EXOCORTIS VIRUS AS A POSSIBLE FACTOR

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1 COHEN: EXOCORTIS VIRUS 115 tions which are much poorer than the average. Experiments such as the one described here can provide much information over a period of years. Plantings of this kind for different citrus varieties could provide a valuable fund of in formation for use in planning future groves. Acknowledgments Virus indexing done by the Citrus Budwood Registration Section of the Division of Plant In dustry was essential to this study and is very much appreciated. Willing and complete coop eration has always been offered by Mr. G. D. Bridges, Chief of the Citrus Budwood Registra tion Section, and by his predecessor Mr. G. G. Norman. LITERATURE CITED 1. Olson, E. O. and A. V. Shull Size and yield of 12-year-old 'Valencia' orange trees on various rootstocks in presence or absence of exocortis and xyloporosis viruses. J. Rio Grande Hort. Soc. 1: Sinclair, J. B. and R. T. Brown Effect of exocortis on four citrus rootstocks. Plant Disease Rptr. 44: EXOCORTIS VIRUS AS A POSSIBLE FACTOR IN PRODUCING DWARF CITRUS TREES Mortimer Cohen University of Florida Citrus Experiment Station I FAS Indian River Field Laboratory Fort Pierce Abstract Certain trees on exocortis-susceptible stocks in a rootstock experiment at Fort Pierce appear to be reduced in size as a result of infection by exocortis virus but are thrifty in appearance and bear fruit of good quality. Growth and productivity of unit grove areas theoretically planted solidly with trees affected by exocortis disease are compared in theoretical performance with unit groves planted with trees propagated from the same budwood sources on exocortistolerant rootstocks. In some situations, exocortissusceptible combinations produced more fruit per acre than combinations tolerant to exocortis. Infected exocortis-susceptible trees of this kind appear to have many of the characteristics of the type of dwarf tree considered desirable by some horticulturists. Introduction Some virus strains are latent or apparently harmless and others modify the size, shape and productivity of citrus trees in various degrees, often without drastically affecting tree longevity. Today, as the pressure of new harvesting tech- Florida Agricultural Experiment Stations Journal Series No nology and the shortage of picking labor changes our concept of what is desirable in tree size and shape, it seems necessary that we study all influences which can modify tree form. Ulti mately, the effects of virus infection on tree growth, productivity and fruit quality must be demonstrated by long-term experimentation but since knowledge of citrus viruses is so new, few such experiments have yet been reported. It is conceivable that trees with exocortis disease might have the dwarf characteristics which certain horticulturists believe could be useful in citrus culture because trees affected by exocortis disease are stunted in size but often continue to produce fruit with no appar ent tree deterioration. An experiment exploring the value of inoculation with different strains of exocortis at various stages in the growth of 'Valencia' trees on Poncirus trifoliata was begun in Australia about 191 (1). Some data on older trees with exocortis disease may be obtained from experiments set up for other purposes. Such an experiment is a rootstock trial begun by Reitz () at Fort Pierce in Description of the Rootstock Experiment The experiment at Fort Pierce involves the scions 'Valencia7 orange (Citrus sinensis) and 'Ruby' red grapefruit (Citrus paraddsi) on 2 soil types:, an acid, sandy soil with an organic hardpan at a depth of 30 inches, and Parkwood, a finer textured soil, close to neutral in ph with considerable marl 30 inches below the soil sur face. Separate plantings of each variety on each

