolde n Delicious fruitsb y Alternaria alternata Nees and Stemphylium botryosum Wallr.occurre d in 1973.I n a field trial control was not achieved with captan sprays. In the laboratory thiophanate methyl

40 increased the rot caused by both fungi. Invegetabl e growing,mane b and mancozeb are recommended for the control of Alternaria diseases.Neithe r was tested for fruit-rot control,becaus e toomuc hvisibl e residue is left on Golden Delicious fruits.

Phytophthora syringae (Kleb.) Kleb.

In thewinter s of 1973/1974 and 1974/1975 young trees of some apple varieties were attacked by P.syringae, and showed dark-brown orblac k patches all round the buds. Trees of thevariet y RodeMante t were attacked themost .Th e disease also appeared in trees of the varieties Cox's Orange Pippin,Jame s Grieve,an d Winston. Soil andweathe r conditions are thought tohav e contributed to the appearance of thedisease . In the laboratory some fungicides were tested for control of P.syringae. Fruits of Golden Delicious were sprayed with fungicide and placed onwet , naturally infested soil. Little rot developed onapple s sprayed with 0.2% a.i. copperoxychloride,0.16 % a.i. captafol,an d 0.095% a.i. fentin-hydroxide,bu t 0.07% thiophanate methyl enhanced the fruit rot.O n thebasi s of these results, two field trials are being carried out.

Powdery mildew

Some new fungicides were tested onpotte d apple rootstock M.7 and in a field trial on Golden Delicious,t o evaluate control of Podosphaera leucotrioha (Ell.& Ev.) Salm.. Bupyrimate and especially triadimefon were very good, and much better than dinocap in controlling apple powdery mildew. Sinceneithe r of the former fungicides acts onmite s and predators,the y canusefull yb e included in spray schemes for integrated control in orchards.Ther ewer e no significant differences in skin russeting of Golden Delicious fruits due to the tested fungicides.

Apple and pear Silver leaf

Antagonistic activity of Triohoderma viride against Sterevm purpureum (Pers. exFr. )Fr . is reported in the literature. The curative action of T.viride on silver leaf-diseased trees of the apple variety Cox's Orange Pippin and the pear variety Conference was tested. Peat moss pellets containing T.viride and also pieces of agar carrying mycelium of T.viride were put inhole s drilled in the stem of the trees inApril , 1975.S o far,n o diminishment of the silver leaf symptoms has resulted from the T.viride activity.

41 Blackberry

Cane blight

Cane blight caused by Leptosphaeria coniothyvivm (Fuck.) Sacc.wa s seen in thornless varieties of blackberry. The disease symptoms were the same as in raspberry. Ina field trial fungicides were tested on diseased plants of the blackberry variety Thornless Evergreen. Before the fungicides were applied to thebas e of the canes in the autumn, the diseased partswer e cut away as much aspossible .A second fungicide application wasmad e in the spring of 1975. Little effect was obtained with 0.25% a.i. copperoxychloride, 0.2% a.i.thirain , 0.14% a.i. thiophanate methyl,o r 0.05% a.i.benomyl .

Strawberry Red stele

Soilswer e investigated with respect to infestation with Phytophthora fragariae Hickman. Sampleswer e taken from the top 20-cm layer with a soil auger. Young plants of thevariet y Senga Senganawer e used as baits.Th e roots of these plants were infected most severely on the flooded soil samples.Root shun g in awate r column on the soil gave fewer infections.Te n to fourteen days after flooding of the soil samples at 15°C,infectio n of roots was maximal and no increase in the number of infections was seen at twenty-one days after flooding. In the glasshouse the influence of anumbe r of crops on soil infestation with P.fragariae isbein g investigated. Also the fungus isbein g sought in ground with a known history of infestation, to investigate the influence of crop rotation. Preliminary results with grass on soilwit h aknow n history of infestation with P.fragariae dono t suggest a significant reduction in the amount of infestation. Inpo t experiments the duration of the preventive action ofprothiocar b is being investigated. A field trial toobtai n information on control of red stele had tob e terminated due toprolonge d waterlogging. A new one has been started in cooperation with the regional advisory service at Tilburg.

