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Variation in Symptom Expression of Exocortis and Gummy Pitting in Trees on Poncirus trifoliata Rootstock in New South Wales

L. R. Fraser and Patricia Broadbent

USAGE OF TRIFOLIATE N.S.W. as a stock, by Crichton in 1893 IN NEW SOUTH WALES (N.S.W.) (quoted by Bowman, 1955), referred to its dwarfing properties. , previously grown to The great demand for trees on root- only a minor extent in N.S.W., came rot-resistant stocks made prominent the into prominence during the 1940's. In variable performance which had pre- the 1930's and 19403, root rot caused by vented general use of trifoliate orange in spp. resulted in heavy the past. losses of trees on rough and sweet The Trifoliate Improvement Com- orange stocks during a succession of mittee (later renamed the Citrus Im- seasons of higher-than-average rainfall. provement Committee) was set up by This was particularly so in the irrigation the N.S.W. Department of Agriculture areas of the Murrumbidgee and Murray in 1942 to solve this problem. Surveys Rivers where the situation was aggra- were made of all known plantings of vated by faulty irrigation practices, lack trees on trifoliate orange in N.S.W., to of adequate drainage, and nonuniform clarify and define the nature and extent soil types (Fraser, 1949). In the 1950's, a of the variability. succession of wet seasons and flooding of rivers in the central coast area and at VARIATION IN OLD BLOCKS OF Narromine also resulted in destruction TREES ON TRIFOLIATE ORANGE of thousands of trees. By 1942, it was evident that a suitable The most striking feature was the root-rot-resistant stock must be found range of tree sizes, from large - ap- to reestablish the industry in areas hard proaching that of trees of the same age hit by Phytophthora diseases and to on stock -to very small; permit expansion. Though sour (Seville) some trees were only a few meters high orange had been used traditionally with at ages of up to 40 years. Tree shape also great success in overseas countries for varied; some dwarfed trees were flat- the control of these diseases, it was a tened and spreading, others were failure in N.S.W., as it also has been in rounded or upright. South Africa since the 1870's. The Phenomena shown by the stock reason was then not known, but was below the bud union also varied. In the later recognized as tristeza virus. most dwarfed trees, the butt was clothed In the search for stocks alternative to with hard, persistent scales (exocortis). rough lemon and sweet orange, all avail- Some large trees also showed scaling. able were screened for resistance Other small trees showed intermittent to Phytophthora. Trifoliate orange was or partial scaling of the stock, with or the only species with required root-rot without pustular corky outgrowths. resistance, but it had a worldwide repu- Associated with this latter type was tation as a dwarfing and unreliable gummy pitting of the wood originating stock. The first mention of its use in at the cambial level. In other trees. the Eighth IOCV Conference

