RESEARCH STATIONFO RFRUI TGROWIN G WILHELMINADORP (GOES),TH ENETHERLAND S

ANNUAL REPORT 1974

Information contained inthi s report may notb e reproduced intota lo r inpar twithou t permission PriceDf . CONTENTS

REMARKS ON THEBRAND S OFPESTICIDE S ANDHERBICIDE S 5

DONORMEMBER S 6

BOARD 7

STAFF 8

POMOLOGY SECTION: 10 Research on top fruit (pruning - planting systems - pollination -windbreaks ) Growth regulators on top fruit (plantmateria l- fruit set- chemicalthin ­ ning) Virus research and clonal selection in fruit trees Rootstocks and interstocks for pit and stone fruits Fruitvariet y trials (apple- pear- plum- second screening inexperimenta l orchards)

SMALL FRUIT SECTION: 21 Cultural experiments with strawberries (glasshouse strawberries - strawberries inth e open) Variety testing of strawberries (glasshouse strawberries - strawberries in the open- second crop) Cultural experiments with raspberries, currants,an dblackberrie s Variety testing of raspberries, currants,an d blackberries Cultural experiments with blueberries

SOILFERTILIT Y SECTION: 25 Soilmanagemen t experiments Influence ofnitroge n onyiel d and quality of fruit crops Relationship between potassium levels in apple leaves and soil Bitter piti n apple Weed control

PHYSIOLOGY SECTION: 31 Investigation into factors underlying the relationship between thevegetativ e and generative development of fruit trees (regulators -nitroge n analyses - theeffec t of root temperature and airhumidit y on thebehaviou r of apple rootstocks) Pre-harvest physiology of fruits as related to fruit quality (theeffec t of temperature and airhumidit y on shoot and fruit growth and on theminera l composition of leaves and fruits of apple - fruit quality, time ofpicking , and storage condition) PHYTOPATHOLOGY SECTION: 34 Apple (canker - fruit rot- powderymilde w- phytotoxity) Blackberry (cane spot) Morellos (blossomblight ) Pear (scab) Strawberry (crown rot- red stele - Vertioillium wilt)

ENTOMOLOGY SECTION: 37 Leafrollers in orchards Integrated control (supervised control - integrated control)

ACAROLOGY SECTION: 39 Chemical control of P.ulmi Integrated control of the fruit tree red spidermit e Panonyahus ulmi (Koch) (ecology ofpredaceou s mites -occurrenc e ofpredaceou smite s onvariou s host plants - influence ofpesticide s on predaceous mites - simulationmodel s on the interactions between P.ulmi and its principal predators - integrated control of P.ulmi) Red spidermit e in strawberries

ACAROLOGY,SOI LFERTILITY ,AN DPHYSIOLOG Y SECTIONS: 41 Influence ofnitroge n level in apple trees on the development of the fruit tree red spidermit e

LIST OF CHEMICALS 42

PUBLICATIONS 44 REMARKSO NTH E BRANDSO FPESTICIDE S AND HERBICIDES

The dosagesmentione d forpesticide s andherbicide s inthi s report refer toth e commercial products.Trad ename s ofproduct s havebee nuse d for the sakeo f convenience,bu t thismake s itunavoidabl e that insom e cases similar products on themarke t under other trade names areno tmentioned . No endorsement ofname d products is intended.A list ofmos t of the commercial productsmentione d inthi s report and their active ingredients isgive no npag e42 . Some of theexperiment s described inthi s report are carried outwit h chemi­ cals and/or concentrations not yet legally approved. DONOR MEMBERS

Thosewh o are interested in thewor k of theExperimenta l Station towis h tokee p inactiv e touchwit h and support its research are invited tobecom e donor members. Donormember swil l receive theAnnua l Report and other publications of the Experimental Stationwithou t charge.Th eminimu m subscription fordono rmember ­ ship is 25guilder s for private individuals inTh eNetherlands ,2 5guilder s for foreign private individuals,an d 50guilder s for institutions,annually . Checks of international postalmone y orders shouldb emad e payable to Proefstation voor deFruitteel t atWilhelminadorp ,wit h the indication 'Donor Subscription'. BOARD (31thDecembe r1974 )

Boardo fth eFoundatio nResearc hStatio nfo rFrui tGrowin g

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.Bo s N.J.J. Costongs Z.Houte r W.d eJage r Mr.J.R.A .Janse n CR.T .Leender s B. Leijs

Advisor:

Ir.H .Sijtstr a STAFF (31thDecembe r1974 )

Research Station for 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.Baarend s (guide field plots) M.L. Joosse F.Nijss e

Soil fertility section: Dr.Ir.P .Delve r (stationedb yth eInstitut e forSoi lFertilit y(IB) ) Ing.P.J .Boldin g (chemicalwee d control) J.d eSchippe r J.Oel e (laboratory) MissC.A .va nKlee f MissJ.S .va nd eVoord e

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

Acarology section: M.va nd eVri e (stationedb yth eInstitut e ofPhytopathologica l Research (IPO)) W. Remijnse

Phytopathology section: Drs.H.A.Th ,va nde rSchee r J.P.A.va nDiere n (statistics) H.J.Wonderge m (stationedb yth eNationa l Board ofAgricultura l ResearchTNO )

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

Small fruit section: Ir.J .Dijkstr a A.A.va nOoste n Economies section: J.Goedegebur e (stationedb yth eAgricultura l Economic Research Institute (LEI))

Administrative and clerical staff: MissJ .Hamelink ,hea d MissK.A .Korshuiz e MissC.C .Meulenber g Miss J.C.Verslui s MissP.C .d eSmid t (housekeeping) MissH .va nZundere n (housekeeping)

Editor/librarian : Ing.M.J .Radem a

Technician: J.va nDale n

Experimental garden Wilhelminadorp (top fruit): J.J. Lemmens,far mmanage r P.Proost ,deput y farmmanage r A.A. Janse M.C.va nMari s J.J. Reinhoudt A.d eWitt e

Experimental garden Kapelle (small fruits): L. Nijsse,far mmanage r J.M. deGroen e A.M.Verhuls t

National Fruit Advisory Service

Ir.R.K .Elema ,nationa l fruit adviser Ir.J .Dijkstra , co-ordinator regional experiments Ir.T.H .Edens ,economis t J.Blommer s (small fruits) Ing.J.H .Bootsm a (stationed atth eexperimenta l gardena tKraggenburg ) Ing.A .Pouwe r (stationed atth eSoi lAdvisor y Servicea tWageningen) , soil scientist A. Scholtens (stationed atth eexperimenta l gardena tGeldermalsen ) L.Uitterlinde n (topfruit ) D.L. Verwijs (stationed atth eexperimenta l gardena tZaltbommel ) POMOLOGY SECTION

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

Research on top fruit

S.J. Wertheim andJ.J . Lemmens

Pruning

SLENDER-SPINDLE SCAFFOLDS Thevalu e of apermanen t frame isbein g investigated in two trials on slender spindles ofGolde nDeliciou s onM.9 .Tw o treatments are compared: 1.remova l of complete framebranche swhe n theybecom e too large for the planting distances involved,an d 2. maintainance of framebranche s by shortening at regularintervals . The results of afour-yea r comparison areno w available forHoek ,wher e the trialwa s started in 1971,afte r the fourth growing season.Thi s trialha s six replicateswit h tentree s ineac h replicate.Tabl e 1show s that pruning method1 leads toreductio n of theyield .However ,prunin g in thiswa y proceeded faster and required lesshand-thinnin g of fruits.Wit h respect togrowth ,metho d1 reduced the development of the lowerpar to f crown.Ther ewere ,however ,n odif ­ ferences between the two treatments in totalgrowt h per tree or ingrowt h inth e upper part of the tree.Economically ,metho d 2prove dmor e favourable.Th e results were confirmed ina secon d trial,wher e the treatmentswer e compared for the third consecutiveyear .

Table 1.Result s of aprunin g trialo nslende r spindles ofGolde nDeliciou s on M.9. Thevalue s representmean s over 4year s for 60 trees per treatment.

Pruning Pruning Hand-thinned Kg/tree/ Mean fruit %K g of method cuts/tree/ fruits/tree/ year weight (g) fruits year O year O >7 0m m 2)

1 19.9 a 33.A a 16.7 158 77 2 31.8 b 38.2 b 18.1 160 82

0 Values indicatedb y different letters differ significantly (P= 0.05). 2)Mean s for 1973 and 1974only .

SUMMERPRUNIN G As in 1973 (see 1973Annua l Report) summer pruning greatly reduced the percentage offruit swit h small crackso n the apple cultivarKarmij n deSonnavill e onM.9 . Thisyea r summer pruning inplanting swit h 4,000an d8,00 0tree s perh a showed a percentage of4. 4 and 1.5%o f fruitswit h small cracks,respectively . 'Normal' pruning inplanting swit h 4,000an d 2,000tree s perh a amounted to9. 1 and 13.4%, respectively.

10 Figure 1.Experimenta l pruningmachin e forhedgero worchard swit hspindles .

MECHANICAL PRUNING In 1974a prunin gmachin e (Fig. 1)wa s developed andmechanica l pruningwa s com­ paredwit hhan d pruning onmatur e trees ofGolde nDeliciou s onM.9 .O n twohold ­ ings three treatmentswer e compared,viz . ' 1. han d pruning, 2.mechanica l pruning, 3.mechanica l pruningwit h additional hand pruning. Thesemethod swil lb e compared for several years.Som e of the 1974result s are giveni nTabl e 2.

Table 2.Result s ofprunin g trials onGolde nDeliciou s onM. 9 tocompar ehan d pruning andmechanica l pruningwit h orwithou t additionalhan d pruning.

Pruning Numbero f Pruning Fruits/ Kg/ Meanf i ruit Kg/tree of method trees cuts/tree tree tree weight (g) fruits >7 0m m

Free spindle 3.5 x 2m 1 16 49 177 22.4 151 15.6 2 16 83 247 33.9 137 17.9 3 16 7 1) 177 27.1 154 17.7 Slender spindle 3x 1m I 50 28 71 11.2 158 8.1 2 3! 41 83 12.5 152 8.5 3 21 6 1) 77 12.2 158 8.8 1)Manua l corrections only. 11 Mechanically pruned trees gavehighe ryields, becaus eo f thegreate r quantity ofwoo d left onth etre e and probably alsodu e to increased set on cutone-year - old twigs.Fo r the latter reason,mechanicall y pruned treesha d ahighe rpropor ­ tiono fnissete d fruits,typica l for one-year-old wood. The trials arebein g continued.

Planting systems

Lightmeasurement s were started in full-field plantings asha s alreadybee ndon e inmulti-ro w systems.Ligh t interception and distribution infull-fiel d plantings differed distinctly from those insingl e rows.Fo r example,i na full-fiel d plan­ ting of theappl eKarmij nd eSonnavill e onM. 9 planted on 1.50 x0.8 0 cm inth e third leaf,onl y 23% of theenterin g lightwa smeasure d onth eground . Inth e single rows on 3.50 x 1.43m thispercentag ewa s 38 (measured under the tree). Themeasurement swil lb emad e every otheryear .Th e light values found atvariou s points inth e canopies are related tovariou s fruitcharacters .

Pollination

PERCENTAGE POLLINATOR TREES In the autumn of 1971,a tria lwa s planted atWilhelminador pwit h Cox's Orange Pippina smai nvariet y and GoldenDeliciou s aspollinato r variety. Twelve small parcels,separate d fromeac h other bywindbreak s of Alnus glutinosa, were planted, eachwit h 100tree s in five rows of 20trees .Thre e plotswer e plantedwithou t pollinators,thre ewit h 10,thre ewit h 20,an d threewit h 60pollinato r trees. The aimo f the trial is todetermin e theoptima l percentage ofpollinators .I n 1974,th e third growing season,yiel dwa s similar in allplot s (9t o 10k gpe r tree).However ,th ehighe r thenumbe r ofpollinator s present,th emor ehan d thin­ ningwa s necessary. In the parcelswithou t pollinators 82fruit s per treeha d to be thinned as against 223fo r thebest-pollinate d plots.Th e question isno wwha t theyiel dwil lb e like ina less favourableyear .

CROSSINGSWIT H ORNAMENTAL MALUS In 1974,man y crosseswer emad ebetwee n Schoneva nBoskoo p and various ornamental Malus species or cultivars.Th e results obtained with M.'Profusion' , M. floribunda, and M.baooata were disappointing. On the otherhand , M.'Liset' and M.'Hillieri' led togoo d fruit set. M.'Aldenhamensis'wa sno t satisfactory in 1974.Sinc e Schoneva nBoskoo p flowers ratherearly , M.'Liset 'seem s agoo d pollinator for thisapple . On thebasi s of theexperienc e accumulated during the last threeyear s concern­ ingpolle n quality,flowerin g period, and scab susceptibility, the cultivars 'Aldenhamensis', 'GoldenHornet' , 'Hillieri', 'Liset',an d 'Prof.Sprenger'ar e worth furtherattention .

GRAFTING ORNAMENTALMALUS In 1974,o ntw oholdings , Malus 'Hillieri'o r M.'Aldenhamensis'wer e grafted on Cox'sOrang e Pippin trees intw odensities ,viz . 1graf t to6 o r to3 trees . Plotswithou t grafts serve ascontrols .Th e aimo f these trials is tose eho w many trees need tob e supplied with apollinatin g treetop .