Vertiaillivm wilt

In 1972,th e influence of temperature on the symptom expression of Vertiaillium wilt in four strawberry varieties was investigated at 13an d 24°C.Repetitio n of the experiment with threevarietie s at 12an d 16°C gave more or less the same results. At 16°C,wiltin g occurred later inplant s of Gorella and Talisman than in those of Elista.A t 12°C,th e artificially inoculated plants of Gorella and Talisman appeared normal,an d those of Elistawer e smaller than uninoculated plants. Inprecedin g years sprayswit h 0.07% a.i. thiophanate methyl did not give adequate control of Verticilliim wilt in artificially inoculated Gorella plants. In 1974/1975,tw o sprayswit h 0.4% a.i. thiophanate methyl applied immediately after planting inAugus t and at the resumption of growth inMa y gave good control. The sprayswer e applied at a rate of 1500 litre/ha over the plant rows.

42 ENTOMOLOGY SECTION

D.J. de Jong Tortricids inorchard s

An insect growth regulator isbein g tested on Archips rosanus L..Th e insect was very susceptible,bu t itwa s necessary for the fifth, i.e. the last, larval instar tohav e been incontac t with theI.G.R. . Despite the low starting level of Adoxophyes orana in spring, this insect built up fairly high populations inAugus t and September. Thewar m summer led to exceptional population development and fruit damage. Inman y years the last part of the second flight is run out by unfavourableweathe r conditions at the end ofAugus t and later,bu t thisyea r the first part did so as a result of the integration of the phenology and the course of the flight with temperature and photo period. It is calculated that about 10 to 20%o f the eggs laid during the second flight period developed tomoth s of a third flight inOctober .Man y moths of this third flight are caught in sex traps,bu t conditions were so cold for them and their eggs, that this flight did not result in surviving progeny. This study has led to abette r understanding of the quantitative population development and an improved basis for the forecasting of population density in control programmes. The development of larvae intomoth s of a third flight resulted increased fruit damage. In an orchard complex near Wilhelminadorp, P.Vollaar d tried sex traps and a release-recapture method to study the relationship between the population density of the summer-fruit trotricid and the daily catches.H e also investigated aspects of the dispersion of moths and problems connected with the spacing of sex traps.

Integrated control in orchards

Research on integrated control programmes isbein g done in seven orchards. Methods for the rapid introduction of predatory mites inJul y and early in September form an important problem. Introduction at the end of the summer has given promising preliminary results;establishmen t and multiplication of the predatory mites originating from apple or cherry leaves,occurre d someweek s after their liberation on the trees. The control programmes were based on integrated sampling methods and critical threshold levels.Thes e methods and levels gave some problems with respect to thewinte r moth Operophthera brunata, the clouded drab moth Orthosia spp, and the phytophagous bug Lygus pabulinus, and consequently toomuc h damage was done by these insects in some orchards.

In the experimental orchard at Bergen op Zoom, Amblyseius fallacis and Typhlodromus pyri from the USA,whic h are resistant to azinphos methyl and some other insecticides, are being liberated on apple trees to study their capacity to survive and regulate the fruit tree red spider mite populations. In relation to this trial, the activity of some pesticides on the resistant predatory mites isbein g tested. Separation of the experimental plots hasprove n tob e difficult.

43 Biological control on strawberries in glasshouses Inglasshous eexperiment so nstrawberr yplant sintende d forautum nproduction , thepredator ymit e Phytoseiulus persimilis wasliberate do nsevera ldate safte r infectiono fth eplant swit h Tetranychus urticae. Regulationo fth e T.urticae populationoccurre di nal lcases ,bu tth emomen to freleas ewa sdecisiv efo rth e rateo freductio no ffrui tproduction .Practica lapplicatio no fbiologica lcontro l of T.urticae insuc hstrawberr ycultur eseem sa possibility .Combinatio nwit ha moderateus eo fspecia lfungicide san dinsecticide sseem spossible .