bark of the stock was smooth without blossoming and fall. Some remain external abnormalities but often with leafy as seedlings over the winter during some gummy pitting of the wood their first year, becoming fully winter- (Fraser et al., 1976). All these symptoms dormant only in the second or third varied greatly in intensity from tree to season. One, a tetraploid, is slightly tree. The budunion varied from smooth larger and coarser. and flat to abruptly benched, with in- HORTICULTURAL TRIALS dentation and some periodic gumming at the cambial level. This indentation The possibility that strains of trifoliate varied from nil or mild to deep. With orange may perform differently as age, the stock greatly outgrew the scion. stocks, and that some could produce The stock varied from slightly enlarged stunted trees because of incompatibility, in diameter compared with the scion, was investigated. Seed-source trees for straight sided, or more or less strongly trial as stocks were selected in a variety fluted. Some dwarf trees showed no of ways and included suckers from butt symptoms at ages up to 40 years, stocks of non-dwarfed Washington other than a reticulum of surface cracks navel oranges, old trifoliate orange on the shoulder below the budunion stock trees in plantations of satisfactory and a few small, gummy pits. growth, a range of stocks imported A few blocks of Valencia and Wash- from other states, and trees showing ington navel orange and Emperor man- minor vegetative differences. The tetra- darin showed no dwarfing; trees were of ploid was included in one series of trials. uniform size and their stocks were con- A total of 33 stock selections was used in siderably enlarged and fluted without trials with scions of non-dwarfing any of the abnormalities associated with Washington navel and , dwarfed trees. These were later deter- Marsh , and Ellendale mined to be of nucellar origin and at Griffith, Yanco, and Somersby. Al- formed the basis of a bud certification though slight differences in size and scheme developed on indexed virus-free performance of trees of the same age on clones. these various stocks developed, none of the trees was stunted. In all these trials, SOURCES OF TRIFOLIATE the seedlings used for stocks were ORANGE selected in the nursery for uniformity As a basis for field trials, a collection before budding, to eliminate possible was made of trifoliate orange sources. hybrid types. Seed was obtained from many old seed- In other trials, seedlings from single ling trees in N.S.W., from other Aus- sources were separated into categories tralian states, and from the U.S.A. on size, deciduous habit and type of Suckers from the stocks of trees branching. When budded with orange showing satisfactory growth were pro- varieties, no significant difference was pagated directly, as cuttings, for inclu- shown between them and no butt ab- sion in the collection. normalities developed. In a trial at A selection of 148 of these is main- Yanco, which included a tetraploid, no tained in the arboretum at Somersby significant differences were shown Horticultural Research Station. between the tetraploid and other variation between clones of trifoliate seedling stocks. orange is not great in the material available in N.S.W., suggesting that TRANSMISSION OF EXOCORTIS original imports of this stock were few 'Trials were set out, first on growers' and of uniform type (Fraser et al., properties and later at horticultural 1961). research stations at Narara and ~kon~selections in the Somersby Somersby on the central coast and collection, minor variations occur in Griffith and Yanco in the hotter, more flower type, autumn leaf color, time of arid Murrumbidgee irrigation areas, to Exocorris and Gummy Pitting 203

TABLE 1 DWARFING SOURCES, AND THEIR REACTIONS ON

Etrog citron Source Symptoms in source tree reaction

Washington navel orange, dwarfed, few old scales, pustules, gummy pitting, discontinuous indented ring Washington navel orange, moderately dwarfed, benched at union, few scales Washington navel orange, moderately dwarfed, slight scaling, severe gummy pitting, discontinuous ring, benched Washington navel orange, non-scaling, moderate benching Washington navel orange, severely dwarfed Washington navel orange, dwarfed, small patches of pustules and scales Valencia orange, dwarfed, not scaling (off-type stock) Marsh grapefruit, intermediate size Marsh grapefruit, dwarfed

* 3539 produced a positive reaction for xyloporosis in Orlando .

TABLE 2 EFFECT OF INOCULATION WITH BUDS FROM DWARFED TREES ON SIZE AND CROPPING OF BELLAMY NAVEL ORANGE ON TRIFOLIATE ORANGE ROOTSTOCK

Cumulative yield Tree size to July 1977 Fruit weight Height Width Inoculation No./tree (kg)/tree (9) (m) Bellamynucellar navel orange 3531 3532 3533 3534 3535 3536 3537 3538 3539 Eighth IOC V Conference