12 ORNAMENTAL MALUSI NPOT S Ina commercia lholdin gwit h anine-ro w planting of Schoneva nBoskoop , flowering trees of Malus 'Aldenhamensis'i npot swer e introduced into theorchar d during thebloom . The potted treeswer e divided in such awa y as toobtai n plotswit h 0%, 10%,o r 33%pollinato r trees,wit h three replicates foreac h density.Th e yield in 1974wa sno t increased by thepollinators .O n the contrary, trees inth e plotswithou t 'Aldenhamensis'ha d 266fruit s per tree,i n the 10%plot s thisnum ­ berwa s 252,an d in the 33%plot s only 241. The trialwil lb e repeated,wit h interplanted 'Liset'.Interplantin g of ornamental Malus willb emor epractical , because trees inpot s requirewatering ,whic h involves considerable labour.

POLLINATION OFNE WAPPL E CULTIVARS In 1974,man y crosseswer emad ewit h thene w apple cultivarsKarmij n deSonnaville , Alkmene,Holstein ,Jamba ,Elstar ,Septer ,Jonagold , and Gloster.Polle no f these cultivarswa s tested on themai n apple cultivars andvic eversa .Th e results of this and previous crossingwor k are summarized in thene w (15th)lis to f fruit cultivars,publishe d inDecembe r 1974.

Windbreaks

In 1969,a tria lo nvariou s poplars,willows ,an d elmswa s planted nearWilhelmi ­ nadorp toevaluat e suitability forwindbreaks .Amon g thepoplars , Populus alba 'Raket'prove d awelcom e addition.Thi s poplargrow sver y rapidly andha s asuit ­ ablehabi t forwindbreaks .Amon g thewillows ,th eselectio n 'Tinaarlo'i spromis ­ ing;amon g theelm s onlynumbe r577 . Inanothe r trialwit h fouralde r species planted atWilhelminador p in1965 , Alnus oordata was satisfactory. This aldergrow smuc h faster than the standards A.glutinosa and A.inaana. The fourth species, A.spuria, obtained fromDenmark , showed very poorgrowt h and is considered unsuitable.

Growth regulators on top fruit

S.J. Wertheim

Plant material

The chemicalN C 9634wa s tested in thenurser y on the apple cultivarsBenoni , Schoneva nBoskoop , andWinsto nwit h respect tofeathe r formation.Shoot s growing from thebud swer e sprayedwit h an0.8 % solutionwhe n theplan theigh t reached 60 or 70cm .Th e resultswer e disappointing forBenoni ;n o featheringwa s induced. ForSchon eva nBoskoop ,th e averagenumbe r of laterals was only increased from 0.2 perplan t inth euntreate d plots to0. 7 after application at 60 cmplan t height.Fo rWinston , featheringwa s augmented from 3.1 laterals per tree onlyt o 4.1 after spraying at 60c mplan theight .Treatment s at 70 cmplan theigh twer e lesseffective .

13 Fruit set Discovery

In a trial on three-year-old trees of the apple Discovery, the ability of Alar-85 and GAit+7 to increase fruit set was investigated. At full bloom, 50 ppm GAi)+7 and 0.30% Alar-85 were applied. Fruit set was good. GAi4+y appeared to decrease fruit set, whereas Alar-85 had no effect at all. Fruit set per 100 flower clusters on June 19th was 23 (untreated), 14 (GA^+y), and 23 (Alar-85). At picking, these values were 18, 12, and 17, respectively. In 1974, flower clusters were counted. The average numbers of flower clusters per tree were, in the same sequence, 127, 152, and 114.

Fruit set Doyenné du Cornice

Autumn treatments with 2,4,5-TP and GAi,+7 , in some cases compared with the normal spring applications of GAi4+7, were evaluated with respect to their effect on fruit set. Untreated trees and trees sprayed only with GAi^+7 in the spring served as controls. Six-year-old trees were sprayed with 20 ppm 2,4,5-TP or 25 ppm GAi,+7 on October 4th 1972. The spring application was made on 4 May 1973 at 50 to 60% open flowers with 15 ppm GAit+7. The idea was that such treatment might raise the hor­ mone content of the ovaries and thus promote fruit set. Autumn treatments with auxins have sometimes raised fruit set in experiments done in other countries. The results of our trial were negative. The gibberellin treatment decreased fruit set slightly; auxins gave a small insignificant increase. After June drop, all trees had very little fruit; untreated trees, for example, had 7 fruits per 100 flower clusters. The 'worst' treatment, GAit+7 in the spring, gave 5 and the 'best' treatment, autumn auxin combined with GA^+7 in the spring, only 9 fruits per 100 flower clusters. These differences were not significant. All GAit+7 treat­ ments applied in the spring of 1973 decreased flowering in 1974: untreated trees bare 615 flower trusses per tree, as against 498 for all trees treated with 15 ppm GA4+7 in the previous spring.

June drop Doyenné du Cornice

To find out whether GAi,+7 would reduce June drop, twelve trees were sprayed with 25 ppm GAi4+7 on 24 May 1973, seventeen days after full bloom. Twelve untreated trees served as controls. On May 28th, before June drop, fruit set was similar, viz. 50 fruits per 100 flower clusters for the untreated trees as against 55 for the treated ones. After a heavy June drop, hardly any fruit remained. At picking, the numbers of fruits per 100 flower clusters were 6 and 3, respectively.

Chemical thinning

PRÉCOCE DE TRÉVOUX Mature trees of the pear cultivar Précoce de Trévoux were sprayed with the flower- thinning chemical Ethrel-A. On April 3rd, 1974, 0.1% or 0.2% Ethrel-A was sprayed when 5 to 10% of the flowers hadopened. On May 9th there was a clear difference in fruit set between sprayed and unsprayed (control) trees. Untreated trees had 384 fruits per 100 flower clusters, trees sprayed with 0.1% Ethrel-A had 350, and those treated with 0.2% had 291. After June drop these values were 171, 184, and 152, respectively. Since thinning by Ethrel-A was too light, higher concentrations will be studied.

14 BENONI Four-year-old trees of the apple cultivar Benoni were sprayed with Ethrel-A for flower thinning. Treatments were carried out with 0.2% in the balloon stage of the old wood, or during the same stage of the one-year-old twigs, or both. Untreated trees served as control. The results are summarized in Table 3.

Table 3. Results of chemical flower thinning with Ethrel-A on Benoni in 1974. Values are means of eleven trees per treatment.

Treatment Flower Fruits Fruits Kg/ Mean clusters/ hand-thinned picked tree fruit tree weight /tree /100 1) /tree /100 clusters clusters (g)

Hand-thinned 210 934 490 a 82 43 11.8 144 0.2% Ethrel-A (22/4) 20209 9 205 108 b 76 40 10.7 141 0.2% Ethrel-A (8/5) 208 346 183 c 92 49 12.0 130 2 x 0.2% Ethrel-A 210 152 79 d 71 37 10.6 150 (22/4 + 8/5)

1) Values indicated by different letters differ significantly (P = 0.05).

The early spray thinned more than the late one, and two successive sprays thinned more than one application. Extensive hand-thinning remains necessary for this very fruitful cultivar. Pre-harvest drop was increased slightly by the use of Ethrel during bloom. For the treatments mentioned in Table 3, the number of drop­ ped fruits were 1, 5, 3, and 10 per tree, respectively.

SCHONE VAN BOSKOOP In 1973, 0.05% Ethrel-A thinned satisfactorily, whereas 0.10% thinned too heavily. The Amid thin product did not thin. In 1974, the trial was carried on with the same trees and treatments except that the 0.1% concentration was lowered to 0.075%. As a result of the heavy thinning of Ethrel-A in 1973, these trees flowered more abundantly than those of the other treatments (Amid thin, hand-thinned, unthinned). Ethrel-A was applied on April 13th, when the first flowers opened. Amid thin (0.072%) was sprayed on May 13th, when the average fruit diameter was 6.3 mm. Fruit set was light. Amid thin did not thin and Ethrel-A thinned too much in both concentrations. Untreated trees had 28 fruits per 100 flower clusters at harvest (24.7 kg), hand-thinned had 23 (21.5 kg), Amid thin-sprayed trees had 24 (25.6 kg), 0.05% Ethrel-A-treated trees had 11 (17.3 kg), and after the use of 0.075% these values were 11 and 20.2 kg for the two compounds, respectively. It may be concluded that when the weather during bloom is unfavourable, as was the case in 1974, the use of Ethrel-A is too dangerous.

GOLDEN DELICIOUS In an experiment at Wilhelminadorp three new chemicals were tested with respect to chemical thinning, viz. PH 30-13 and the morphactins CME 72200 and CME 73130. PH 30-13 was applied in a concentration of 0.3% at full bloom (May 1st) or four days after the end of flowering (May 20th). The morphactins were used two weeks after full bloom (May 15th). The concentrations were 0.24% for CME 72200 and 0.0015% for CME 73130. PH 30-13 proved phytotoxic and when used during flowering it thinned too strongly and after flowering too lightly. CME 72200 was phytotoxic as well and the thinning effect was too heavy. CME 73130 was slightly harmful and did not thin sufficiently. Only the last of these compounds will be tested further.

15 In a second trial Ethrel-A was tested further on Golden Delicious. This chemi­ cal was applied both early (0.05 and 0.075%) and late (0.10%). The early applica­ tion was made on April 23rd, when the first flowers opened, the late application on May 29th, when the fruits on old wood averaged 12.2 mm diameter. On the latter date, 0.15% AArupsin (carbaryl) was applied as a comparison. The results of this trial are given in Table 4.

Table 4. Results obtained with chemical thinning on Golden Delicious in 1974. Values are means of 10 eleven-year-old trees on M.9 per treatment.

Treatment Flower Fruits/100 Kg/ Mean %K g of clusters/ flower clusters tree fruit fruits tree weight Before At 1) > 70m m drop harvest (g) 7/6 2/10

Unthinned 337 125 85 a 32.7 117 37 0.05% Ethrel -A (23/4) 334 40 31 c 18.1 151 73 0.075% Ethrel -A (23/4) 342 21 19 d 10.9 165 82 0.10% Ethrel -A (29/5) 338 129 67 b 24.4 111 27 0.15% AArups in (29/5) 338 127 70 b 28.8 122 49 Hand-thinned I) 344 131 65 b 28.3 132 51

1) Values indicated by different letters differ significantly (P 0.05). 2) Per 100 flower clusters, 25 fruits; per tree 84.

Both of the early Ethrel-A treatments thinned much too strongly. In view of these and previous results, this chemical can be written off for Golden Delicious as a flower thinner. The late Ethrel-A spray thinned well and just as well as carbaryl or hand-thinning, but here the negative effect on fruit growth rules out its adoption for practical use on this variety (Table 4).

In a third experiment on ten-year-old trees, carbaryl and NAAm were investi­ gated separately or successively on the same trees. NAAm (0.072% Amid thin) was applied on May 17th, four days after flowering. The fruits on old wood then aver­ aged 4.6 mm and on one-year-old branches only 3.0 mm. Carbaryl (0.15% AArupsin) was sprayed on May 29th, 28 days after full bloom, when the mean fruit diameter on old wood amounted to 12.6 mm, and that on one-year-old branches 6.6 mm. Amid thin used alone gave only slight and insignificant thinning. In 1973 this chemical did thin, possibly because at spraying time the fruit size was somewhat larger (7.3 mm). Carbaryl thinned significantly. When the two compounds were used successively, thinning was equal to that of carbaryl.

A fourth experiment on Golden Delicious concerned the exact application time for carbaryl. On eight-year-old trees on M.9, 0.15% AArupsin was sprayed 15, 20, 24, 29, 35, or 49 days after full bloom. On these dates fruit diameter on old wood averaged 3.9, 6.3, 8.0, 11.5, 14.9, and 26.7 mm and on young wood 2.8, 3.7, 4.5, 7.5, 10.2, and 20.8 mm respectively. The strongest thinning effect occurred after spraying on fruits with a diameter of 11.5 (7.5) mm. Application on June 18th had no effect. The results confirm those obtained in previous years, i.e., in this cultivar thinning is strongest when carbaryl is applied at a mean fruit diameter in the range of 10 to 15 mm and on old wood.

LAXTON'S SUPERB On four-year-old trees on M.9, Ethrel-A was tested as a flower-thinning agent. On April 25th, when 0.25% Ethrel-A was applied, about 30% of the flowers had

16 opened.Ligh t thinning occurred. On the untreated trees,7 8fruit sha d tob e hand-thinned,o n treated ones 66pe r tree.Pe r 100flowe r clusters,thes evalue s were 56 and 47,respectively . Theeffec t ofhighe r dosageswil lb e investigated.

WINSTON On nine-year-old trees on M.9, 0.3% Ethrel-A, which had been a favourable flower- thinning treatment in previous years, was tested once again. In addition 0.25% was tested. Both concentrations were sprayed on April 25th, when 40% of the king flowers had opened. Both applications gave too much thinning. The light fruit set in 1974 may have played a role here ( for example, hand-thinning was not necessa­ ry). Untreated trees had 79 fruits per 100 flower clusters (23.2 kg/tree), and trees sprayed with 0.25% Ethrel-A had 38 fruits per 100 clusters (13.9 kg/tree). The highest concentration thinned to 32 (12.4 kg). Thus, when the weather is cold and unfavourable during flowering, the use of Ethrel-A on this cultivar should be avoided.