44 SOIL FERTILITY AND ACAROLOGY SECTIONS

P. Delver and D.J. de Jong Influenceo fnitroge n level inappl e treeso nth e development ofth e fruit-tree red spider mite

Ina n earlier field experiment and ina po t trialwit hdifferentiall y ferti­ lized apple trees thedevelopmen t of red spider mite populationswa s found tob e strongly retarded by a lownitroge n status of the tree.Th e egg production decreased and themortalit y of themite s increased. Thus the limiting population density atwhic h theproductivit y of the tree is affected isreache d late orno t at all.Thi s diminishes the necessity to applyacaricides . Nitrogen deficiency is undesirable as itunfavourabl y influences the production of apple trees.Howeve r there is astron g tendency to cutnitroge n dressings in modern orchards especially under circumstances ofvigorou s growth (as influenced by variety and soil conditions) and dense planting. Field experiments have shown that under such circumstances thenitroge n status of the treema y be reduced to well underwha t is considered "normal"withou t affecting theproductivity . It is therefore interesting tokno w howmuc h spidermit e populationswil l be affected by slight deviations of thenitroge n status under "optimal". In 1972 anexperimenta l fieldwa s planted with WinstonM. 9 apple trees.Th e following treatments have been applied since 1974:

A. 250k g Npe r ha given in one single dressing inFebruary ; B. 100k gN per ha,ditto ; C. 250k g Npe r ha spread over the growing season in sevendressings ; D. 100k g Npe r ha,ditto ; E. white clover sown in 1972; F. clean cultivation, organic manure and greenmanure .

The experiment isdivide d into fourblocks ,tw o ofwhic hwil l be infected by winter eggs inApri l 1976.Th e remaining blocks will be kept free ofmite s by acaricides. The aim of this experiment is to create smallnutritiona l differ­ ences and to relate them to the development of themites .Th e relationship between the population density of themite s and thedamag e to the trees can also be investigated. Until June 1975 rathernarro w tree strips,5 0 cmwide ,wer e kept free from weeds. The remaining areawa s covered with a grass sward. Because a rather strong competing systemwa s thus created theyiel d remained low. InJun e 1975 the tree stripswer ewidene d to 1metr e (treatments A-E). Table 11show s some data, indi­ cating the slight differences innitroge n status thatwer e aimed at,an d the response of the trees.Becaus e of thever ywe t summer and autumn of 1974an d the succeeding mild winter, the cropwa s rather heavily infected by canker.Th e table shows that distinct differences innumbe r of cankers were related to the nitrogen status of thetrees . Of each of the 864experimenta l trees data on the growth and production have so far been recorded.

45 Table11 .Dat ao nth einfluenc eo fdifference si nfertilize rtreatments .

Treatment %N i n leaf Kgpe r tree Yield 1975 December 1975 6 13 1974 1975 %fruit s Treepi t number Aug. Aug. with number of 1974 1975 deformed of cankers stalks fruits pertre e

A.25 0k gN pe rh a 3.03 2.36 3.8 6.3 13.5 27 1.9 3.6 1.4 B. 10lu0ku gK N gpe Hpe rh ran a 2.73 2.11 6.7 9.5 5 250k gN (spread ) 2.81 2.31 3.1 6.3 11.5 6 2.2 100k gN (spread ) 2.67 2.11 2.9 5.5 8.8 5 1.4 Whiteclove r 3.05 2.25 2.9 5.5 15.3 6 1.3 2.91 2.25 3.2 5.9 16.2 8 1.9

46 ECONOMICS SECTION

J. Goedegebure Research onth e economic significanceo fne wplantin g systemsan dtree-shape s in top fruit growing

Incooperatio nwit h thepomolog y section figures on theproductio n capacity of high density orchards were collected and interpreted. Especially the relation between production and thenumbe r of trees per hectare and the significance of multi-row systemswer e emphasized. Also differences in investments and costs were estimated so that the different densities,coul d be compared economically. An increasing number of trees per hectare - the research covered the range from 2000-4000 trees per hectare - resulted in amarke d increase of production (Table12) .

Table 12.Productio n increase of 3000an d 4000 trees per hectare compared with 2000 trees per hectare (x 1000kg) .

Production increase 3000 trees 4000 trees

- 1st - 8th year 28.3 56.1 - 1st - 15thyea r 48.3 92.6

Besides the production increase,th e financial result depends on other factors, ofwhic h the establishment costs and the fruit prices are themos t important ones. Under the present circumstances (rather high costs of trees and stakes and moderate fruit prices) the economic advantages of these largenumber s of trees are questionable. After 7o r 8year s the extra trees showa positiv e balance but about the performance (production and quality) in later years there is still uncertainty. Relatively small changes in establishment costs or in fruit prices have a great impact on the economic results.A decrease in the costs of trees and stakes or an increase inprice s soonmak e these largenumber s of trees attractive.