determine whether a transmissible Of the trees inoculated with budwood agent present in the scions of dwarfed from the apparently healthy tree, one is trees was responsible for dwarfing and showing a small area of flaky scales and scaling. pustules in 1979. In 1959, budwood of The dwarfing agent associated with two dwarfing selections used in other the scaly butt condition proved to be trials was used to inoculate some of bud-transmissible in the symptomless these trees. By 1979, not all of the inocu- scion (Benton et al., 1950). Trees propa- lated control trees had reacted to the gated from dwarfed, exocortis-affected challenge inoculation. Thirty-three per sources reproduced the characters of cent of trees inoculated with one dwarf- the parent tree. Size differences between ing budline now known to carry xylo- affected and healthy trees, and onset of porosis, and 60 per cent of trees inocu- scaling, started to show up at about 4 lated with the other budline, which is years. not known to carry other viruses (except The variation shown by trees in old tristeza), have remained free of scaling. commercial blocks on trifoliate orange Inarching of dwarfed and exocortis- which had been propagated from bud- affected trees at Dooralong. Trees in a wood from trees on rough lemon stock commercial orchard, showing a full was reflected in the transmission trials, range of dwarfing and butt characters, though in lesser degree. and including non-dwarfed trees, were Highest percentage transmissions of inarched in 1942 and 1943 about 30 cm the pathogens causing scaling were above the bud union with selected obtained from old-line lemon varieties. seedlings of known parentage (two per Over 90 per cent of these showed strong tree). Inarches developed early symp- exocortis symptoms at about 4 years. toms of scaling at 4 to 6 years. Onset of Transmission of exocortis, using symptoms varied between the two in- Washington navel and Marsh arches on each tree, and between trees. grapefruit sources for inoculation of The exocortis reaction has so far been nucellar varieties, varied considerably the only symptom observed. and was generally less than 75-80 per Dwarfing transmission trial - cent. Somersby H.R.S. Bellamy nucellar navel oranges on trifoliate orange were TRANSMISSION OF EXOCORTIS inoculated as nursery trees with nine AND DWARFING dwarfing budlines of Washington navel Griffith Viticultural Research Station. and Valencia orange and Marsh grape- In a trial at the Griffith Viticultural fruit (table 1). Four trees of each propa- Nursery in 1947, trees of two bud gation were planted in 1955 at Somers- sources from an orchard at Dooralong by. Dwarfing was evident by 1960. The were compared: 1) a dwarfed tree with dwarfing budlines differed in tree size, exocortis scaling; and 2) an apparently shape, cropping, and vigor. To date, normal tree in the same orchard. gummy pitting has not developed in any Scaling was recorded on half the trees of these trees. One budline (3539) has budded with the exocortis budline at 4 been found to carry xyloporosis. Some years, and in a total of 67 per cent of the inoculations have had little effect (e.g. trees after 13 years. During these years, 3537 -the parent tree - was found to a portion of the original scaling trees be on an off-type stock); others (e.g. showed periodic remission of symptoms 3532,3538,3539) have markedly reduced and finally developed flaky scales and tree size and total crop compared with pustules. In most remaining trees, at 32 uninoculated control trees. There has years of age, gummy pitting of the wood been little effect on fruit quality or fruit has developed; however, 15 per cent of size. Records of cropping and tree char- these trees developed no external butt acters are given in table 2. symptoms, though minor gummy pit- Inoculum from the selections with the ting occurred and all were dwarfed to best cropltree size ratios (3532, 3538, varying degrees. 3539) was used in replicated dwarfing Exororris and Gummy Piltinl: trials planted at Yanco in 1961, 1963 BEHAVIOR OF THOMPSON and 1964. GRAPEFRUIT Thompson grapefruit was introduced THE ALLEN VALENCIA ORANGE from California in 1938, as budsticks from a single-source tree, by Mr. R. G. Variation in transmission from a Benton of the Division of Horticulture, single bud source is illustrated by the N.S.W. Department of Agriculture. It performance of direct propagations of was propagated on trifoliate orange, Allen Valencia orange. Three selec- and trees were grown in several localities. tions, free of exocortis, were made from A high proportion of the first propaga- a plantation over 40 years old. Few trees tions developed severe exocortis scaling iithe block were affected with exocortis and dwarfing, but some appeared and some partially dwarfed trees were normal. Subsequent propagations were present. Plantings were made at Somers- made only of trees of good performance. by, Yanco, and Dareton in 1951, 1953, In a trial on a grower's property at 1955 and 1959. Progeny trees of each Curlwaa (Lower Murray River, N.S. W.), selection showed a range of sizes. Varia- second-generation progeny trees had tions occurred between selections in the separated into two size classes, large percentage of trees in the different size and vigorous and small and less vigor- groups, in the numbers showing gummy ous, after 7 years. The size differences pitting, and in the severity of the gummy became more pronounced with age. NO pitting (table 3). Tree size and intensity exocortis scaling had developed when bf the gummy pitting symptom were not the trees were removed after 32 years. closely correlated. One tree in the A trial at Somersby has shown that Dareton planting developed a mild the Thompson grapefruit carries exocortis reaction. gummy pitting. Lambert nucellar Eureka lemon was propagated on tri- foliate orange with Thompson grape- WATERSHOOT BUDWOOD fruit as an interstock. In this comb- A trial was initiated in 1954 to deter- ination, gummy pitting symptoms mine whether rapidly growing shoots developed in the trifoliate orange stock, could temporarily outgrow invasion by indicating that gummy pitting had been exocortis from the parent tree. Sham- introduced by the Thompson grapefruit. outi orange and mandarin trees on trifoliate orange, which showed INFLUENCE OF SCION typical severe exocortis, were cut back VARIETY ON SYMPTOMS in spring to induce strong shoot growth IN THE STOCK and, before growth had ceased and the wood hardened, buds were taken from Lemon scions induced earlier and the base to the apex and propagated on more severe gummy pitting, when com- trifoliate orange stock. Exocortis pared with grapefruit, on trifoliate developed in the stocks of the Shamouti orange stock. There is also evidence orange trees at 3 to 5 years from budding, from an old trial of a nucellar lemon on and only 4 per cent remained free at the trifoliate orange inoculated with several end of the trial after 7 years. The Clem- dwarfing selections and exocortis that entine mandarin trees remained free of the lemon scion had enhanced scaling scaling and were transferred to a per- and gummy pitting in the stock. In this manent block at Narara. No exocortis trial, the exocortis inoculum caused has developed, but differences in tree scaling in all inoculated trees, but size startedto show at 7-8 years of age. scaling was intermittent in the early Half the trees are now showing mild years, appearing in two trees at 5 years, dwarfing and mild gummy pitting has then disappearing and developing again been detected in one tree, but none has at 8 years. Gummy pitting developed in shown bark scaling. all trees. Eighth IOCV Conference