Virus research and clonal selection in fruit trees

H.J. van Oosten

Until recently, it was only possible to compare growth and production of virus- free and virus-infected trees of Golden Delicious on MM.104 and M.26 in two expe­ rimental gardens (Werkhoven and Horst, respectively). During the last two years several other trials with Golden Delicious, Cox's Orange Pippin and Red Boskoop on our most important rootstock, M.9, have come into production. Although the duration of the majority of the trials is relatively short, some preliminary conclusions can be drawn. Some of the reports in the literature could be confirmed. Virus-free apple trees grow more strongly than virus-infected ones, but produc­ tion is not negatively affected. The virus-free trees usually produce more per unit of growth (cm stem circumference) (table 5). There are indications that growth of virus-free trees will persist stronger than of virus-infected trees also in later years (with full cropping).

Table 5. Growth and production of virus-free Golden Delicious clone B on virus- free (v-f), virus-testedl) (v-t) , and virus-infected (v-i) M.9 at Wilhelminadorp between 1971 (year of planting) and 1975.

Rootstock Virus Kg/tree Kg/cm stem Increase in ctem circumference circumference (cm)

M.9 v-f 26.2 1.9 9.8 M.9 v-t 22.1 1.7 9.0 M.9 v-i 21.9 1.7 9.1

1)A t leastknow n tob e free of rubberywoo d andappl emosaic .

The results of the trials indicate that the stronger growth of virus-free trees may form only arelativel y small problemwhe n the trees are planted onol d apple soil. In some casesmor e growth is acceptable,bu t on fresh soil (for apple) growth seemed tob e too strong forou rplantin g system. It isexpecte d that under these conditions at least the usual planting distanceswil l tob e increased. However, the trials are still tooyoun g foral l aspects of this problem tob e evident.

17 The apples of virus-free Golden Delicious trees have proved to have a smoother skin than apples from virus-infected trees. This was also found for apples of Cox's Orange Pippin and Red Boskoop. Usually, the fruits of these three varieties were larger on virus-free trees. In last year's report some problems concerning tip grafts of heat-treated sources of Cox's Orange Pippin and Red Boskoop Bieling were described. This year, a tip graft from another source of Cox's Orange Pippin gave similar problems: some trees had red fruits, whereas the fruits of other trees were considered 'normal'. At the Institute for Phytopathological Research in Wageningen a tip graft with striped fruits occurred on Red Boskoop Bieling, whereas other tip grafts from the same source were 'normal'. The mutant was similar to those found last year. A new and unexpected observation was the occurrence of bronze-coloured fruits on trees of a heat-treated tip graft of Doyenné du Comice. It is, however, uncertain whether the original virus-infected mother tree had had also bronze- coloured fruits or if this is a mutant.

Rootstocks and interstocks for pit and stone fruits

H.J. van Oosten

In 1974, the production of Doyenné du Comice and Beurré Hardy on quince MA (two sources, one Dutch and the other English), quince MC, and quince Adams was very high. The differences in production between trees of the same variety on these rootstocks were rather small, but for both varieties the trees on quince Adams had the highest production. The trees of both varieties were the smallest in size on quince MC. The rootstocks differed as to suckering. Quince Adams produced more suckers than quince MA or MC, and quince MC produced hardly any suckers. Suckering was influenced by the scion variety, being more abundant in Beurré Hardy than in Doyenné du Comice. As mentioned in last year's report, M. 27 layers caused more difficulties than other rootstocks after planting in the nursery. Several layers were dead at budding time or grew very poorly. The same occurred this year, although all rootstocks were carefully graded before planting, and, moreover, all of the M.27 layers were rooted at planting time. In a small experiment an attempt was made to root cuttings of apple rootstocks (3426, M.9, M.27, M.13-chimaere) according to the East Mailing method (cuttings given an IBA treatment followed by bottom heat). No rooting at all was obtained in M.9 and 3426, but a few plants of M.27 and M.13-chimaere did root, especially at an IBA concentration of 2500 ppm. In the glasshouse an attempt was made to propagate M.9 by root cuttings. The thickest roots (3-4 mm in diameter) were planted vertically and completely covered with soil. These cuttings produced shoots quite readily, and the new shoots also formed new roots. Several experimental plum rootstocks were budded with Victoria M314 . Bud-take was high on Prunus pumila and P.beeseyi as well as on the old Dutch rootstock 'Tonneboer', and nice maiden trees were obtained. On P.triloba and P.tomentosa, bud-take was low and this was also the case for the varieties Czar and Opal on P.pumila. The few trees obtained had small, yellow leaves which became red during the summer, possibly as an indication of incompatibility. The same seemed to be the case for Victoria on P.pumila, because many trees broke at the union during a heavy storm at the end of the summer. Fruit variety trials

P.D. Goddrie

Apple

In the 1973/1974 planting seasonninetee nne w varietieswer e set out forth e first screening trials atWilhelminadorp . Some new varietieswer e very productive in 1974,e.g .Alnarp s Favorit,Honeygold , and Spijon.Jonagol d tooha d aver y good yield,an d this could alsob e said forGloste r 69 in 1974,afte r arathe r lowinitia l production.Th e yield ofMuts uwa s extremely high in allyears .Tha t ofKarmij n deSonnaville ,plante d onM. 9 in the spring of 1966,remain s very constant ata level of 25 to3 0k g per tree eachyear .I n 1974,th e productive variety Odin again showed cracking and spots.Roger sMcintos h appeared tob e very susceptible to canker: after fourgrowin g seasonshal f of the treeshav e been lost due to this disease.Varietie s growing in thevicinit y of Rogers Mcintosh didno t show symptoms of canker at all.Becaus e of itsver y nice dark red colour,Rod aMante t mayb e considered as aver y good improvement on the standard Mantet.Thre e spur types ofSchon eva nBoskoo p of the Institute for Horticultural PlantBreedin g (IVT)a tWageningen ,obtaine d by irradiation,wer e disappointing due either to low production or tolac k of aspur-typ e character. The collection of sweet varieties onM.9 ,plante d mainly in the spring of 1972,gav e the first yields.A s far asgrowth ,tre e shape,an d production are concerned, there seem tob e suitable sweet varieties.I n the trialwit hWinsto n mutants the average fruitweigh t ofDubbel e Winstonwa s only 13g highe r than thato f standardWinston , andyield swer e roughly the same after three growing seasons. In 1974,severa l taste experiments were carried out onne wvarieties .I n these experiments Karmijn de Sonnaville,Gloste r 69,an d Jonagold obtained highscores . Itwa s striking that in all these experiments the standard varieties Cox's Orange Pippin,Golde nDelicious ,an despeciall y Schone vanBoskoo p received rather low scores. On thebasi s of the results obtained in the last fewyears ,thre e varieties were grubbed at the end of1974 .

Pear

Only onene w varietywa s planted inth e spring of 1974.Sinc e thepea rvarietie s in the first screening trials are allver y young,n o information canb e given about thesevarieties . Anexperimen twit h Précoce de Trévoux and fourmutation s of thisvariety , planted in the spring of 1974,ha d tob e grubbed atth e end of 1974.Th e records since 1970showe d thatSupertrévou xwa s thebes tmutation ,wit h 103.5k g per tree between 1970an d 1974.I n the same period, the average fruitweigh t of thismuta ­ tionwa s 215g . The yields of the other doublemutation swer e appreciably lower. Standard Précoce deTrévou xyielde d 75.0k g per treewit h an average fruit weight of 121 g.

Plum

The yields of allvarietie s were very good in 1974.Nevertheless , the quality of somevarietie s was very poor as aconsequenc e of cracking caused by frequent rain in the ripening period.

19 Second screening trials in experimental orchards

In apple trial nr. 71.0.3, planted in the spring of 1971, the yield of Karmijn de Sonnaville amounted to 25.0 kg per tree from 1972 to 1974. In the same period of the same trial, the standard variety Cox's Orange Pippin yielded 22.2 kg per tree. Alkmene was much less productive, with 12.2 kg per tree. However, the growth of this spur type is very weak on M.9, so that this variety can be planted much closer together than the other varieties in the trial. If this is done, yields per ha should become much higher. At the end of 1974 it was decided to terminate trial nr. 72.0.1, planted in the spring of 1972 with Elstar and three IVT spur types of Schone van Boskoop. This was done because of insufficient production and lack of the spur-type char­ acter in the three mutatants. Furthermore, many of the fruits showed bitter pit on the tree every year. After two growing seasons of apple trial nr. 73.0.1, virus-free Cox's Orange Pippin (T 61) gave the highest yield in 1974, amounting to 5.6 kg per tree. Standard Cox's Orange Pippin yielded 2.8 kg per tree. Good yields were also ob­ tained from virus-free Karmijn de Sonnaville and Elstar, with 5.0 and 5.3 kg per tree, respectively. Three IVT varieties showed cracking around the stalk. In the 1974/1975 growing season a new trial will be planted in the various experimental orchards, including the new varieties Jamba 69, Gloster 69, Jonagold, and Septer to be compared with the standard varieties of James Grieve and Golden Delicious.

20 SMALL FRUIT SECTION

J. Dijkstra,J .Blommers ,L .Nijss e andA.A .va nOoste n

Cultural experiments with strawberries

Glasshouse strawberries

Waiting-field plants of Glasa were stored at -1°C for 8, 6, 4, or 0 weeks before planting in the glasshouse on December 6th. As in 1972/1973, flower development stopped during cold storage, which led to differences in growth and in the amount of flowers. This year, a 6- or 8-week period of cold storage reduced production. The same trial was done with Redgauntlet, using both fresh and cold-stored propagation-field plants. Here, too, cold storage before planting in the glass­ house had a negative effect on the yield. Cold storage after the plants had been removed from the waiting-field was particulary unfavourable for the cold-stored propagation-field plants. The Redgauntlet plants gave a second yield in July, and this time there were no differences between the treatments. The average first yield was 3.5 kg/m2 and the average second yield was 3.6 kg/m2. The general conclusion to be drawn is that flower development in the autumn can be influenced easily, but the effect on production is not predictable because of the importance of (minimum) temperature and light intensity in the winter and spring, which can scarcely be modified. In the autumn of 1972, marked differences in flower development were found between plants of the same variety. In 1973, an attempt was made to determine whether the time at which the young runner is formed has an effect on flower de­ velopment. To this end the first, second, and third plants on a runner were studied. Within these group of plants the same differences were found as in 1972, specially for Redgauntlet. Of the Glasa variety the youngest plants gave the high­ est yield. For Gorella in an unheated glasshouse, four plant densities of cold-stored runner-plants were compared with one plant density of fresh runner-plants. On the basis of the same number of crowns per m2, the yield of cold-stored propagation- field plants was lower than that of the fresh material. Lower plant densities gave higher yields per plant but lower yields per m2. In 1974, only a few of the fresh plants wilted during the harvesting period. A second crop with everbearing strawberries failed due to a severe red spider infection. Later, control of the red spiders was obtained with predators. As in 1973, the second crop yielded by cold-stored waiting-field plants was small. This culture method seems very risky.

Strawberries in the open

Research was done on the second crop with cold-stored waiting-field plants. In Redgauntlet, the combined influence of planting time and plant density was inves­ tigated. Planting dates were May 25th, June Ilth, and June 25th. Plant distances in a two-row system were 70 + 40 x 20, 27.5, and 35 cm, respectively. The later the planting date, the lower the production. Average yields per planting date

21 were 2,4, 1.3, and 0.9 kg/m2, respectively. Thehighes t yieldswer e obtained at thehighes t plant densities.N o good explanation canb e offered yet for themuc h loweryield swhe n planting occurred only 14day s later,bu tw ehav e the impression that the quality of the plants deteriorates after a longer storage period.Fo r instance,mor e flower-bud desiccation seems tooccu r after longer storage.Also , thedrie r andwarme rweathe r conditions usually prevailing after later planting mightb eunfavourable . Ostara and Rabundabot h gave equallyhig h yields on ablac k polythene anda straw soilcover .

Variety testing ofstrawberrie s

Glasshouse strawberries

Forearl y forcing purposes,th ene wvarietie s Riva (Zb 6415),Mid a (Zb 6443), Zb 6712,an dIV T680 8wer e comparedwit h Glasa.A s in 1973,Riv aan dMid awer e intermediate between Glasa andGorell awit h respect tobot h ripening period and productivity. Riva is abi t earlier thanMid a andmostl y abi tmor eproductive . The IVTselectio n 6808 ripens only afe w days later thanGlasa ,bu t productivity ismoderate .I n this trialZ b 6712wa s the latest andmos t productive selection. In acol d glasshouse anumbe r ofne w varieties were comparedwit h Gorellaan d Redgauntlet.Al l selections ripened 1o r 2week s before the standardvarieties , but productivity was low tomoderate .Therefore ,non e of thene w selections seems tob e avaluabl e acquisition. The varietiesBelrubi ,Holyday ,Sequoia ,an dTiog awer e notvaluabl e inth e glasshouse either.Productio nwa s onlymoderat e and ripeningwa s late. In smallnarro w polythene tunnels,IV T6710 0gav e agoo d crop.Ripenin gwa s a bitearlie r than forRedgauntlet .Th e fruits are large ando fgoo d quality.I nth e polythene tunnels Primella and IVT 6808wer e theearlies t varieties,bu tproduc ­ tionwa s low and the fruitswer e rathersmall .