Farm comparative research into the differences infinancia l resultso ffrui t holdings

Thepurpos e of this research is toexplai n themutua l connections and the influence of different factors on the financial results. Soa bette r quantitive knowledge of the factors causing differences in financial results canb e obtained. The research is carried out by factor analysis and isbase d on thebusines s accounts of over 80holdings . A largenumbe r of parameters (±55 )whic h probably have some influence on the results have been drawn up.Althoug h the research is still inprogress ,th e first results arepromising . For the total group ofholding s about 70%o f thediffer ­ ences in financial results appeared tob e dependent on theparameter s chosen. For a group of about 30holding s more parameterswer e available and these were analysed separately resulting in apercentag e of90% .

47 LIST OF CHEMICALS,mentione d in this report

Commercial product Contains:

Growth regulatorsan dcompound s for chemical thinning

Amid thin (NAAm) 8.4%ot-naphtylaceetamid e

BerelexA^/A. , (GAk+J) 0.89% gibberelline A4+A7 + 0.06% gibberelline Aj3 Curbiset 5% chloroflurecol Ethrel-A 480g ethephon/litre Fruitone-T (2,4,5-TP) 64 gphenoprop/litr e MSB 25,105 not released for publicity NC 9634 25% (3-phenyl-l,2,4-thiadiazol-5-yl) thioaceticacid

Off-Shoot-0 45% methylesters of C6~C12 fatty acids PP 528 ethyl 5-(4-chlorophenyl)-2H-tetrazol-2-ylacetate Ehodofix 1,1%a-naphtylaceticaci d

Fungicides, Insecticides, Acaricides

Bayer 6681 25% triadimefon Benlate 50% benomyl Duphar DithaneM-4 5 80% mancozeb powder Du-Ter Spuitpoeder 47.5% fentin-hydroxide Extra Koper Bayer 50% copperoxychloride Koperoxychloride powder Liro-Maneb-80 80% maneb powder Luxan Captan83 % 83% captan powder LuxanTMT D80 % 80% thiram powder Nimrod 25% bupyrimate Ortho-Difolatan-80 80% captafol Previcur Vloeibaar 700g/ 1 prothiocarb TopsinM 70% thiofanaat-methyl 48 PUBLICATIONS

Publications in197 4

Delver, P. Les conséquences du désherbage chimique dans les vergers traités selon le système desbande s engazonnées.L eFrui t Belge 43(1975)369:50-58. (French.) Consequences of the chemical weed control.Belgisch e Fruitrevue 27(1975)7/8:203- 207. (Dutch.) Application of leaf analysis in orchards.Bedrijfsontwikkelin g 6(1975)9:751-760. (Dutch.) Leaf analysis and the storage advice.D e Fruitteelt 65(1975)28:791-797. (Dutch.)

Dijkstra, J. Top fruit research in 1975.Bedrijfsontwikkelin g 6(1975)6:553-559. (Dutch.) Small fruit research in 1975.Bedrijfsontwikkelin g 6(1975)3:275-279. (Dutch.) Small fruit,cultur e and research.D eFruitteel t 65(1975)5:103-105. (Dutch.)

Dijkstra, J. and A.A. van Oosten Small fruit research on the Research Station for Fruit Growing. Kleinfruitteelt- adviezen (1975):11-16 . (Dutch.)

Elema, R.K. Topics at the Research Station forFrui t Growing. De Fruitteelt 65(1975)5:88-89. (Dutch.) Practical research for fruit growing after 1945.D e Fruitteelt 65(1975)10:260- 262. (Dutch.) The competitiveness of fruit growing in theNetherlands .Versla g Nationaal Tuin- bouwongres 1974:101-112. (Dutch.) Woran arbeitet dieVersuchsstatio n für Obstbau 'Wilhelminadorp'. Obstbau/Weinbau 12(1975)6:171-172.(German.) La recherche en arboriculture fruitière aux Pays Bas.L e Fruit Belge 43(1975) 371:176-190. (French.)