COMMERCIAL APPLICATION mal spacing, were planted at Yanco in OF DWARFING 1964. Sources of dwarfing used were exocortis and four dwarfing selections. The first experiment to determine the Yield and size of individual trees on transmissibility of the dwarfing agent, the are greater than twice planted at Somersby in 1955, prompted that on trifoliate orange. With adjust- an examination of efficiency of crop must for spacing, production is satis- production in relation to tree size (Long factory for the smaller trees. Trifoliate et al., 1972). orange stocks inoculated with exocor- Trials have been underway since 1961 tis have normally scaled at 4 to 5 years, to examine the feasibility of using, on a but no scaling has yet appeared on the commercial scale, trees which have been . Fruit size and quality are deliberately dwarfed by inoculation, unaffected by stock and dwarfing and to examine spacing and manage- inoculations. ment requirements. Close-planted, A trial comprising nucellar Bellamy dwarfed trees in trials at Yanco and navel orange on trifoliate orange stock Dareton have given higher yields per was planted at Yanco in 1966. Sources hectare in their early years than normal of dwarfing used were four lines of trees planted at normal spacing, permit- Unshiu mandarin. No dwarfing has ting considerable savings in manage- occurred to date, but cropping has ment costs by more efficient use of irri- been enhanced (Bacon and Bevington, gation, fertilizers and herbicides (Long 1977). et al., 1972; Stannard et al., 1975; Bevington and Bacon, 1977). A trial comprising nucellar Bellamy navel orange on trifoliate orange, Five trials are current: Troyer citrange, Carrizo citrange, Nucellar Washington navel orange , Cleopatra mandarin, trees on trifoliate orange were planted sweet orange, and rough lemon was at Yanco in 1961. Two sources of planted at Dareton in 1966. Two dwarfing were compared - exocor- sources of dwarfing budlines used in tis (033) and moderate dwarfing previous trials - 3539 and 3532 - (3538). Trees were inoculated in the were compared with uninoculated nursery and at 1,2,3, and 5 years. The controls. Percentage of reduction in dwarfing effect has been greatest on canopy surface area and yield has trees inoculated in the nursery, and it been 5 1, 25, 25 and 19 for trifoliate appears that tree size will stabilize in orange, Troyer and Carrizo citranges, five classes according to age at inocu- and Rangpur lime, respectively. To lation. Exocortis has caused greater date no significant difference has size reduction than has the moderate- developed in trees on sweet orange or dwarfing budline (Stannard et al., rough lemon (Bevington and Bacon, 1975). 1977). Nucellar Bellamy navel orange trees on trifoliate orange rootstock were ~lantedat Yanco in 1963. Five sources RESULTS OF INDEXING of dwarfing were compared -exocor- Seven dwarfing budlines (one Marsh tis (budline 033) and four sources of grapefruit, five Washington navel, one moderate dwarfing (budlines 353 1, Valencia orange), showed positive 3532, 3538, 3539) in high- and low- exocortis reactions on Etrog citron density plantings. Cumulative yields (Broadbent et al., 1971). One dwarfing per hectare of high-density dwarf trees budline (grapefruit) was also positive have been higher than those of normal for xyloporosis. All were negative for trees at low density. Fruit quality psorosis. All carry the strains of tristeza remains unchanged. virus normal for the variety. Most of the Nucellar and old-line Valencia orange, dwarfing budlines cause creasing of the on trifoliate orange, Troyer and Car- wood of Orlando tangelo trees, but the rizo citrange at high density and nor- cause of this is unknown. Exocortis and Gummy Pitting DISCUSSION The constant association in old com- mercial blocks of trees with exocortis scaling, pustules, dwarfing, and gummy pitting, the range of field symptoms of 0 .