Variation in Symptom Expression of Exocortis and Gummy Pitting in Citrus Trees on Poncirus Trifoliata Rootstock in New South Wales
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Variation in Symptom Expression of Exocortis and Gummy Pitting in Citrus Trees on Poncirus trifoliata Rootstock in New South Wales L. R. Fraser and Patricia Broadbent USAGE OF TRIFOLIATE ORANGE 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. Trifoliate orange, 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 Phytophthora spp. resulted in heavy the past. losses of trees on rough lemon 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 rough lemon 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 species 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 leaf 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 Valencia orange, dwarfed trees. These were later deter- Marsh grapefruit, and Ellendale tangor 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 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 tangelo. 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.