Fvankenhuyzen, A. and D.J. de Jong Leaf rollers (Tortricidae)a n interesting group ofMicrolepidopter a (5). Enarmonia formosana Scop (Grapholitha Woeberiana Schiff.). De levende natuur 78(1975):144 - 150. (Dutch.)

Goddrie, P.D. The taste of somene w apple varieties.D eFruitteel t 65(1975)30:848-849. (Dutch.) Fruit variety trials in top fruit.Bedrijfsontwikkelin g 6(1975)9:779-883. (Dutch.)

Goddrie, P.D. and J.P.A. van Dieren The taste of Schone van Boskoop. DeFruitteel t 65(1975)44:1154-1155. (Dutch.)

Goedegebure, J. Economic considerations on replacement of fruit plantations.D e Fruitteelt 65 (1975)48:1236-1237. (Dutch.)

49 Jong,D.J. de and P. Gruys Practical approach to the leaf roller problem in integrated control inorchards . CR. 5e Symp. Lutte Intégrée en vergers.OILB/SRO P 1974(1975):227-235 .

Lemmens, J. J. Summer pruning with apple.D eFruitteel t 65(1975)29:823-826. (Dutch.)

Meyneke, C.A.R., H.J. van Oosten and H. Peerbooms Growth, yield and fruit quality ofvirus-infecte d and virus-free Golden Delicious apple trees.Act a Horticulturae A4(1975):209-212.

Minks, A.K. and D.J. de Jong Determination of spraying dates for Adoxophyes orana by sexpheromon e traps and temperature recordings. J.Econ.Ent. 68(1975)5:729-732.

Nijsse, L. Report on some trials with small fruit crops on the Experimental Garden for Small Fruits at Kapelle. Kleinfruitteeltadviezen (1975):5-10. (Dutch.) Oosten, H.J. van Dutch experiences withvirus-fre e apple trees. I.Golde n Delicious onM. 9 and M.26. DeFruitteel t 65(1975)16:436-438. (Dutch.) Dutch experiences withvirus-fre e apple trees. II.Cox' s Orange Pippin and Red Boskoop onM.9 .D e Fruitteelt 65(1975)17:466-468. (Dutch.) Dutch experiences with virus-free apple trees. III.Conclusion s and remarks on affecting growth. De Fruitteelt 65(1975)18:499-501. (Dutch.) Research on clones of Cox's Orange Pippin. De Fruitteelt 65(1975)Kerstnummer: 1310-1313. (Dutch.) Some pornologica l aspects of the top fruit virus project. Bedrijfsontwikkeling 6(1975)11:945-948. (Dutch.) Susceptibility of somewood y plant species,mainl y Prunus spp. to sharka (plum pox)virus .Neth.J.Pl.Path . 81(1975):199-203 . Fruit virus research and the introduction ofvirus-fre e trees to the fruit industry in the Netherlands.Frui t notes 40(1975):1-6 . Oosten, H.J. van and J.L. Baarends Growth and production of two pear varieties on three quince types.D e Fruitteelt 65(1975)39:1056-1058. (Dutch.) Peerbooms, H., C.A.JR. Meyneke and H.J. van Oosten Comparison ofvirus-fre e and virus-infected origins ofGolde n Delicious.D e Fruitteelt 65(1975)35:970-972. (Dutch.) Scheer, H.A.Th, van der Obstbaumkrebs und seine bekämpfung !.Mitteilungsblat t des Obstbauberatungs­ ringes Südoldenburg e.V. 29(1974)6:124-126. (German.)

Scheer, H.A.Th, van der and J.P.A. van Dieren Fruitrot onGolde n Delicious.D e Fruitteelt 65(1975)28:792-793. (Dutch.) Scheer, H.A.Th, van der and E. Rijnders Experiences with fungicides in trials for control of canker {Nectria galligena). De Fruitteelt 65(1975)39:1054-1055. (Dutch.) Scheer, H.A.Th, van der, H.J. Wondergem and J.P.A. van Dieren Spur-an d blossomblight of cherry and morello (.Monilinia laxa). De Fruitteelt 65(1975)7:160-161. (Dutch.)

50 Scheer, H.A.Th, van der, H.J. Wondergem and B.J. Luimes Evaluationo fbenomy lan dthiophanate-methy lfo rth econtro lo f Vertioillium wilt ofstrawberr y inth eNetherlands .Neth.J.PI.Path .81(1975)3:94-101 .