-5 .g+- intermediate type and the variation in 2 3.~- Q)%-o(DmQ)(D transmission from single sources suggest ",I- ,>S- br-(~(~mma, that one highly variable agent is in- 0 ?: volved in this disease complex, with g I-$ components causing the exocortis reac- u tion at one end of the spectrum and aZ u gummy pitting at the other. There is o am7oocum~ - 7 evidence of strain mixture in field trees W - I- ? and of competition between strains. 9 ", J Variable transmission of the symptoms $OOOO(DWCU from single sources and from single bud- E sticks indicate uneven distribution in I-: the tree and uneven concentration of z'=e 0 0 ~(O~OO~CU~ virus strains in different tissues within .~PE.- (U the . Budwood may carry few or ;uz .: a many components. In the inarching :_a: 2 trial, trifoliate orange inarches grafted 5 ,soV) $~NN-~(Dom onto trees with a variety of symptoms, 9o a,a as yet, have shown only the exocortis 5 C scaling reaction. aJ &(u(u~~~-cu-~ Challenge by tissue inoculation of " > a 7 P trees carrying nonscaling dwarfing, @J a 5 * with exocortis, usually results in domi- ~OOOO~T-o 2 2 Z 7 7 nance of exocortis symptoms at first, LL 0 though there are indications that with 0 w time exocortis scaling becomes less, and z flaky scaling and pustules tend to pre- J + n om: bQ)mmmCO7 dominate. A degree of competition be- 3 7 7707m 6EZ 7 m z-.C tween strains is also shown by the re- W vi sults of challenge inoculation and in the wu 5 variations in visible symptoms with I vi I- ;E time which have been a feature of the z a older trials. a .-F E, W .-= * The purification and characterization W a o, u > of the viroid causing exocortis by I- E .E E a Semancik and Weathers (1968) pointed z> 2 F the way for a further study of the disease W u m 5 5 complex. If, as seems likely, exocortis m 2 0, g 6, m er and gummy pitting are expressions of a m,--m,-ya,* 3.5 U. mmmm -mp 2 v, different strains and combinations 6,6,6,~~6, 0 .- 7777n7 2 a within a complex, it might be possible to z c- t 2: ddddgdO 0- purify a dwarfing isolate for consistency 2 gs oooo ma $&L o in performance for commercial use. 5 rrrcEr u -:E camcarnocam Sf This would also eliminate the chance of >>>>~,>0 > g z; interaction with mild, symptomless, as =U bbbb777m-Lo^cdcdcdcdcd :I 0000000 za yet undetectable, strains of other m m cucucucummm + viruses. The work of Weathers (1963) with citrus vein enation virus and citrus yellow vein virus has indicated that inter- Eighth IOCV Conference action between viruses can have far- stem pitting in trees on trifoliate orange reaching effects on the host. Such a stock infected with a mild strain of possibility has commercial significance gummy pitting. An as yet unexplained for dwarfing propagations where, of crop enhancement factor has been necessity, tissue inoculation is used. The detected, in a dwarfing trial, in trees problem could be avoided if purifica- inoculated with Unshiu mandarin bud- tion of the chosen inoculum can be wood. This factor is under further achieved. But, having accepted the study. Other viral interactions could place of purposeful virus dwarfing, perhapsimprove fruit quality or size. could there be other virus reactions or Should we be looking to a future where interactions which may be worth con- virus-free citrus varieties are inoculated sideration for future improvement of with a selection of interacting viruses to the commercial tree? A trial with produce a particular result as specified grapefruit stem-pitting strains at by the grower? Narara indicates improved control of