2 11 FLORIDA STATE HORTICULTURAL SOCIETY, 198 soil type were set out in December All plantings contained trees on these rootstocks: sour orange (Citrus aurantium), rough lemon (Citrus jambhiri), 'Parson Brown' sweet orange (Citrus sinensis), Poncirus trifoliata, and '' lime (Citrus limontia). In this report, in addition to results on Poncirus and '' lime, performance of trees on sour orange and rough lemon will be described for purposes of comparison. Trees in both the 'Valencia* orange and 'Ruby' grapefruit plots on soil are planted according to a modified Latin square arrange ment. Trees are arranged in 7 pairs of replica tions, that is, 14 trees on each rootstock. On Parkwood soil, trees are arranged in a random ized block design with 3 replicated pairs of 'Ruby' grapefruit trees on each rootstock and 4 replicated pairs of 'Valencia' orange trees, except for 'Valencia' on sour which has only 3 replicated pairs. Trees on both soil types are planted in rows 30 feet apart. On soil, trees are 25 feet apart in the row, while on Parkwood soil they are planted at 20 foot intervals in the row. Reitz and Knorr 5,) have already described early observations on exocortis in the 2 plots on soil. formance to 193 of all trees on the different rootstocks in these plantings has also been described (3). 'Valencia' budwood was taken from 2 trees at Lake Alfred, Florida, but buds from both trees were used indiscriminately in the nursery. The appearance of rootstock bark scaling and stunting in some 'Valencia' trees on P. trifoliata and '' rootstocks indicated that one of the bud source trees must have been carrying a strain of exocortis virus. The smooth rootstock bark, and normal appearance of the remaining 'Valencia' trees on P. trifoliata and '' lime stocks suggested that the second bud source tree was free of exocortis. However, re indexing of such normal-looking trees, using the citron technique (2,4), demonstrates that they, and therefore the second bud source tree, carry a strain of exocortis. 'Ruby' grapefruit budwood was similarly taken from 2 trees at Lake Alfred. All grape fruit trees on Poncirus trifoliata on both and Parkwood soil are ly stunted and all show rootstock bark scaling symptoms. All grapefruit trees on '' lime are at least moderately stunted and bark scaling of rootstocks appears on most of the trees. Citron in dexing of grapefruit trees on P. trifoliata and '' rootstocks, whether or not showing bark scaling, produces strong symptoms. This is taken to mean that both of the grapefruit budwood source trees carried strains of exocortis and all experimental grapefruit trees are infected. In the 'Valencia' plantings, the number of trees on '' lime and Poncirus trifoliata carrying the strain is greater overall than the number with the strain. Table 1 provides a breakdown of 'Valencia' trees on exocortis-susceptible stocks which are carrying and strains respectively. On soil, all trees, except one on '' lime rootstock, carried the strain. Quality and yield data are not presented in this paper on the sin gle tree with exocortis. The 'Valencia' trees on Poncirus trifoliata in the Parkwood plot require special mention. All 8 trees originally were carrying the strain of exocortis. In 1957 one tree in each of the 4 pairs of P. trifoliata was inoculated with budwood from a single-tree source of the strain of exocortis. Buds were inserted into 3 branches of each inoculated tree. The inoculated trees are listed in the Tables as carrying the and strains of exocortis. By 190, all 4 inoculated trees were showing symptoms of bark scaling. The 4 uninoculated paired trees are still free of bark scaling. In the orchard, all 'Valencia' orange trees on P. trifoliata stock in the Parkwood area were normal in appear ance for some years after inoculation. Since 195, inoculated trees have shown more chlorosis, indicative of iron deficiency, than the uninocu lated trees. Of the 15 trees planted in 1950, most of the trees on P. trifoliata rootstock plus a number of others died and were replanted during the first 18 months. In the subsequent 17 years only 3 trees have died. A 'Valencia' tree on '' rootstock was killed on soil, apparently by lightning, and 2 'Ruby' grapefruit trees on P. trifoliata, ly stunted on soil, are also dead. Except for 7 trees on rough lemon rootstock, including trees in all 4 plots, which have been rendered unproductive by citrus blight, all other trees are alive. Two 'Valencia' trees on '' lime rootstock on soil are also affected by the citrus blight disease. Trees otherwise are generally uniform in size