Soholtens, A. Replant problems inth erive r clay area.D eFruitteel t 65(1975)5:106-108. (Dutch.) Experimental Garden forFrui t Growing "Midden-Nederland" atGeldermalsen .D e Fruitteelt 65(1975)30:846-847. (Dutch.)

Tromp, J. The effecto ftemperatur e ongrowt h andminera l nutritiono ffruit so fapple , with special reference toCalcium . Physiol.Plant. 33(1975):87-93. International Horticulture Congressa tWarscha u (I).D eFruitteel t 65(1975)1: 10-12. (Dutch.) International Horticulture Congress atWarscha u (II).D eFruitteel t 65(1975)2: 30-32. (Dutch.) Effect oftemperatur e ongrowt h andminera l nutrition offruits .D eFruitteel t 65(1975)50:1320-1323. (Dutch.)

Uitterlinden, L. and J. van Eyden Handgrafted andbudde d trees.D eFruitteel t 65(1975)20:568-570. (Dutch.)

Verwijs, D.L. Leaf-eelwormmiser y with strawberries solved. Groenten enFrui t 31(1975)1:37. (Dutch.)

Wertheim, S.J. and M.L. Joosse Experiences with the flower-thinning chemical Ethrel-A. De Fruitteelt 65(1975) 17:464-465. (Dutch.) Experiences with fruit thinningwit h carbaryl onGolde nDelicious .D eFruitteel t 65(1975)21:588-590. (Dutch.)

Wertheim, S.J., M.L. Joosse and F. Nijsse Fruit russeting bytensio n cable masts inth eorchard .D eFruitteel t 65(1975)5: 93-95. (Dutch.) Results with 2,4,5-TP sprayso npear .D eFruitteel t 65(1975)45:1182-1184. (Dutch.)

Wertheim, S.J. and F. Nijsse Report ofwin d break trials.D eFruitteel t 65(1975)44:1157-1160. (Dutch.)

Wertheim, S.J., F. Nijsse and M.L. Joosse Howd ow e 'make'one-year-ol d feathered apple trees.D eFruitteel t 65(1975)50: 1298-1301. (Dutch.)

Wertheim, S.J. and J. Tromp Results of the third national storage trial with Golden Delicious in 1974-1975. De Fruitteelt 65(1975)46:1198-1202. (Dutch.)

Wertheim, S.J., C. van de Vrie and H. Wiedenhoff Mechanical pruning further examined. De Fruitteelt 65(1975)5:100-102. (Dutch.)

51 Bulletins ofth e Research Station for Fruit Growing

9. S.J. Wertheim: Carbaryl;a reliabl echemica l thinnerfo rGolde nDeliciou sappl etree s (Englishwit ha Dutc h summary), 19p p August 1970 ƒ2, —

10. S.J. Wertheim: Thetrainin go fth eslende r spindleo ffou rappl evarietie s (Dutchphoto - graficrepor twit ha nEnglis hsummary),15 8pp . November 1970 outo fprin t

11. H.A.Th, van der Scheer: Fruitro tan dfrui t skininjurie si nappl e (Dutch),4 0p p April 1972 ƒ2,5 0

12. S.J. Wertheim and J.J. Lemmens: Planting systemsfo rappl ean dpea r (Dutchwit ha n Englishsummary) ,7 7p p February 1973 ƒ7,5 0

13. M. van de Vrie: Miteso nfrui ttree s(Dutc h witha nEnglis hsummary) ,6 8p p December 1973 outo fprin t

14. 15thLis to fFrui tVarietie s (Dutch), 136pp .. Novembe r 1974 ƒ 6,—

Publications ofth e National Fruit Advisory Servicean dth e Research Stationfo r Fruit Growing

1. Strawberries inth eope n (Dutch),4 8pp . ... March 1973 ƒ3,5 0

2". Glasshousestrawberrie s (Dutch),6 4p p March 1974 ƒ5,5 0

3. Bushan dCan efruit s (Dutch),7 2p p February 1976 f 7,50

TheBulletin san dPublication s listedo nthi spag ewil lb esen tpostpai dupo n receipto fa ninternationa lposta lmone yorde rfo rth eappropiat eamount . Pleaseindicat eyou rorde rclearly .

52