Strawberries in the open

IVT 67100di dno t give as good acro p in the open asunde r polythene tunnels. Zb 6695 inparticula r gave ahig h yieldwit h large fruits.Unfortunately , the taste ofthi svariet ywa s onlymoderatel y good.Z b 6655 toowa s very productive, not only inKapell e but also inothe rtrials .

Second orop

Cold-stored waiting-field plants ofRedgauntlet ,Gorella ,Sivetta ,Tago ,lamella , and Tenirawer e comparedwit h respect toth e second crop.Yield swer e reasonably good, amounting toabou t 1.5 to 1.8kg/m 2 at the relativelywid e plantdistance . Therewer e only small differences in themea nharvestin g date.Tamell awa s the highest yielding variety, followed by Redgauntlet,Gorella ,an dSivetta .

22 Cultural experiments with raspberries, currants, and blackberries

The soil-improvement experiment on the raspberry variety Mailing Promise gave no differences in yields in the second year. Yield was also good in the untreated object, amounting to 166 kg/m2. Because Mailing Promise often shows poor fruit setting, a pollination trial was started. During the blossoming period, cages with and without honeybees were placed in a raspberry plantation. Fruit set was close to zero in the cages with­ out bees, but good those with bees. In the upper part of the plant, i.e., where the earliest flowers occur, fruit set was better in the presence of bees than with free pollination, resulting in about 90 and 45% good fruits, respectively. In the lower parts of the plant fruit set was good both in the cages with bees and outside, amounting to about 90%. Thus, the study showed that fruit setting of Mailing Promise can be improved by good pollination and windsheiter.

The pruning trial on blackberries (pruning in the autumn and spring) was con­ tinued, but showed no differences in 1974. In a ten-year-old plant-distance trial on Himalaya, yields were equal for wide (2.00 m) and narrow (1.00 m) spacing of the plants in the rows. Yield was about 2.1 kg/m2 for all objects.

The pruning trial on the red currant varieties Jonkheer van Tets, Rondom, and Stanza was continued for the fourth year. Light pruning gave the highest yields, but mechanical and intermediate pruning also gave satisfactory production. Only with severe pruning was the yield distinctly lower. The hedge-height experiment (red currant) was terminated. The influence of the hedge-height on production proved to be dependent on the growth capacities of the varieties. When this capacity is low it is useless to try to obtain a high hedge. For the weakest-growing varieties, Maarses Prominent and Red Lake, a hedge-height of 1.25 m gave the same production as a hedge-height of 2.00 m. For Stanza, the best results were obtained with a hedge-height of 1.50-1.75 m, and for the most vigourous varieties, Jonkheer van Tets and Rondom, a hedge-height of 1.75-2.00 m gave the best result.

In the fifth year of the plant-distance trials on black currants the cuttings planted 30 cm apart in the row again gave the best results for the varieties Baldwin Hilltop and Black Reward (60 and 97 kg/are, respectively). For Wellington XXX the highest yield was obtained from the material planted at the normal dis­ tance of 1.50 m (67 kg/are).

Variety testing of raspberries, currants, and blackberries

The raspberry variety Glen Clova planted as a buffer in the soil-improvement trial gave a high yield (184 kg/are). The fruits of this variety are proof against rainy weather; fruit rot did not occur. Picking can be done easily, but the fruits are somewhat smaller than those of Mailing Promise. For Glen Clova it seems nec­ essary to remove new shoots for a longer period than is the case for Mailing Promise. The variety trial with Mailing Promise, Schönemann, Glen Clova, Sirius, and Spica gave the first yield in 1974. Growth of Sirius and Spica was disappointing,

23 andth eyiel do fSiriu s inparticula rwa slow .Th emea nharvestin g dateo fSiriu s was closet otha to fth elat evariet y Schönemann,bu tSchöneman nwa sfa rmor e productive (174 kg/are). Spicaha drathe r small fruitswhic h seemt ob esuscepti ­ blet ofrui trot . Ina ninitia l screeningo fth evarietie s Mailing Orion,Mailin g Admiral,Veten , andMultiraspa ,bot h growthan dyiel dwer e disappointing.Virus-teste d plantso f Mailing Promise,Rod e Radboud,an dSchöneman nwer e judgedwit h respectt ofrui t quality.Poo r fruitse twa sno tfound .

The variety trialo nblackberrie swa scontinued .Harvestin go fth elat evarie ­ tiesha dt ob eende d earlydu et oheav y rainfalli nth eautumn .A sa result ,th e yieldo fth elat evarietie s Smoothstem andThornfre ewa sdisappointin g (126an d 131kg/are ,respectively) .Th eyiel do fHimalay awa s33 8an do fThornles sEver ­ green 234kg/are .Th ehig hyiel do fHimalay ai ssomewha t unrealisticbecaus eo f the limited distance of2.0 0m betwee n therows ,whic h canonl yb emaintaine d underth econdition so fth ewea k growtho fth ethornles svarieties .

Thehedge-heigh t trialo nre dcurrant s alsoprovide d information aboutth e productivity ofth evarieties .Stanz a again proved tob eth emos t productive,bu t Jonkheerva nTet s also gave good production.Th eweak-growt h varieties Maarses Prominent andRe dLak ewer emuc h lessproductive .Th eproductio n shownb yRondo m was also disappointing,du et oa numbe ro fsteril ebushes .

Inth eblac k currant plant-distance trialth ehighe r productiono fBlac k Reward is striking.Th eaverag e yield overth efiv e yearswa s8 2kg/ar e forBlac k Reward, 54kg/ar e forBaldwi nHilltop ,an d5 6kg/ar e forWellingto nXXX .

Cultural experiments with blueberries

In the plant-distance trial at Horst the highest plant density again gave the highest yield. The sale of some of the berries in consumer units resulted in a price per kg that was 2 guilders higher than for berries sold in 5-kg crates. In the I.S.H.S. variety trial the best results were again obtained with H 1 and G 71. Converted on the basis of 1,600 bushes per ha, yields were 6,700 and 4,000 kg, respectively. The American varieties were far less productive because many branches died after being infected by Godronia aassandrae.

24 SOIL FERTILITY SECTION

P. Delver Soil management experiments

The influence ofplan t density and of thewidt h of theweed-fre e tree stripo n thenitroge n fertilizer requirement of Schoneva nBoskoo p M.9 apple trees,i s being investigated in anexperimen t planted in 1972.Benon iM. 9 isuse d as pollinator. The differences inplan t density,rangin g frombed s (six rows,3,43 0 trees perh a of orchard area including alley-ways) to thesingle-ro w systems planted at30 0x 102c m (2,930 trees), 355x 136c m (1,860 trees), and 395x 205 cm (1,110 trees) involve differences inpruning :widel y separated trees are pruned more strongly than closely planted trees,i norde r to achieve early closing of the rowsb y promoting growth.Fo r instance,afte r planting the systems mentioned abovewer e pruned on average toa heigh t of 99,94 ,87 ,o r 73c mabov e the ground, respectively. As in 1973,th e dressings of0 ,70 ,140 ,210 ,o r 280k gN/h a caused distinct differences in leaf colour and inth enitroge n percentage in the leaves.Th emor e nitrogen given,th ehighe r thepercentage s innarro w tree strips ina gras s cover (ascompare d withwid e strips) and the further apart the treeswer e planted (or, which comes toth esam e thing foryoun g trees,th emor e theywer e pruned). In 1973,thes enitroge n levelsvarie d from 1.73% (intree s planted at30 0x 102c man d strips 30 cmwid ewit h no fertilization) to 2.32% (bed system given 280k gN per ha). In 1974,th eyiel d per tree already showed arespons e toth e treatments.Bed syielde d 10.3kg ;wit hbar e tree strips (150-200c mwide ) the plant systems 300x 102,35 5x 136,an d 395x 205 cmyielde d 8.4, 11.0, and 10.0 kg, respectively, andwit h narrow treestrip s (30-40 cm)thes e valueswer e only 4.8, 6.5, and 6.0 kg.Th e differences are attributed to competitionb ygrass , differences in shootgrowt h inpreviou s years causedb y pruning and,withi n the beds, to aneffec t ofwin d protection. Finally, theyiel d per treewa s related to thenitroge n status as influenced by thenitroge n dressings.Fo rnarro w tree strips it increased by 42%whe n the N leveli nth e leaves increased from 1.75 to2.15% , and trees inwid e strips yielded 11%mor e at 2.30%N as compared to2.15% . InNumansdor p in anexperimen t on theeffec t ofplant-hol e treatments,Cox' s OrangePippi nM. 9 trees planted in 1971 gave aslightl yhighe ryiel d underonl y one of the treatments: farmyard manure spread around the stem,whic h alsole d tohighe rK andN percentages in the leaves.Th e other treatments (blood-an d bone-meal dressings applied in the planthol e or around the stem;peat )ha d no effect in the thirdyear . The soil-moistening effect of trickle irrigationwa s studied in ayoun g orchard at theexperimenta l station.Wate rwa s applied in daily amounts varying between 1.5 and 15litres ,regulate d by combining differentnumber s ofmicro-tube s ('Volmatic'system )an ddifferen twaterin g times.Afte r application for twoweek s in thedr y period inMay-June ,moistur e determinations were carried out in alarg e number of soil samples.Th e soilwa s amarin e silty clay loamoverlyin g non-rooted finesan d ata dept h of 60 cm.Th e diameter of themoistene d spot (soilwit h an increase inmoistur e of 2%o rmore )wa s atmos t 120cm , and only asmal l increase in the volume ofmoistene d soilwa s obtainedwhe nmor e than 8-10 litres perda y was given.Wit hhighe r quantities,e.g . 20litre s ormor e asofte n used in

25 practice,a considerabl e part of thewate rma y leach to thesubsoil ,especiall y under conditions of shallow rooting.Moreover ,du e to cracks andwor mhole s great differences inmoistene d soilvolum e occurbetwee n the trickle spots.Fo r instance,wit h thehighes t quantity ofwate r (!5 litre/day) thisvolum e varied between 100an d 600litres . In the long-term fertilizer experimentwit h pears (varieties Conference and Doyenné du Comice),whic hwa s terminated in 1973,n onitroge nwa s given in1974 . Since 1965,dressing s of 0o r 240k g N perh a on thebar e tree stripsha dbee n combinedwit h 0,90 ,180 ,270 ,o r36 0k gN onth e grass strips.I n 1972an d 1973 thenitrat e level in the soil ofunfertilize d tree stripswa s found tob e related to thenitroge n dressings on thegrass .Thi swa s due to the release ofnitrat e from thevariabl e quantities of grassmulche d on the tree strip.I n 1974,thi s nitrate levelwa s very low and no longer showed any relationshipwit h the earlier differences innitroge n dressing and grass production, thus indicating that the nitrate effect of grassmulc h is ashort-tim e effect resulting mainly fromrapi d decomposition. Before the treeswer e grubbed, afina l investigationwa s carried out onth e degree ofvertica l perforation of the soilb ywor mhole s in the tree strips and grass strips.I twa s found that at adept h of 10c mth ewor mhole s in the tree strips greatly outnumbered those under the grass,probabl y as aconsequenc e of mulching and ahighe rmoistur e content of the tree strip.A t the same time,man y morehole swer e counted onbot h strips,wher e over anine-year-ol d periodhigh - nitrogen dressings were given on the grass,a s comparedwit h unfertilized strips. This demonstrates the importance of grass production forwor m food. Thepermea ­ bility of the soil,estimate d from theinfiltratio n rateo fwate r poured into wide plastic rings drilled afe w centimetres into the soil, showed good correla­ tionwit h the localnumbe r of largewor mholes .