3 COHEN: EXOCORTIS VIRUS 117 and productivity within each group. Fruit quality samples were collected each year in March or April for 'Valencia* oranges and in December for the 'Ruby' grapefruit. Tree canopy diameters were measured diagon ally across the axis of the rows. All trees are fertilized at the same rate and receive similar care. Irrigations are applied when trees approach wilting, averaging about 4 irrigations per year of trees on soil com pared to a single irrigation for trees on Parkwood soil. Results A summary of tree productivity, growth, and fruit quality is given in Tables 1 and 2. Data on 'Ruby1 grapefruit covers the time when trees first began bearing in to Data for 'Valencia' oranges cover only the period 193 to 198 so that the effect of the inocula tion of 'Valencia* trees on Poncirns trifoliata in the Parkwood soil plot can be properly assessed. Fruit quality of 'Ruby' grapefruit and 'Va lencia' oranges from exocortis-affected trees generally differed little from quality of fruit from "healthy" trees (Table 1) although 'Ruby' grapefruit from trees on P. trifoliata on soil had a lower perage of juice and smaller size than fruit from trees on other stocks. soluble solids of juice from trees on '' was substantially higher than per solu ble solids of fruit from trees on rough lemon in both grapefruit plots and in 'Valencias' on Parkwood, but was somewhat lower for juice from 'Valencias' on. TABLE 1 Average Fruit Quality in the 1950 Rootstock Experiment at Port Pierce Average Fruit Quality 1 Variety Ruby Soil Type Rootstock Exocortis Virus Strain No. Trees Of Juice Sol. Slds. Acid Ratio Sol. Slds/ Acid Wgt. Fruit (gins) Grapefruit Parkwood * * 8.1*2 9.2 l.ll* * * * ll* Ik Ik *29.2 Valencia Orange Parkwood & k k * H # * * 170o * ll* * * Oranges for the years

4 118 FLORIDA STATE HORTICULTURAL SOCIETY, 198 TABLE 2 Average Productivity and Size of Trees in the 1950 Rootstock Experiment. Productivity of a fully Planted Acre, (l) Theoretical Var. (2) Rstk. Exo(2) (2) Vlr- K ' Str. Average Tree Diameter Cumul. Cumul. of Canopy Yield Lbs. of 198 Height (Boxes) Solids (Feet) (Feet) Theoretical Fully Planted Acre (3) Tree Cumul. Cumul. Yield Lbs", of Acre (Boxes) Solids Ruby Park. RgPo U o ,432 4,43 4,178 4,150 14,221 18,52 15,231 17,285.k 1*1.1 U ^.5 I83.I ^ U ,351 5,14 4,03 4,473 9,844 23,837 17,70 19,731 Val. Park. RgPo S & M h ^ , ,922 1,7 1,778 1,2 1,99 12,43 10,14 11,153 8,427 9,377 11,14 7* , lk ^ , *l ,734 (1) Figures for Grapefruit are an averi data for the 14 years to 197-0"; for Oranges for the years I93-I! (2) See Table 1 for full titles. (3) See text for method of determining number of trees per acre. 8,942,95 13,98 12,597 12,38 In contrast to the small differences seen in fruit quality, data in Table 2 show great differ ences in average tree productivity, average can opy diameter and average height in the different categories. Trees on Parkwood were larger than those on and produced more fruit, with the exception of 'Ruby' grapefruit on '' lime where the reverse was true. The largest and most productive trees were those on rough lemon rootstock. It is obvious that more of the smaller, lowyielding trees could be planted in a given area than the larger high-yielding ones. Table 2 contains some calculations relating to theoretical plantings in which the number of trees per acre varies with the size of the trees in 198. Trees are visualized as being planted in rows in which they just touch, with an 8 foot space between rows. To calculate the total area occupied by the tree and the adja between-the-rows space, one need only obtain the quantity D2 + 8D where D is the average canopy diameter of trees on the rootstock in question. This quantity di vided into the area of an acre (43,50 sq. ft.) gives the number of trees per acre. Assuming that the average tree planted at this closer spacing would still produce the same amount of fruit as trees in the present rootstock block, we can calculate the theoretical cumulative average for a fully planted acre. Table 2 reveals that in closely spaced plant ings of this kind, much of the previously noted yield difference between stocks would disappear. 'Ruby' grapefruit plantings on '' lime would have a higher yield, despite exocortis dis ease, than plantings on any other stock. 'Va lencia* orange trees carrying the strain of exocortis on P. trifoliata would constitute the highest-yielding combination on Parkwood soil. However, other closely spaced plantings on P. trifoliata carrying exocortis, 'Valencia' on soil and 'Ruby' grapefruit on either soil type,