LITERATURE CITED BACON, P. E., and K. B. BEVINGTON 1977. Inoculation with dwarf mandarin buds increases cropping of Washington navel orange, p. 570-71. In Proc. 1977 Int. Soc. Citriculture Vol. 2. Lake Alfred. BENTON, R. J., F. T. BOWMAN, L. FRASER, and R. G. KEBBY 1950. Scaly butt and stunting of citrus. Dept. Agric. N.S.W. Sci. Bull. 70. BEVINGTON, K. B., and P. E. BACON 1977. Effect of rootstock on the response of navel orange trees to dwarfing inoculations, p. 567-70. In Proc. 1977 Int. Soc. Citriculture Vol. 2. Lake Alfred. BOWMAN, F. T. 1955. A history of citrus growing in Australia 1788-1900. Citrus News. Vol. 31 (7): 88-89; (8): 102-03; (9): 1 15; (10): 128-29, 136; (1 1): 149-5 1; (12): 165-68. BROADBENT, P., L. R. FRASER, and J. K. LONG 1971. Exocortis virus in dwarfed citrus trees. Plant Dis. Rep. 55: 998-99. FRASER, L. R. 1949. A gummosis disease of Citrus in relation to its environment. Proc. Linn. Soc. N.S.W. 74: 5-19. FRASER, L. R., P. BROADBENT, and J. E. COX 1976. Gummy pitting of Poncirus trifoliatatits association with dwarfing of citrus in New South Wales, p. 147-51. In Proc. 7th Conf. IOCV. IOCV, Riverside. FRASER, L. R., E. C. LEVITT, and J. COX 196 1. Relationship between exocortis and stunting of citrus varieties on Poncirus trifoliara rootstock, p. 34-39. In Proc. 2nd Conf. IOCV. Univ. Florida Press, Gainesville. LONG, J. K., L. R. FRASER. and J. E. COX 1972. Possible value of close-planted, virus-dwarfed orange trees, p. 262-67. In Proc. 5th Conf. IOCV. Univ. Florida Press, Gainesville. SEMANCIK, J. S., and L. G. WEATHERS 1968. Exocortis virus of citrus: Association of infectivity with nucleic acid preparations. Virology 36: 326-28. STANNARD, M. C., J. C. EVANS, and J. K. LONG 1975. Effect of transmission of exocortis dwarfing factors into Washington navel orange trees. Aust. J. Exp. Agric. Anim. Husb. 15: 136-41. WEATHERS, L. G. 1963. Use of synergy in the identification of strains of Citrus yellow vein virus. Nature 200: 812-13.