Influenceo fnitroge no nyiel d and qualityo ffrui t crops

Gekro,a norgani c nitrogen fertilizer of animal originwhic h is some five times more expensive thanordinar y fertilizer, cannotb e considered atru e slow-release nitrogen source.Thi swa s concluded from anexperimen t inNumansdor p ina n orchardwit h Cox's OrangePippi n and GoldenDeliciou s M.9 trees,wher e Gekro 10-5-0 and Gekro 12-4-6hav ebee n compared since 1970wit h calcium ammonium nitrate andwit h nofertilization .Ther ewer e nodifference sbetwee n the three treatments,bu t in 1972-1974omissio n of fertilization gave asligh t reduction inyiel d amounting to 1.5% and 6.5% for thevarietie s mentioned.Moreover ,Cox' s showed lessbitte r pit and GoldenDeliciou s had fewer russeted fruits.Repeate d determinationso f soilnitrat e levels showed thatGekro' sactio ni s only slightly prolonged as comparedwit h calcium ammoniumnitrate . Due toa well-balance d recycling ofnitroge n frommulche d grass on treestrips , the fertilizer requirement of fruit trees ingrass-stri p culture is sometimes extremely low.Thi swa s again evident from anexperimen t in theWilhelminapolde r with James Grieve M.9 apple trees,wher e since 1964fou rdressing s on thegras s strip (0,120 ,240 ,o r 360k gN perha )hav ebee n combinedwit h four dressings on theweed-fre e tree strip (0,60 ,120 ,o r 180k gN per ha). As in the previous tenyears ,th e averageyiel d of fertilized plots (27.2k gN pe r treei n 1974)wa s nothighe r than that ofunfertilize d plots (27.7k g per tree).Lea f analysis again confirmed that,i n this orchard, trees inunfertilize d plotsmaintai n a relatively goodnitroge n status.I t isbelieve d that especially inmoderatel y productive

26 orchards pruningma y have astron g leveling influence on suchnitroge n effects as shoot growth. \ In the lastyea rbefor e grubbing, the94 0 13-year-old pear trees of theferti ­ lizerexperimen tmentione d in the preceding chapterwer e used for a comparative study of foliar spray applications of urea andWuchsa l (acomplet e liquid ferti­ lizer for foliarnutrition) .Afte r fertilizationwa swithhel d in 1974,th e pre­ dominantly normal nitrogen status of the trees showed only slight differences dependent onearlie r treatments.Spray s containingWuchsa l (6litre s perha) , urea (0.11% and 0.50%), orwate r+ detergentwer e applied eight times between April 26th and July 8th.Althoug h some slight increase in theN content of the leaveswa s observed,especiall y for trees sprayedwit h 0.5%urea ,ther ewer en o differences inyield ,no t even for treeshavin g arelativel y lownitroge n status owing to the omission of fertilization since 1965. In another spray experiment on apple and pear trees,crystallin e urea incon ­ centrations between 0.5 and 1.5%wa s compared with urea coatedwit h aderivat e of formaldehyde.Compare dwit h the concentration effect,th e coating effect, expressed in leafburning ,wa s very small andnegligible .

Relationship between potassium levelsi nappl e leaves and soil

Inbar e tree stripswher eherbicide s areusuall y used to controlweed s and soil tillage isomitted , 'potassiumprofiles 'develop ,i.e. , theuppe r 3-5 cmo f the soilbecom e extremely rich inpotassiu m as compared with deeper layers,becaus e potassium released fromdecomposin g organicmateria l such asgras s and leaves is adsorbedb y clayparticles .A s iseviden t from the effect of rotary tillage on the tree strips,thi s shaltow upper layer iso f great importance forth e potassiumnutritio n of the tree.Tillag e results in alowe rK status of the leaves and alowe r incidence ofbitte r pit inCox' s Orange Pippinapples . The relative importance of the upper soil layer for the potassium nutrition of apple treeswa s studied ina po texperiment .Pot sholdin g 35 litreswer e fil­ ledwit h soiloriginatin g from different layers (between 0 and 30c mdeep )o f tree strips.Thes e strips,whic hwer e located in the fertilizer experimentwit h pearsmentione d above,ha d never received any potassium fertilizerbu t as acon ­ sequence ofdifferentia l nitrogen fertilization on thegras s andmulchin g onth e tree strips,th eK2 O content in theuppermos t 2c mvarie dbetwee n 40 and 48m g per 100g soil . In the subsoil ata dept h of 20-30 cm,K2 Ovarie dbetwee n 18an d 25mg . Two-year-old Schoneva nBoskoo p treeswer e planted in thepot s and soon showed distinct differences in leaf size and leaf colour indicating that the soil inth e uppermost 0-3 cmrelease s farmor e nitrogen than the deeper layers.Preliminar y leaf analysis suggest that there are also great differences inK status.Th e leaves of trees in '0-2cm ' soil showed 1.86%K ,wherea s for '20-30 cm' soil this valuewa s only 0.93%. It goeswithou t saying that cultural practices that disturb the accumulation of potassium in tree strips canhav e astron g influence on theK nutrition of fruit trees.Thi sha sbee n demonstrated repeatedly in the soilmanagemen t experi­ ment inOosthuizen ,wher e (seebelow ) chemicalwee d controlwithou t soil tillage isbein g comparedwit h rotary tillagewithou t herbicides,i ncombinatio nwit h mulching either onth e tree strips oro n the grass strips.I n addition,nitroge n isbein g given on the tree strips (10)o ro n thegras s strips (01)o ro nbot h(11) . In 1974,potassiu m deficiency symptoms occurred insom e Cox's Orange Pippin trees

27 Standingnea r a competing poplarwindbrea k ina plo t given treatment MM.11 (soil tillageo n tree strips,mulchin g on the grass strips,nitroge n onbot h strips). A combination of factors,i.e. ,drought ,th e omission ofK-fertilizatio n forman y years, the absence of roots inth e0- 6 cmsoi l layer of the tree strips,n o potassium supply to the tree stripsb ymulc h and nitrogen fertilization,mus t havebee n responsible for the substantially reducedK uptakeb y the fruittrees .

Bitter pit in apple

P.Delve r and A.Pouwe r

Inplan tnutritio n studiesmuc h attention isbein g given to the relationships between thenutritiona l status of the tree and the susceptibility of fruits to storage disorders such asbitte r pit andblackdown .Thes e investigations include the influence of soil properties,fertilization , and soilmanagement .Consider ­ able attention is alsobein g given toresearc h concerning thepredictabilit yo f bitterpi t on thebasi s of datao n tree growth,cro p size,an d cropanalysis . So far,thes e relationships havebee n studied only on thebasi s of leafanalysis . At present theK+Mg/C a (eq/eq)rati o isbein g appliedmor e or less successfully as acriterion .On e advantage of the analysis of leaves sampledbetwee n 1an d 10 August is that it also provides abasi s for fertilization recommendations. In the future,however ,ou r storage experimentswil l also include the analysis of fruits tofin d outwhethe r the correlationbetwee n the prediction andoccur ­ rence ofbitte r pit after storage canb e improved.Fo r this purpose,fo rinstance , datawer e collected from fifteenCox' s OrangePippi norchard s insevera l fruit­ growing areas.Th e data comprised measurements of the rate of fruit growth in the periodbetwee n fullbloo m and the average 60gram-per-frui t stage (atth e end ofJuly) . Furthermore, fruits and leaves sampled aten d ofJul y and atpickin g timewer e analysed and three picking dateswer e applied.Th e fruitswer e stored at the Sprenger Institute.Preliminar y analyses showed avariatio n of 4t o6. 5 mg in the Ca levelpe r '60-gram' fruit,whic hwa s negatively correlatedwit h the incidence ofbitte rpit . BetweenMa y 24th andAugus t 25th,fruit s ina Cox' s Orange PippinM. 9 orchard inWilhelminador p were sampled at 10-day intervals according tofrui t size and position on the tree.Th e aimo f this study is todetermin ewha t factors inth e crown of the tree influence theminera l composition of the fruits.A similar studywa s carried out on 300Winsto n apples sampled atpickin g time.A number of growing conditions,suc h as position onth ebranches ,vicinit y of otherfruits , colour, and size,wer e registered; the fruitswil lb e analysed individually. The preliminary results suggest that the levelsvar ybetwee n0.6 0 and 1.20%K , 0.023an d 0.046%Mg ,an d 0.018 and 0.045% Ca,expresse d ondr ymatter .Frui t size accounts for only asmal l part of thevariatio n in the Cacontent . Some of theGolde nDeliciou s apples showing severe treepi twhe n picked in 1973,wer e stored.U p toth een d ofNovember , the degree ofpittin g only increased very slightly. The fruitswer e then sorted according to thenumbe r ofvisibl e pits and sampled for analysis.A ver y close correlationwa s foundbetwee n theK and Ca contents and thedegre e ofpitting . The drymatte r of fruits sized 75-80m man d lacking pits, showed on average 0.71%K and 0.022%Ca .Severel y pitted fruitsha d 0.99%K and 0.015% Ca.Th e various parts of the fruitswer e also analysed sepa­ rately.Th e lowest Ca level occurred in the calyxhal f and in theoute r parts of the fruit flesh.A s forCa ,th eK percentages decreased fromth e core toward the

28 outer part of the flesh,bu t inpitte d fruits theK %unde r the skinwa s relative­ lyhigh . An experiment on fruit thinningwa s carried out in cooperationwit h the Physiology Section.Thinnin gwa sbrough t aboutb y the spraying ofEthre l0.08 % fourday sbefor e fullbloo m (normal concentration:0.05% )o r carbaryl 0.15% three weeks after fullbloom ,o rb yhan d thinning onJun e 25th.Th e thinning effect of Ethrelwa s very stroigon treeswit h few openblossom s at themomen t ofspraying , andwa smuc h lower on treeswit h 70%o rmor e openblossoms .Onl y theEthre ltreat ­ ment led tomuc h larger fruits,th emea nweigh t atpickin gbein g 149 ga s against 120g foruntreate d trees, 125g fo r the carbaryl-treated trees,an d 130g for hand-thinned trees.Fruit s fromEthrel-spraye d treesha d ahighe r potassiumcon ­ tent (0.84%K , drymatter )tha n those from unthinned trees (0.76% K). Carbaryl andhand-thinnin g hadn o effect on theminera l composition of thefruits . Preparations weremad e fora nexperimen t concerning the influence ofsoi l management factors onth e occurrence of storage disorders inapple s (e.g.bitte r pit andbreakdown) . The trees -Cox' s Orange Pippin and Schoneva nBoskoo p grafted onvirus-fre e rootstocks M.9 -wil lb e planted in 1975,an d the treatments will comprise chemicalwee d control or rotary tillage on tree strips,mulchin g ontre e strips or grass strips,trickl e irrigation, and fertilizationwit h potassium.

Cox's OrangePippi n fruits from the soilmanagemen t experiment in Oosthuizen were stored under cool conditions until January 22nd in 1974.Bitte r pit,whic h had developed in asmal lpercentag e of the fruits,showe d the following response to the treatments applied in the orchard.Unde r chemical weed control (no tillage) and grassmulche d on the tree strips,6.4 % of the fruitswer e affected, and under thesam e treatment butwit hmulchin g on thegrass ,3.6% . Of the fruits fromplot s with rotary-tilled tree stripswit h grassmulche d on the tree strips or onth e grass strips,3. 4 and 1.8%showe dbitte r pit,respectively . Good correlations were found betweenbitte r pit and theC a enK contents of the fruits and theK contents of the leaves.Fruit swer e entirely free ofbitte r pit onlywhe n theK level inth e leaveswa s as lowa s0.9% . Although rotary tillage results ina lowe r incidence ofbitte r pit,i tals o depresses theyield . Inthi s experiment tillage up to 1974gav e onaverag e a2 %lowe r yield compared with chemicalwee d controlwithou t tillage.Probabl y owing toth edr y spring,thi s reduction,amounte d to8 % in1974 .

In 1973,tw o co-workers (A.Pouwe r and S.Bommeljé )collecte d orchard dataan d leaf samples from over 100mixe d orchardswit h Cox'sOrang ePippi n and Golden DeliciousM. 9 apple trees in the southwestern part of the country,an d thiswor k was repeated inth e same orchards in 1974.Th e aim of the surveywa s to introduce leaf analysis tofrui t growers and toobtai na n impressiono f the soil-fertility level in thisregion . TheK levels in the leaves showed good correlations bothbetwee n the twovarie ­ ties (always in the same orchard) andbetwee n the twoyears ,an dwer e about the same inbot h years.I n spite ofver ymoderat e dressings oreve n continued omission of potassium fertilization over aserie s ofyears ,th eK contents in atleas t half of theorchard s in this regionmus tb e considered toohig h invie w of the risk ofbitte rpit . The calcium level inth e leaveswa s considerably higher (onaverag e 0.3%) in 1974,tha ni n 1973,possibl y due to the lower temperatures in 1974.C a percentages were 0.3 lower inCox' s OrangePippi n leaves thani nGolde nDeliciou s leaves.

29 Weed control

P.J. Bolding

The long-term trial on the influence of different amounts of simazin,atrazin , AAkarmex (each 3.5 and 8kg/ha) , and Prefix (60,80 ,an d 100kg/ha )unde rJame s Grieve Lired (MM.106),whic hwa s started in 1965,wa s terminated in 1974.Ove r the 10-year period,overdosag e of simazin,atrazin ,an dPrefi x seemed tohav e had someharmfu l effect,bu t thesignificanc e of the difference could notb e determined because thenumbe r of treeswa s toosmall . In atria lunde r TydemanE.W. ,wit h amitrol,Gramoxone ,Roundup ,an d untreated, adistinc t effect ofRoundu p and Gramoxonewa s seen threeweek s after theappli ­ cation inNovembe r 1973.I n the autumn of 1974, the effects of amitrol,Gramoxone , and Roundupha d disappeared,bu tmor eweed s occurred where no treatmentha dbee n applied. In atria l to compare the effect of acombinatio n ofRoundu p and simazinwit h that of simazin followedb y Roundup, the latter treatment gavebette rresults . Instrawberrie s (Redgauntlet) a trialwit h lenacil (1.5kg/ha) , simazin (0.75 kg/ha), Bladex (1 and 2kg/ha) , and no treatment showed that the application of 2k gBlade x perh a led to reduction of theyield .

30 PHYSIOLOGY SECTION

J. Tromp

Investigation into factors underlying the relationship between the vegetative and generative development of fruit trees

Regulators

In 1973,a po texperimen twa s startedwit h one-year-old Cox's OrangePippi n trees toevaluat e theeffec t of someGAi t+7an d SADH treatments onunfertilize d trees and trees given anitroge n dressing either in the spring orlat e in thesummer . The level of flower-bud formationwa s ratherhigh .Ther ewa s no clear effect of thenitroge n treatments on flowering,possibl y due to the use of arathe r rich soilmixture .Remarkably ,SAD H reduced flowering somewhat.A s expected,GAi t+7 also reduced the percentage of flower-buds.I ngeneral , the differences between treatmentswer e small.Th e experiment didno t contribute toa bette r understanding of the process of flower-bud formation. To find outwhethe r the supply of nitrogenwoul d affect the flowering response to application of GAi,7+ i nvertica l andhorizonta l trees,a ne w experimentwit h Cox's Orange Pippinwa s started.

Nitrogen analyses

In the autumn of 1973,a po t experimentwit h apple rootstocks M.7wa s started to follow the effect of an autumn application ofeithe r nitrate or ammonium onth e level and the composition of thenitroge n reserves in the tree during the autumn, winter, and spring.Nitrificatio n of ammoniumwa s preventedb y the application of N-Serve (2-chloor-6(trichloormethyl)pyridrine) to the soil. During the autumn thenitroge n level in thevariou s tissueswa s almost thesam e in the two treatments.I n the spring,however , totalnitroge nwa s somewhathighe r in the trees fertilizedwit h ammonium. This concerned the levels ofbot h protein and soluble nitrogen.Wit h respect to the amino acid composition of thesolubl e fraction,onl y the data for the roots are available.Qualitatively , therewer e no differences between treatments.A s found earlier,asparagin e and arginine predom­ inated.Th e asparagine contentwa smuc hhighe r in the 'ammonium trees'tha ni n the 'nitrate trees'.Th e sameheld ,bu t to a lesser degree,fo r arginine,whic h implies that thehighes t asparagine/arginine ratioswer e found in the ammonium- treated trees.Ammoniu m couldbarel yb e demonstrated in the roots of treesferti ­ lizedwit h ammonium. Obviously, ammonium, likenitrate ,i s incorporated into amino acids and amides immediately afteruptake .

The effect of root temperature and air humidity on the behaviour of apple root- stocks

In the controlled-environment rooms the shoot growth, leaf development,dry - matter production,wate r use,an dminera l composition of three types ofroot - stocks (M.9,M.26 , andMM.106 )durin g aperio d of 11week swa s estimated ata

31 low (40-50%) and ahig h (85-90%)humidit y in anumbe r of combinationswit h three root temperatures (10 ,17° ,an d 26°C). In accordancewit h expectation,shoo t growth and dry-matter production increased in the sequence M.9,M.26 ,an dMM.106 . Generally speaking,a t the lowhumidit y shoot growth and drymatte r production weremarkedl yhighe r at aroo t temperature of 17°C than at 10°C;a furthe rin ­ crease up to 26°Cha dhardl y any effect.A t thehig hhumidit y therewa sn odif ­ ferencebetwee n 10°an d 17°C (the 26°C treatmentwa s omitted) for theM. 9 and theM.2 6 rootstocks,bu t theMM.10 6 response resembled that seen at the low humidity. With respect to theminera l composition of thevariou s tissues,onl y theana ­ lyses forM. 9 are available so far.I fw e restrict ourselves to thene wgrowth , the results indicate thatbot h the concentration and the total amount ofK in ­ creasedwit h root temperature.Th e Ca concentration,o n the contrary,decrease d with increasing root temperature at lowhumidity ,wherea s the total amount ofC a washighe r at 17°C and 26°C than at 10°C.Th e concentrations ofK and Cawer e lower atth ehig h than at lowhumidity ,bu t for the total amounts ofK and Ca this patternwa s reversed. TheK/C a ratio increased with increasing roottempe ­ ratures andwit h increasing airhumidity .

Pre-harvest physiologyo ffruit sa srelate d tofrui t quality

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

In 1973,tw oexperiment s with Cox's Orange Pippinwer e done incontrolled-envi - ronmentroom s tostud y theeffec t of two temperatures of the air (24°an d 19°C) in combinationwit h tworoo t temperatures (the air temperature and 10°C).Experi ­ mental details and the results concerning shoot and fruit growth havebee n given in the 1973Annua l Report (pp.61).Th eminera l analyses,whic hwer e done in1974 , indicate generally that the amounts ofK per fruit increase linearlywit h the increase indr yweight ;th e levelwa s somewhat lower at the low root and air temperatures.Th e amounts of Cape r fruit also increased, but a linearrelation ­ shipwit h the increase indr yweigh twa sno t found.Th e low root temperature led toa highe rC a level in the fruits.Th eK/C a ratiowa s clearly lower at the low root temperature. Increasing the air temperature raised theC a level inon e of the experiments buthardl y affected it in the other one.Fro m these data and the earlier results of similarexperiment s itma yb e concluded that as long as the fruits are small the amounts ofC a per fruit arenegetativel y correlatedwit h the rate of fruit growth. For leaves,lowe rK concentrations were found at the low root temperature, irrespective of the air temperature.Th e temperature of the airha d little influ­ ence onth eK level.Th e spread in the Cadat awa s too large tojustif y the drawing of anyconclusions . In ane wexperimen twit h Cox's Orange Pippin at aconstan t air temperature, the effect onshoo t and fruit growth of a low (40-50%) and ahig h (85-98%)rela ­ tivehumidit y of the air in anumbe r of combinations with three root temperatures (10°, 17°,an d 26°C)wa s studied. The treeswer e poor innitroge n at thestar to f theexperiment ,bu t ahig h nitrogen dressingwa s applied during leafing-out.Th e trees showed little growth ingeneral ,bu tmor e atth ehig hhumidit y and atin ­ creasing root temperature.A t the lowhumidit y fruit size increasedwit h increas­ ing root temperatures but at thehig hhumidit y no differencebetwee n 10°an d 17°C

32 was found (the 26°C treatmentwa s omitted). The lowhumidit y reduced fruit size only at aroo t temperature of 10°C.Fruit s and leaveswil lb e analysed forN ,K , Mg, Ca,an dP .

Fruit quality, time of picking, and storage condition

In cooperationwit h theExtensio n Service,a ne w storage experiment onGolde n Deliciouswa s started in the autumn of 1973 (see 1973Annua l Report,pp.62-63) . Fruitswer e picked in several places inTh eNetherland s on three dates (25/9, 9/10, and 23/10)an d stored under three sets of conditions,viz . cell 1:ordinar y CA-storage (6-7%C0 2, 14-15%0 2)a t3°C ; cell 2:scrubbe d CA-storage (3-5%C0 2, 3%0 2)a t3°C ;an d cell 3:scrubbe d CA-storage (3-5%C0 2, 3%0 2)a t1°C . A few results concerning thevariou s ripening testswil lb e givenhere ,bu t dis­ cussion of the storage data fall outside the scope of thisreport . In 1973,th e respiration level and fruit colourvalue swer e distinctly higher than inth e preceding year,bu t the iodine test showed no difference in the stage of ripeningbetwee n the twoyears .A s couldb e expected, the intensity ofrespi ­ ration increased the later the picking time,bu t the changewa s sogradua l that thebeginnin g of the climacteric couldno tb e determined accurately.Frui t colour changed rather slowly during the picking period,whic h reduces the reliability of the fruit colour as criterion for determination of the picking time.I ncontrast , the starch level decreasedmarkedl y during the picking period. In the 1973/1974 season the sugarvalu ewa s somewhathighe r and the acid level slightly lower than in the preceding season.Th eestimatio n of the ethylene productionb y the fruits proved tob e abette r criterion fordeterminatio n of thebeginnin g of theclimac ­ teric than the production of C02. InMarc h andMa y of 1974th e taste of the fruitswa s evaluated. In contrast to earlier findings,pickin g time didno t affect taste.Fruit s stored in cell 2ha d a distinctlywors e taste than those from cells 1an d 3.A positive correlation was foundbetwee n taste and sugar content and anegativ e correlationbetwee n taste and acid content. To find outwhethe r the colour of fruits ofGolde n Delicious canb e estimated reliablywit h a colour chart,th e colour of anumbe r of fruitswa s estimated by a panel composed ofexperience d fruit growers.A Belgia n colour chart (scoring 1-8) was used. Statistical analysis of the data showed thatwhe n the same applewa s estimated twiceb y the same person themea n spreadwa s about 0.50.Whe n thesam e applewa s estimatedb y anumbe r of persons themea n spreadwa s about 2.30.Thi s studywil lb e continued in 1975,usin g apples from the same orchard picked on the samedate .

33 PHYTOPATHOLOGY SECTION

H.A.Th,va n der Scheer

Apple

Canker

On cankered Cox's Orange Pippin trees artificialwound s weremad e every forteen days toinvestigat e the development ofnatura l infection by Nectria galligena Bres. Infection occurredmainl y in the autumn andwinter .Wound s normallyorigi ­ nating in this period, including leaf scars,prunin gwounds ,an d frost orhai l injuries, should be sprayedwit h an appropriate fungicide.

Fruit, rot

Research on the susceptibility ofGolde n Delicious fruits to Pezicula malioortiois (H.S.Jacks.)Nannf .an d P.alba Guthriewa s continued.A s in the precedingyear , fruits of Cox's Orange Pippin and Golden Deliciouswer e equally susceptible to F'.malioortiois, but the latterwer emor e susceptible to P.alba than the former. More Gloeosporium rotdevelope d onth e side exposed toth e sun and onth e calyx side than on the shaded and stem sides,bu t themos t rot developed on thetransi ­ tional areabetwee nboth . During storage nest rot can occur,spreadin g from one rotten apple usually infected with Botrytis cinerea Pers.e x Pers..Th e promotion of such infections by dirty containerswa s investigated. Cold storage ofGolde nDeliciou s apples in boxes smearedwit h pulp of apples carrying Botrytis rotdi dno t lead tomor enes t rot than occurred in cleanboxes . In the laboratory, 0.05% TopsinM gave good control of F.malioortiois when applied 22day s after inoculation ofGolde n Delicious apples.I n ayiel dexperi ­ ment, 0.1%Topsi nM gave good control of Gloeosporium rotwhe n the GoldenDelici ­ ous treeswer e sprayed once twoweek s before picking.

Powdery mildew

Somene w fungicideswer e tested onpotte d apple rootstock M.7 toevaluat e control of Podosphaera leuootrioha (Ell.& Ev. )Salm. .Whe n applied inhig hdosages , Ridomilgav e control of the powdery mildew.Th e fungicide JF 4260 in the same a.i. dosage controlled the fungus farmor ebette r as astandar d spray ofdinocap . Since JF 4260 does not act onmite s and predators,i t is ausefu l fungicide in spray schemes for integrated control inorchards . Tokil l the fungus ininfeste d terminalbuds , sprayswit h 10%Off-Shoot- 0wer e applied at thebeginnin g and end of leafdro p in the autumn.N o reduction ofa limited primary mildew infectionwa s obtained.

34 Phytotoxicity

In a field experiment on Golden Delicious, 0.04% JF 4260 caused as much fruit russeting as 0.06% dinocap. Golden Delicious fruits sprayed with 1% Golclair showed the same russeting as those sprayed with 0.25% Luxan Captan-Zwavel wettable powder. The addition of 0.02% Tuzet (5x) or 0.1% Luxadim 40 (3x) to 0.25% Luxan Captan-Zwavel wettable powder reduced the amount of russeting. In 1973, no damage was seen after two-year-old Golden Delicious trees were sprayed with suspensions of Goldion wettable powder or Luxan Dodine 65% wettable powder in fluoridated water. There was no effect of this treatment on the amount of rotting during cold storage.

Blackberry

Cane spot

In a field experiment on the blackberry variety Himalaya, pre-blossom sprays with 0.1% Topsin Mgav e good control of cane spot caused by Rhabdospora ruborum (Lib.) J(6rst. and to a minor degree by Septovia rubi Westend. The sprays were applied at two-week intervals starting when the young canes reached a length of 50 cm and continued until blossoming began. Pre-blossom sprays with 0.25% Duphar Ferbam and sprays with 0.5% Koper Bayer Koperoxychloride wettable powder in the autumn (from the end of picking in October until the middle of December) also gave con­ trol of the cane spots. The copper oxychloride sprays, which were phytotoxic, caused a reddish discoloration of the canes.

Morel!o s

Blossom blight

In afiel dexperiment , four applications of0.1 %Topsi nM befor e and duringblos ­ soming gave excellent control of Monilinia laxa (Aderh.& Ruhl )Hone y exDennis . Blossombligh t attackwa s veryheav y inunspraye d trees.Tw o applications of0. H TopsinM justbefor e and duringblossomin g gave poor control.Th e first applica­ tion shouldb emad e asearl y as thebu d swelling stage.

Pear Saab On Beurré Hardy pear trees, Venturia pirina Aderh. was controlled equally well by seven curative applications of 0.1% Topsin M, applied only after conditions for heavy infection (according to Mills' table) were present, or by 18 preventive applications (every ten days) with 0.05% Topsin M. Scab control by 18 preventive applications of 0.25% Luxan TMTD 80% wettable powder was slightly less effective.

35 Strawberry

Craim rot

Inoculation of Glasa strawberry plants with zoospores of Phytophthora oaotorwn (Leb. & Cohn) Schroet. by means of soil percolators generally failed to give diseased plants.

Red stele

No outgrowth of mycelium of Phytophthora fragariae Hickman was observed from roots of Glasa strawberry plants with oospores after storage for two years at -20°C. Soil infestations could be demonstrated by the use of young strawberry-plant roots held in water-saturated soil samples in petri dishes. Detached Glasa roots showed the characteristic red discoloration of the stele less often than attached ones. The soil samples were taken from fields with a known history of infestation. Fungicides were tested in infested fields to evaluate control of red stele in plants of the variety Senga Sengana. A soil drench with 1 ml Previcur in 500 ml water per plant applied immediately after planting in August, controlled the dis­ ease. A spray application of 0.2% Previcur at a rate of 1500 litre/ha gave no control.

Verticillium wilt A number of isolates of Vertioilliwn species were tested with respect to patho­ genicity and virulence for three strawberry varieties. Only the isolates of V.dahliae Kleb, and V.albo-atrum Reinke &Berth , were pathogenic; none of the isolates of V.nigrescens Peth., V.nubilum Peth., or V.tricorpus Isaac were. The pathogenic isolates differed in virulence from moderate to strong. Some of the isolates were not equally virulent for the three strawberry varieties. In a greenhouse, artificially infected plants of the variety Gorella matured without disease symptoms when a soil drench of 0.2 g Benlate in 100 ml water per bucket was applied after potting. Plant growth was better in peat-sand than in silty clay-loam, and Benlate had a stronger positive effect on plant size in the latter soil type. In a field experiment, three applications of 0.1% Topsin Mgav e some control of disease symptoms in artificially inoculated Gorella plants. The applications were made one and four weeks after planting (September) and at the onset of growth in the spring (April).

36 ENTOMOLOGY SECTION

D.J. deJon g

Leafroller s in orchards

The project is intended tosuppor t the improvement of supervised and selective integrated control in orchards.Fo romissio n of theus e of insecticides inAugust , adequate control of caterpillars in thesprin g proved tob enecessary . The effect of azinphos-methyl insimpl e supervised schemes and of Bacillus thuringiensis in selective integrated control schemeswa s often insufficient in the spring (see page 38).Therefore ,fiel d trialswit h juvenilehormon e analogs are planned. Pre-blossom sprays probablyha d asligh t influence on the sex ratio inth e surviving Adoxophyes population.Al l eggs laidb y moths of the first flightdevel ­ oped andbecam emoth s of the second flight.Som e larvae of Adoxophyes, which star­ ted tohatc h onJul y 15th,wer e induced todiapause ,an d from thebeginnin g of August almost 100%o f thehatchin g larvaewer e induced todiapaus e in theautumn . The significance of the second flight of Adoxophyes was analysed inrelatio n to temperature.Th e first larvaehatche d onAugus t 19th fromegg s laid onAugus t 7th. Only the larvaehatchin g fromegg s laid up toAugus t 28th,develope d into thenorma lhibernatin g instarL 3. Embryonic developmentwa sno t completed inegg s laid afterSeptembe r 6th.Therefore ,onl y the firstpar to f the flight period (which lasted fromAugus t 7th tillSeptembe r 23th)wa s of importance forfrui t damage.Fro m the end ofAugust ,th emoth s dideve nno t produce offspring that developed toL3 .Thi smean t that controlmeasure swer e unnecessary inman ycases , evenwher e critical population densities of caterpillars were observed inJuly . Starting onSeptembe r 20th,th e first larvaemad e theirwinte rwebs . OnOctobe r 2nd andNovembe r 7th, 15an d 90% of the larval population, respectively,wer e present inhibernacles .Th ewinte rmortalit ywil lb e studied in thewinte r of 1974/1975. The transport and survival ofegg-larva e of Adoxophyes isbein g studied quan­ titatively on apple trees in theexperimenta l orchard.Becaus emos t of theman y freshlyhatche d larvae thatwer e transportedb ywin d and reached the soil died due to lack of food plants,th emortalit y in this period seems tob e aver yimpor ­ tant factor for the populationdynamics . In cooperationwit h theLaborator y forResearc h onInsecticide s (LIO), quanti­ tative relations between the captures inse xpheromone-trap s and the larvalpopu ­ lationdensit y asdetermine d by earlier countings of the larval population are being studied in Adoxophyes. The numbers captured differed appreciably,bot h between theman y traps and onsuccessiv e dates for the same trap.Th e influence ofweathe r factors seems tob e very complicated. The results of recaptures of markedmoth s indicated that dispersion into adjacent plots is anorma l occurrence in the orchard complex.

37 Integrated control

D.J. deJon ge nM .va nd eVri e

Supervised control

Since supervised controli sexpecte d tobecom e thenorma lwa yo fcombattin g insect damage inth enea r future,extensio n officers arebein g instructed.A sa resulto fthi s program, such schemeswer e appliedb yabou t6 0grower si nth e southwestern parto fth ecountr y andb y 175grower s throughout country.I fneces ­ sary,w eassiste d thesuperviser s andinvestigate d special problems relatedt o improvement ofth etechnique s (seepag e37) . Inth elatte r connectionth etortrici d populationwa ssample d quantitatively ina norchar d complexa tWilhelminadorp .I nal lplots ,th eleafroller sha dt ob e controlled inJun ean di nAugust ,a swel la sth ewinte rmot h (Operophtera brumata) andth ecloude d drabmot h (Orthosia spp.)i nth espring .I nspit e ofth eapplica ­ tiono fazinphos-methy l inth espring ,th elepidopterou s larvae damaged4.3 %o f the fruits,th etota l insect damage amounting to5.1% .Th ewindbreak sha da strong influence onth epopulatio n densityo fth etortricid s inadjacen t rows oftrees .

Integrated control

Selective integrated schemeswer e applied forth ethir d successive yeari nBerge n opZoo man dfo rth efirs t timei nNoordwelle . InBerge n opZoom , Bacillus thuringiensis gave insufficient controlo fleaf - rollers andothe r lepidopterous larvae inth espring . Pirimor gave excellent results against aphids.Predaceou smite s (Phytoseiidae) were released duringth e 1972seaso n forcontro lo f Panonychus ulmi. During 1973an d1974 ,chemica l control wasno tneeded .A tth ebeginnin go fth e197 4season ,predaceou s miteswer e present but rather scarce;durin g thesecon d parto fth eseaso n theirnumber s increased considerably.Th eevolutio ni nth ecompositio no fth etortrici d population contin­ ued; Rhopobota naevana and Pammene rhediella were also observed in1974 .Contro l ofth ecodlin gmot h Laspeyresia pomonella wasnecessary .Th eleafmine r Stimella malella waspresen ti nacceptabl e numbers onapple ,an dthi s alsohold sfo r Psyllid spp.o npear .Onl y 1.2t o3.8 %o fth epear swer e attackedb yinsect san d 2.1 to5.2 %o fth eapples ,wit h theexceptio no fLombartscalvill e (14.5%). Nearly two-thirdso fth efrui t damagewa sdon eb ywinte rmoths . InNoordwelle ,predator y mites were liberated onappl e trees inJun eand , for the first time,als oo nplu m trees.A lo wpopulatio nwa spresen t inth esumme r and autumn.Throughou t theseaso na smal l populationo fth efrui t treere dspide r mitewa spresent .Onl yon espra ywa snecessar y tocontro lth ewinte rmot h andth e rosaappl e aphid Dysaphis plantaginea inth espring .Almos t 5%o fth efruit swer e damagedb yinsects ,mainl yb yth ewinte rmoth .

38 ACAROLOGY SECTION

M. van deVri e

Chemical control of p.ULMl

Thehatchin g period ofwinte regg s of P.ulmi was studied. Considerable variation was foundbetwee n samples fromdifferen t orchards stored under uniform conditions. Anexplanatio n for these differences cannotb e given.

Integrated control ofth efrui t tree redspide r mite PANONYCHUS ULMI (Koch) Ecology of pvedaoeous mites

The influence of different temperatures (15,20 , 25,an d 30°C)a t aconstan t air humidity (85%)o n the developmental period of Amblyseius potentillae was studied. The stage of the developingmite swa s checked every 8hour s until the females werematur e and started oviposition, afterwhic h the daily egg productionwa s recorded. This information isneede d for the completion of simulationmodel s for the interactions between A.potentillae and P.ulmi.

The occurrence of pvedaceous mites on various host plants

The surveys of the occurrence ofpredaceou s mites onvariou s host plants (fruit trees, ornamentals,etc. ) and their prey specieswer e continued. Samples sentb y colleagues inNorway ,Sweden ,Belgium , Switzerland, andNorther n Italywer e received forindentification .

Influence of pesticides on predaceous mites

Since frequent applications of dinocap are disastrous forpredaceou smites ,sever ­ alpotentia lmilde w fungicideswer e tested on A.potentillae. Two of these com­ pounds (Bayer 6447 and ICI 588)wer ewithou t effect on this predator in laboratory experiments. In field experiments with apple trees the influence ofSpruzi t andFunda lo n the species compositionwa s studied and compared with theeffec t ofparathio n and water.Bot h of the former proved tohav ewide-spectru m activity.Th e effectwa s evident immediately after the application,especiall y with Spruzit.

39 Simulation models on the interactions between P.ulmi and its principal predators

Several elements with relevance for the development of a simulation model of the interaction between P.ulmi and A.potentillae were studied, including the influence of the host-plant quality on the reproductive capacity of P.ulmi; the macroclimate and its relation to the microclimate in the orchard; the relationship between the colour of the predaceous mite and its behaviour; the influence of temperature and air humidity on the rates of development and reproduction of the prey and the predators, and the influence of the predator on the stage-composition of the prey. Some of these elements were studied in both the laboratory and the field.

Integrated control of P.ulmi

In several experimental plots a study was carried out on the development of P.ulmi populations under the influence of A.potentillae. In the Schuilenburg plot predaceous mites were released in 1970; in the release plots P.ulmi controlled perfectly. In the Bergen op Zoom plot predaceous mites were released during 1973; here, too, no chemical control was needed during 1974. In the Wilhelminadorp plot the predaceous mites released in 1973 became estab­ lished during 1974 and kept P.ulmi at reasonably low levels during the summer. Redistribution occurred at the end of the summer; and the predators were observed on trees adjacent to the release plots. In the Numansdorp plot very low densities of P.ulmi occurred at the time of the release of A.potentillae, and therefore additional releases of P.ulmi were made. However, the predator and prey populations only became established in low densi­ ties.

Red spider mite in strawberries

The population development of Tetranyahus urtiaae on strawberry in greenhouses was studied in relation tolea f development.Whe n the leaf developmentwa s reduced , the averagenumbe r ofmite s increased drastically. Preliminary experiments on the release ofpredaceou s mites showed thepoten ­ tialso f Phytoseiulus persimilis and A.potentillae toreduc ean dkee p T.urticae below theeconomi c threshold level.Th emos t suitable period for releasingbot h species depends on thenumbe r ofpre y present,th e control potentials ofbot h predator species,an d the period of theyear .

40 ACAROLOGY, SOIL FERTILITY,AN DPHYSIOLOG Y SECTIONS

M.va n deVrie ,P .Delve r andJ . Tromp

Influence of nitrogen level in apple trees on the development of the fruit tree red spider mite

In 1973,a po t experimentwa s startedwit h young Schoneva nBoskoo p appletrees . The trees received differential nitrogen dressings (see 1973Annua l Report)an d inMa y young impregnated femalemite swer e placed on the leaves.Th e development of thepopulation s was followed on thebasi s of regular counts ofmite s and eggs. In leaves sampled at short intervals,th e levels of total-N,soluble-N ,protein-N , and anumbe r of amino acidswer e determined. The treeswer e repotted in 1974an d thenitroge n treatments repeated.Ne wmit e populations developed from the 1973winte r eggs,bu t leafgrowt h onmos t of the treeswa s very poor due tonitroge n deficiency atstar t of the experiment and throughout 1973an d 197A.Thi s deficiencyha d anunfavourabl e influence onth e development of themites ,an d the experimentwa s terminated aten d of1974 . For an investigation of the same problem, atria l fieldwa s planted in 1972 withWinsto nM. 9 trees.Th e soili s coveredwit h grass,an d theweed-fre e tree strips are only 50 cmwide .Startin g in 1974,eithe r 100o r 250k g N perh ai s given annuallyeithe r inon e spring dressing ordistribute d over theseaso n to simulate aslo wnitroge n source.Th e treatments also comprise aperennia l cover ofwhit e cloverwithou t fertilization and clean-cultivated soilwit h annuallegu ­ minous crops fertilizedwit h farmyard manure.Accordin gt oplan ,hal f of the plots willb e infectedwit hmite s in 1975an dn o acaricideswil lb euse d there.Th e relationshipbetwee n the development ofmit e populations, thenitroge n status of the leaves (described indetai lb y determination of amino acids), and thesoi l treatments,wil lb e studied.N omite swil lb e placed on the trees in the other half of the plots,an d ifnecessar y mite controlwil lb e carried out inthem . It ishope d that comparison ofyield s from similar fertilizer treatmentswit h andwithou tmite swil l permit theevaluatio n ofmit e damage inrelatio n toth e population density. In 1973,th e lowyiel d of about 3k g per tree,indicate d that some response toth enitroge n treatmentsha d already occurred.

41 LISTO F CHEMICALS,mentione d in this report

Commercial product Contains:

Growth regulators and compounds for chemical thinning

AArupsin 50% carbaryl Alar-85 85% N-dimethylaminosuccinamide Amid thin (NAAm) 8.4% a-naphtylaceetamide

Berelex kklk7 {Gkh+1) 0.89% gibberelline A^+Ay + 0.06% gibberelline Aj3 CME 72.200 6.25% dichloroflurenol-methyl CME 73.130 75% chloroflurenol-methyl Ethrel-A 480 g ethephon/litre Fruitone-T (2,4,5-TP) 64 g phenoprop/litre NC 9634 25% (3-phenyl-1,2,4-thiadiazol-5-yl) thio acetic acid

Off-Shoot-0 45% methylesters of C6-C12 fatty acids PH 30-13 5% 5,7-dichloro-4-ethoxycarbonylmethoxy-2,1,3- benzothiadiazole

Herbicides

AAkarmex 80% diuron Bladex 50% cyanazin Brabant MCPA-30 25%MCP A Gramoxone 20% paraquat Luxan 2,4-D 50% 2,4-D LuxanMCP P 50%MCP P Orga-Atrazin 50% atrazin Orga-Simazin 50% simazin Prefix 7.5% chlorothiamide Roundup 360g glyphosate/litr e Venzar 81% lenacil Weedazol 250g amitro l + 220g ammoniumthiocyanate/litr e

42 Fungicides, Insecticides, Acaricides

Benlate 50%benomy l Dipel HD-2 strain ofBacillu s thuringiensisBerline r DupharFerba m 76% ferbam wettable powder Golclair sulfur + borax + marl + ? Goldion 60% sulfur + 20% mancozeb wettable powder Gusathion 25% azinfos-methyl wettable powder JF 4260 25% butrimate KoperBaye r 50% copperoxychloride Koperoxychloride wettable powder Luxadim4 0 40% dimethoate Luxan Captan-Zwavel wettable powder 45% captan + 19% sulfur Luxan Dodine65 % wettable powder 65% dodine LuxanTMT D80 % wettable powder 80% thiram Pirimor 50% pirimicarb Previcur 70% prothiocarb Ridomi 1 50% chloromethoxycarbonyl propyl benzimidazole Spruzit flüssig 2% pyretrinen + 2.5% rotenon + 12% piperonylbutoxide TopsinM 70% thiophanate-methyl Tuzet 40% thiram + 20% ziram + 20% urbazid

43 PUBLICATIONS

Publicationsi n197 4

Blommers, J. (Glasshouse strawberries.)D e teeltva n aardbeien onder glas.Wilhelminadorp , Netherlands.Nationa l FruitAdvisor y Service and Research Station forFrui t Growing. 1974.6 4pp .Publikati e no.2. (Dutch.)

Boon, J. van der and A. Ponwer (Bitter pit inJame s Grieve apples as influenced by nitrogen topdressing , cal- ciumnitrate sprays,frui t thinning, and picking time.)Invloe dva n stikstofover- bemesting, spuitenme tkalksalpeter , dunnen enpluktijdsti p opsti p in appelsbi j James Grieve.Haren ,Institut e forSoi lFertility . 1974.Rappor t 3-74. (Dutch.)

Delver, P. (Soil factors in relation tobitte r pit.)Bodemfactore ne n stip.D e Fruitteelt 64(1974)20:542-545. (Dutch.) (Trends in thenitroge n fertilization of fruit orchards.)Ontwikkelin g ind e stikstofbemesting vanboomgaarden .Stiksto f 77(1974):136-145 . (Dutch.)

Dijkstra, J. (Rubus stunt in blackberry.) Heksenbezemziekte bij braam. Groenten en Fruit 30 (1974)5:195. (Dutch.)

Elema, U.K. (The role of theResearc h Station forFrui tGrowin g in thenationa l research pro­ gram for fruit production.)D e plaats vanhe t Proefstation voor deFruitteel t in het fruitteeltkundig onderzoek.D eFruitteel t 64(1974)8:222-223. (Dutch.) (Fruit-growing inFranc e in 1974.)D eFrans e fruitteelt in 1974.D e Fruitteelt 64(1974)49:1212-1215. (Dutch.)

Frankenhuyzen, A. van and D.J. de Jong (Leafrollers (Tortricidae), aninterestin g group ofMicrolepidopter a (3).Th e codlingmot h (Laspeyresia pomonellaL.). ) Bladrollers (Tortriciden),ee ninteres ­ sante groepmicrolepidopter a (3).D e fruitmot (Laspeyresia pomonella L.). De levende natuur 77(1974):160-166 . (Dutch.) (Leafrollers (Tortricidae),a n interesting groupo fMicrolepidopter a(4) . Grapholitha tenebrosanaDup. )Bladroller s (Tortriciden),ee n interessante groep microlepidoptera (4).D e rozebottelmot (Grapholitha tenebrosana Dup.). De levende natuur 77(1974):221-224 . (Dutch.)

Goddrie, P.D., J.J. Verhaegh and T. Visser (External and internal quality of somene w applevarieties. )Uiterlijk e eninner ­ lijkekwalitei t van enigenieuw e appelrassen.D eFruitteel t 64(1974)18:484-485. (Dutch.)

44 Gruys, P., D.J. de Jong and M. van de Vrie The development of an integrated control programme for apple orchards inth e Netherlands: problems and prospects.Proceeding s FAO Conference onEcolog y in relation toPlan t Pest Control,Rome , 1973 (1974):133-144 . (English.)

Jong, V.J. de and J.P.A. van Dieren Population dynamics of thesumme r fruit tortricid Adoxophyes oranaF.v.R .i n relation toeconomi c threshold levels.Meded .Fakultei tLandbouwwetenschappe n Gent 39 (1974)2:777-788. (English.)

Jong, D.J. de and H.J. Vlug Clef d'identification des chenilles de tordeuses dupommie r auxPay sBas .OILB / SROP. 1974.Brochur e no.3 (Les organismes auxiliaires enverge r de pommiers): 35-37. (French.)

Lemmens, J.J. (Notes on theExperimenta l Garden atWilhelminadorp. )Nieuw s van de proeftuin teWilhelminadorp . DeFruitteel t 64(1974)9:224-227. (Dutch.) (Apple pruning still invogue. )Snoeie nbi j appelno g steeds in.D e Fruitteelt 64(1974)47:1152-1157. (Dutch.)

Nijsse, L. (Blackberry growing.) De teelt van bramen. Kleinfruitteeltadviezen (1974)1:8-12. (Dutch.) (Strawberry growing innarro w plastic tunnels.)Aardbeie n onder plastickappen . Kleinfruitteeltadviezen (1974)1:2-4. (Dutch.) (Strawberry varieties.)Rassenkeuz e bij aardbeien.Kleinfruitteeltadvieze n (1974) 2:5-7. (Dutch.) (Small fruitvarieties. )Rassenkeuz e bijkleinfruitgewassen .Kleinfruitteeltad ­ viezen (1974)3:7-13. (Dutch.)

Oosten, H.J. van (Studies onwea k rootstocks forswee t cherry.)Onderzoe k naar zwakke onderstammen voor dezoet ekers . DeFruitteel t 64(1974)9:231-233. (Dutch.) (Viruses andmycoplasmas. )Virusse n enmycoplasma's .D e Fruitteelt 64(1974)37: 969-970. (Dutch.)

Pouwer, A. Leaf analysis forpredictio n ofbitte r pit.Act aHorticultura e 45(1974):33-38. (English.) Fruit analysis forpredictio n ofbitte r pit.Act aHorticultura e 45(1974):39-43. (English.)

Scheer, H.A.Th, van der Susceptibility of strawberry toisolate s of Phytophthora cactorum and Phytophthora citricola.Meded .Fakultei t LandbouwwetenschappenGen t 38(1973):1407-1415 . (English.) (Sources ofinfectio n of fruit roti n apple.)Infektiebronne nva nvruchtro t bij appel.D e Fruitteelt 64(1974)9:234-235. (Dutch.)

Scheer, H.A.Th, van der and J.P.A. van Dieren (Mortality ofplu m trees causedb y Diaporthe eres.)Pruimebomensterft e veroorzaakt doorDiaporth e eres.D eFruitteel t 64(1974)7:174-175. (Dutch.)

45 Tromp, J. The interactiono fgrowt h regulators and treeorientatio n on fruit-bud formation in apple.Act aHorticultura e 34(1973)1:185-188. (English.) (Experiencewit h ripeness tests.)Ervaringe nme t rijpheidstoetsen.D e Fruitteelt 64(1974)20:546-548. (Dutch.) (Colour,suga r content,an d acidity ofGolde nDeliciou s fruits.)Kleur ,suiker ­ enzuurgehalt eva nvruchte n vanGolde n Delicious.D eFruitteel t 64(1974)33:876- 877,886. (Dutch.)

Uitterlinden, L. (Experimental results on correction of soil fatigue in apple.)Resultate nva nd e proeven terbestrijdin g vanbodemmoehei d bij appel.D eFruitteel t 64(1974)36:943 enGroente n enFrui t 30(1974)19:824-825. (Dutch.)

Verhaegh, J.J., P.D. Goddrie and T. Visser (New stars in the apple sky ?)Nieuw e sterren aand e appelhemel ?D e Fruitteelt 64(1974)19:510-512. (Dutch.)

Wertheim, S.J. (Checking thesplittin g of cherries.)He t tegengaanva nhe tbarste nva nkersen . DeFruitteel t 64(1974)24:648. (Dutch.) (Results of the second national storage trialwit h GoldenDeliciou s in 1973/1974.) Resultatenva nd e tweede landelijkebewaarproe f metGolde nDeliciou s in1973 / 1974.D e Fruitteelt 64(1974)33:874-875,878. (Dutch.) (Experimental planting systems.)Experimentel e beplantingen.D eFruitteel t 64 (1974)49:1198-1203. (Dutch.) Chemical control of flower and fruit abscission inappl e and pear.Act aHorticul ­ turae 34(1973)1:321-331. (English.) L'éclaircissagechimiqu e des fruits.Frui tBelg e 42(1974)367:183-190. (French.)

Wertheim, S.J. and J.L. Baarends (Light in relation tofrui t colour.)D ebelichtin g end evruchtkleur .D eFruit ­ teelt 64(1974)9:228-230. (Dutch.)

Wertheim, S.J. and M.L. Joosse (Chemical flower thinningwit hEthrel- A onWinston. ) Chemische bloemdunning met Ethrel-Abi jWinston .D eFruitteel t 64(1974)15:410-411. (Dutch.)

Wertheim, S.J. and F. Nijsse (Internalbrow n discoloration inpea rbuds. )D ebruinverkleurin g onderpereknop ­ pen.D eFruitteel t 64(1974)6:142-143. (Dutch.) (Thepollinatio n ofKarmij n deSonnavill e apple.)D ebestuivin g vanKarmij nd e Sonnaville.D eFruitteel t 64(1974)38:984-985. (Dutch.)

Wertheim, S.J., F. Nijsse and M.L. Joosse (The chemical thinning ofGolde nDelicious. )D e chemische dunning vanGolde n Delicious.D eFruitteel t 64(1974)15:402-403. (Dutch.)

Remaining publications (15thLis t ofFrui tVarieties. ) 15eRassenlijs t Fruit.Wilhelminadorp ,Nether ­ lands.CR F andResearc h Station forFrui t Growing.Editor :P.D . Goddrie.1974 . 136pp .Mededelin g no.14 .(Dutch. )

46 Bulletins of the Research Station for Fruit Growing

9. S.J. Wertheim: Carbaryl;a reliabl e chemical thinnerfo rGolde n Delicious apple trees (Englishwit h aDutc h summary), 19p p August 1970 ƒ2, —

10. S.J. Wertheim: Thetrainin g ofth eslende r spindle offou r apple varieties (Dutch photo- grafic reportwit ha nEnglis h summary),158pp . November 1970 outo fprin t

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

12. S.J. Wertheim and J.J. hemmens: Planting systems forappl ean dpea r (Dutchwit ha n English summary), 77p p February 1973 ƒ7,5 0

13. M. van de Vrie: Miteso nfrui t trees (Dutch with anEnglis h summary),6 8p p December 1973 outo fprin t 14. 15thLis to fFrui t Varieties (Dutch), 136pp . .Novembe r 1974 ƒ6, -

Publications of the National Fruit Advisory Service and the Research Station for Fruit Growing

1. Strawberries inth eope n (Dutch),4 8pp . .. . Marc h 1973 f3,5 0

2. Glasshouse strawberries (Dutch),6 4p p March 1974 ƒ5,5 0

TheBulletin s andPublication s listed onthi s pagewil lb esen t postpaid upon receipto fa ninternationa l postalmone y orderfo rth eappropiat e amount. Please indicate your order clearly.

47