5 COHEN: EXOCORTIS VIRUS 119 would not come up to the production level of trees on other rootstocks. The relation of canopy diameter and tree height to theoretical productivity per acre can be checked in Table 2. In no case is the tallest or widest tree of a group also the most produc tive. It has already been noted that yield per tree in almost all categories is higher on Parkwood soil than on. Table 2 reveals that in con trast cumulative yield per acre would have been higher on soil for many combinations if trees had been set out in closely spaced plant ings. Theoretical yield per fully planted acre is higher on soil than on Parkwood for trees on rough lemon, '', and sour stocks for both 'Ruby' grapefruit and 'Valencia* oranges. Table 2 also shows that pounds of solids per acre would follow the same trend. Discussion The experiment reported here describes the performance of citrus trees on exocortis-susceptible rootstocks over an 18-year period, in com parison with others, propagated from the same exocortis-infected bud sources, on rootstocks tol erant of this virus. During this period some of the combinations with exocortis-susceptible stocks have produced reasonable yields of fruit of good quality and size and, from present ap pearance, will continue to do so. Trees on susceptible stocks in this planting were gen erally more compact than trees on exocortistolerant rootstocks. Calculations were pre sented to demonstrate that if they had been set out in a planting plan which took advantage of their compactness, some combinations would have outyielded trees on the exocortis-tolerant rootstocks by a considerable margin. These cal culations also demonstrate, regardless of exocortis disease, that trees which are smaller in can opy diameter and height often exceed larger trees in productivity per acre. Now that the need for improved efficiency of labor, improved spray coverage, and the increased use of me chanical aids for picking all point to the desir ability of dwarf trees in the grove, it is encour aging to see that reduction in tree size need not mean reduction in yield per acre. It is obviously not correct to suggest that all trees on exocortis-susceptible rootstocks would have the favorable characteristics described for some combinations in this experiment. Many strains of the exocortis virus exist. This study alone involves 3 strains and one strain of exocortis which were carried in the 4 original bud-source trees. It is likely that the 3 strains are not identical but that each influences inoculated trees somewhat differently. It is noteworthy, however, that the trees on lime and Pondrus trifoliate carrying exocortis virus which appear here to have pro duced trees meriting some commercial considera tion would not even have been tested in many parts of the world but would automatically have been rejected as diseased. More research is needed to confirm and ex pand the observations made here. Strain differ ences must be investigated. The great influence of soil type should be explored. Actual field trials must test the validity of the hypothetical calculations presented. Information should be obtained on the longevity of exocortis-affected trees, and trees free of exocortis must be studied in comparison with those carrying the virus. However, the idea that the precise influence of different strains of exocortis on the growth and productivity of citrus trees is a proper subject for serious investigation is unchallengeable. LITERATURE CITED 1. Anonymous. 19. Dwarf orange trees. The Agric. Gazette of New South Wales. 77: Calavan, E. C, et al Rapid indexing for exocortis of citrus. Phytopathology. 54: Cohen, Mortimer and Herman J. Reitz Root stocks for Valencia orange and Ruby red grapefruit: re sults of a trial initiated at Fort Pierce in 1950 on two soil types. Proc. of Florida State Hort. Soc. 7: Garnsey, S. M. and Mortimer Cohen Response of various citron selections to exocortis infection in Florida. Proc. of Florida State Hort. Soc. 78: Knorr, L. C. and H. J. Reitz Exocortis in Florida, p In James M. Wallace (Ed). Citrus Virus Diseases, proceedings of the conference on citrus virus diseases, held at Riverside, Calif. Nov , Univ. of Calif., Div. of Agric. Sciences.. Reitz, H. J. and L. C. Knorr Occurrence of lime disease in Florida and its concurrence with exocortis. Plant Disease Reporter. 41: