RELATIONSHIP OF CITRUS ROOTSTOCK TO PHYTOPHTHORA ROOT ROT AND POPULATIONS OF PHYTOPHTHORA PARASITICA

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1 on (Egel et al., 99; Graham et al., 99; Graham and Gottwald, 99). In a recent outbreak, we observed substantial leafspotting and stem necrosis on 'Henderson' and 'Flame' red grapefruit budlings that were exposed to the moderately aggressive strain from adjacent rows of lightlyinfected (unpublished observations). In some cases, infected stems of grapefruit budlings were severely weakened by necrosis and broke off at the base. Currently, Florida citrus packinghouses are required to surfacedisinfest all fruit shipped from citrus canker quarantine areas in the state (Anonymous, 987). As previ ously shown with the surrogate, X. campestris pv. vesicatoria (Brown and Schubert, 987), PP added during the washing of the fruit was very effective in reducing the number of viable bacteria on the fruit surface to a low level if not in eradicating X. c. citrumelo. Brown and Schubert (987) discussed the advantages of adding the disinfectant during the washing process as the physical action of the brushes disrupt and remove surface organic matter that improves the exposure of the fruit to the disinfectant. We found that the washing alone removed over 99% of the bacteria but did not possess the eradicant action of PP. Brown and Schubert (987) also demonstrated that treat ments of PP applied during a sec period were as effective as longer exposures, and therefore would not dis rupt the orderly flow of fruit through the packinghouse. Also, PP is a proven fungicide (Eckert and Sommer, 967) and could be used for the dual purpose of decay control and bacterial eradication. Based on current knowledge, fruit treatment for X. c. citrumelo is unnecessary for several reasons. CBS has never been found on commercial citrus fruit cultivars, only on the rootstock cultivar 'Flying Dragon' trifoliate orange in a nursery (Gottwald et al., 988). Fruit cultivars develop a resistant reaction when the fruit rind is artificially inocu lated with X. c. citrumelo (Graham et al., 992). The bac terium does not survive in the lesions more than to 6 days and, therefore, are not present on the fruit at harvest (Graham et al., 992). Literature Cited Anonymous Citrus canker action plan for the state of Florida. Fla. Dept. Agr. Consumer Serv., Div. Plant Ind. and USDA/APHIS. 5 pp. Brown, G. E. and T. S. Schubert Use of Xanthomonas campestris pv. vesicatoria to evaluate surface disinfectants for canker quarantine treat ments to citrus fruit. Plant Dis. 7:92. Eckert, J. W. and N. F. Sommer Control of diseases of fruits and vegetables by postharvest treatment. Annu. Rev. Phytopathol. 5:9 2. Egel, D. S., J. H. Graham, and T. D. Riley. 99. Population dynamics of strains of Xanthomonas campestris differing in aggressiveness on and grapefruit. Phytopathology 8: Goto, M Survival of Xanthomonas citri in the bark tissues of citrus trees. Gottwald, T. R. and J. H. Graham. 99. Spatial pattern analysis of citrus bacterial spot epidemics in Florida citrus nurseries. Phytopathology 8:89. Gottwald, T. R., J. H. Graham, and S. M. Ritchie The relationship of leaf surface populations of strains of Xanthomonas campestris pv. citrumelo to development of citrus bacterial spot and persistence of disease symptoms. Phytopathology 82 (accepted). Gottwald, T. R., J. C. Miller, R. H. Brlansky, D. W. Gabriel and E. L. Civerolo Analysis of the spatial distribution of citrus bacterial spot in a Florida citrus nursery. Plant Dis. 7:297. Graham, J. H. and T. R. Gottwald. 99. Variation in aggressiveness of Xanthomonas campestris pv. citrumelo associated with citrus bacterial spot in Florida ciltrus nurseries. Phytopathology 8:996. Graham, J. H. and T. R. Gottwald. 99. Research perspectives on eradi cation of citrus bacterial diseases in Florida. Plant Dis. 75:92. Graham, J. H., T. R. Gottwald, and D. Fardelmann. 99. Cultivarspecific interactions for strains of Xanthomonas campestris from Florida that cause citrus canker and citrus bacterial spot. Plant Dis. 7: Graham, J. H., T. R. Gottwald, T. D. Riley, and M. A. Bruce Susceptibility of citrus fruit to citrus bacterial spot and citrus canker. Phytopathology 82 (in press). Muraro, R. P Potential economic benefits of defoliation vs. com plete destruction for the eradication of citrus canker infected trees. Food and Resource Economics EN, Univ. Fla., IFAS, Gainesville. Pohronezny, K., M. A. Moss, W. Dankers, and J. Schenk. 99. Dispersal and management of Xanthomonas campestris pv. vesicatoria during thin ning of directseeded tomato. Plant Dis. 7:885. Timmer, L. W Evaluation of bactericides for control of citrus canker in Argentina. Proc. Fla. State Hort. Soc. :69. Timmer, L. W., T. R. Gottwald, and S. E. Zitko. 99. Bacterial exudation from lesions of Asiatic citrus canker and citrus bacterial spot. Plant Dis. 75:9295. Proc. Fla. State Hort. Soc. : RELATIONSHIP OF CITRUS ROOTSTOCK TO PHYTOPHTHORA ROOT ROT AND POPULATIONS OF PHYTOPHTHORA PARASITICA L. W. Timmer, J. P. Agostini, J. H. Graham, Additional index words. and W. S. Castle University of Flordia, IFAS Citrus Research and Education Center 7 Experiment Station Road Lake Alfred, FL 85 Abstract. Inoculations of citrus rootstocks with chlamydospores Florida Agricultural Experiment Station Journal Series No. N72. This research was supported in part by CibaGeigy Corp., Greensboro, NC 279. We gratefully acknowledge the excellent techni cal assistance of S. E. Zitko and H. A. Sandier. Proc. Fla. State Hort. Soc. : 99. of P. parasitica in the greenhouse produced the most fibrous root rot on sweet orange (), sour orange (), Carrizo citrange (CC), and Cleopatra mandarin (CM), less on Volkamer lemon (VL) and least on trifoliate orange (TO) and (). Propagule densities from rootstock seedlings grown in pots of infested soil were greatest on and, less on CM, and least on TO and. The effects of inoculum density and metalaxyl treatment were evaluated in a pot test on,, and budded with 'Pineapple' sweet orange. Inoculation of with P. parasitica produced little root rot and had no effect on growth. Fungicide treatment did not affect growth of trees on this rootstock. On and, root rot increased and growth decreased as inoculum density increased. Metalaxyl treatment reduced root rot and increased growth of trees on these 2 rootstocks. In field rootstock trials 7

2 in Avon Park and St. Cloud, propagule densities of the fungus were highest on Palestine sweet lime,, and VL; lower on CM; and lowest on TO and. Most of the common rootstocks used in Florida with the exception of sweet orange are toler ant to bark infection; however, all except and TO are sus ceptible to fibrous root rot. Root rot, caused by Phytophthora parasitica Dast., is a common problem in Florida citrus groves and is occasion ally severe in young plantings. Most of the commonly used rootstocks in Florida are tolerant to bark infection whereas most scion varieties are moderately to highly susceptible (Castle et al., 989). Bark infection of citrus is seldom seen below the budunion except where sweet orange is used as a rootstock. The degree of susceptibility of most commer cial rootstocks to root rot is wellestablished (Castle et al., 989; Graham and Timmer, 99). Phytophthora parasitica also causes fibrous root rot which affects most citrus rootstocks. Traditionally, this has been considered primarily a problem in seedbeds and nurseries where frequent irrigation and high planting densities create favorable conditions for the disease. Fibrous root rot also occurs in bearing orchards but its importance there has been difficult to assess. Longterm fungicide treat ments have reduced Phytophthora populations, increased fibrous root densities, and in some instances, increased yield, and the size and juice content of fruit (Sandier et al., 989; Timmer et al., 989). Thus, in some situations, fibr ous root losses caused by Phytophthora may be significant. Evaluations of the susceptibility of citrus species and hybrids to fibrous root rot caused by Phytophthora have been conducted in the past (Carpenter and Furr, 962; Grimm and Hutchison, 977; Smith et al., 987; Whiteside, 97). Many of the techniques used produced severe infection which served to eliminate susceptible en tries in breeding programs; however, ratings often did not correspond well to field experience. The purpose of the studies reported herein was to as sess the susceptibility of commercial rootstocks to fibrous root rot using greenhouse, screenhouse, and field experi ments. Portions of these studies have been previously pub lished in greater detail (Agostini et al., 99; Graham, 99). Materials and Methods Greenhouse inoculation. Seedlings of the following rootstocks were grown for 6 months in soilless medium: trifoliate orange (TO) (Poncirus trifoliata (L.) Raf), Ridge Pineapple sweet orange () (Citrus sinensis (L.) Osb.), Carrizo citrange (CC) (C. sinensis x P. trifoliata), Swingle citrumelo () (C. paradisi Macf. x P. trifoliata), sour orange () (C. aurantium L.), Cleopatra mandarin (CM) (C. reticulata Blanco), and Volkamer lemon (VL) (C. volkameriana Pasq.). For inoculation, chlamydospores were produced by the method of Tsao (97) and mixed with moist, autoclaved Candler fine sand. The inoculum mix was incubated for several days and propagule densities determined by plating on the selective medium, PARPH, developed by Kannwischer and Mitchell (978) using the methods de scribed by Timmer et al. (988b). The inoculum was mixed with autoclaved Candler fine sand to achieve a density of propagules/cm. Seven seedlings of each rootstock were transplanted to infested soil in about 2.5liter pots and arranged in a ran domized block design on the greenhouse bench. Pots were flooded days each week by placing a dish under the pot and fililng it with water. After 6 weeks, root rot was rated on a scale of = no root rot to = all fibrous roots rotted. Roots were dried to a constant weight and data expressed as the ratio of root weight of inoculated seedl ings to the noninoculated controls of the same variety. Screenhouse experiment. Seedlings of TO,,, CM, and were grown for 8 months in an artificial potting mix. Nine seedlings of each rootstock were transplanted to about 5liter pots in a soil naturally infested with P. parasitica collected from a grove near St. Cloud. Nine pots of soil were prepared as unplanted controls. Rootstocks were arranged in a randomized complete block design on benches in a screenhouse and watered as needed. Soil cores were removed from the pots monthly and assayed for propagule densities on PARPH selective media (Kannwischer and Mitchell, 978; Timmer et al., 988b). Mean propagule densities were calculated over the 8 months of the experiment. At the end of the experiment, the percentage of root rot was determined by counting the number of rotted roots of 5 roots selected at random in each quadrant of each root system. Inoculum density and fungicide effects. This experiment was established to determine the effects of inoculum den sity and fungicide application on the growth of rootstocks of differing susceptibility to fibrous root rot. It was de signed with factors each at levels: rootstock sweet orange, sour orange, and ; inoculum density of Phytophthora parasitica,, and propagules per cm soil and frequency of metalaxyl application,, and 8 times per year. Infested soil was collected from a citrus grove near St. Cloud and a portion of it was autoclaved. Propagule deter minations in infested soil were made by plating on PARPH and batches of soil with the desired propagule densities prepared by mixing sterilized and infested soil. Fiftyfour uniform, small seedlings of each rootstock were selected and onethird planted in each inoculum density in 5cm diameter pots. Onethird of each group was then not treated or received metalaxyl every 6 weeks (8 times/yr) or every 2 weeks ( times/yr) using a solution of 5 mg/liter and about ml/pot. Treatments began days after the seedlings were potted. Six singleplant replicates of the in dividual treatments were used in a x x factorial ar rangement. The experiment was established in June 989. Plants with different propagule densities were placed on separate benches in a screenhouse to avoid cross contamination and rootstock and metalaxyl treatments were randomized on each bench. Pots were flooded for 2 days immediately after planting. In Apr. 99, all seedlings were budded with Tineapple' sweet orange and the seedling tops lopped off, dried, and weighed. When the experiment was terminated in Sept. 99, the percentage of rooted roots was deter mined by counting the number of tips rotted on roots per plant. Shoots and roots were collected, ovendried, and weighed. Determinations of propagule densities were made by collecting cm diameter soil cores from each pot and plating on PARPH in Nov. 989 and in Mar. and Sept. 99. Since propagule densities were still low in Mar. 7 Proc. Fla. State Hort. Soc. : 99.

3 Table. Effect of greenhouse inoculation of citrus rootstock seedlings with chlamydospores of Phytophthora parasitica on root rot severity and root growth. Rootstock Sour orange Carrizo citrange Cleopatra mandarin Sweet orange Volkamer lemon Trifoliate orange Root rot rating2 9. a 8.7 a 8. ab 7.9 ab 6. be.9 c 2. d Reduction in rootwt (%) zroot rot on a scale of = healthy to = all roots rotted; means separated by Duncan's multiple range test, P ^.5. Table 2. Propagule densities and root rot on citrus rootstock seedlings potted in a field soil naturally infested with Phytophthora parasitica. Rootstock Sweet orange Sour orange Cleopatra mandarin Trifoliate orange Unplanted control Propagules2 cm 2 a 87 a 9 b c c 7 c Root rot (%) 7 a 55 b 26 c c c zpropagule densities are the means of 7 sampling dates from June 988 to Mar. 989; mean separation in columns by Duncan's multiple range test, P <.5. 99, all plants were flooded for 2 days every 2 weeks from Apr. to Aug. 99. Field studies of rootstock effects on propagule densities. Two citrus rootstock experiments with 2yrold 'Valencia' sweet orange on various rootstocks in Avon Park and St. Cloud, Florida were selected as study sites. Both experi ments were designed as split plots with preplant fumiga tion as the main plot treatment in Avon Park and irrigation as the main plot treatment in St. Cloud. Fumigation and irrigation had only minor effects on populations of Phytophthora (Agostini et al., 99), and are not considered in this report. There were replications of the main plots with 6 and 2 trees per subplot (rootstock) in Avon Park and St. Cloud, respectively. Four soil cores were collected from each of 2 trees per replicate, the 8 cores combined, and a single determination of propagule density made for each replication in Avon Park. In St. Cloud, 8 cores were collected from each of the 2 trees per replication, compo sited, a single determination made for each tree, and the mean of the 2 trees calculated for the replicate mean. In Avon Park, rootstocks sampled were: TO,,, CM, and Palestine sweet lime (PSL) (C. limettioides Tanaka) and in St. Cloud they were: TO,, CM, PSL, and VL. Samples were collected 5 times from Mar. 988 to Feb. 989 in Avon Park and from Dec. 987 to Dec. 988 in St. Cloud. Results Greenhouse inoculation. When rootstock seedlings were inoculated with propagules/cm in the greenhouse,, CC, CM, and had the most severe root rot (Table ). All of the above suffered severe reduction in root weight compared to the noninoculated control. VL had a lower root rot rating than or CC, but still had 66% root loss. had less root rot and loss than most of the others and trifoliate orange was essentially unaffected. Screenhouse experiment. When rootstock seedlings were transplanted into naturally infested soil and grown for 9 months, and maintained the highest propagule densities and had the most root rot (Table 2). CM sup ported intermediate propagule densities and root rot was less than on or. Propagule densities were no higher on TO and than on nonplanted pots of soil. Inoculum density and fungicide effects. Conditions in this experiment were not highly favorable for disease develop ment. Propagule densities on inoculated plants not treated with fungicide were less than per cm in Nov. 989 and about per cm prior to budding in Mar. 99. Biweekly flooding after budding increased propagule densities, but densities on inoculated, nontreated plants averaged less than per cm at the end of the experiment. Neverthe less, significant treatment effects were observed in many cases. Rootstock and inoculum density significantly affected most growth variables and the percentage root rot (Table ). Metalaxyl application affected only root rot and root dry weight. Significant interactions were observed in many cases. The rootstock x inoculum density interactions were probably attributable to the fact that inoculum density af Table. Analysis of variance of the effect of rootstock, inoculum density of Phytophthora parasitica, and metalaxyl application on plant growth, root rot, and final propagule densities. Variable Factor Seedling dry wt Scion dry wt Root dry wt Root rot Final propagule density Main effects Rootstock (R) Inoculum density (I) Metalaxyl application (M) Interactions Rxl RxM IxM RxIxM _ significant atp<., <., and <.5, respectively or = not significant using analysis of variance of the x x factorial experiment. Proc. Fla. State Hort. Soc. :

4 Table. Effect of inoculum density of Phytophthora parasitica on growth of citrus rootstocks, root rot, and final propagule densities. Inoculum density (propagules/cm)z Variabley Rootstockx Seedling dry wt ns 2.6ns Scion dry wt ns ns Root dry wt ns Root rot (%) ns Final propagule density/cm ns.ns.ns zdata included in this table represents only plants not receiving metalaxyl to discern the effect of inoculum density on rootstocks in the absence of fungicide application. yseedling dry wt of tops removed after budding; all other parameters measured when experiment terminated. x = sweet orange; = sour orange; and =. wcoefficient of determination for the linear regression with inoculum density; +,, = significant at P <., P <.5, P <.; ns = not significant. fected and but had no effect on (Table ). The rot increased, but other variables were not significantly afrootstock x metalaxyl interaction likewise was attributable fected with increasing propagule densities. There was no to little effect of inoculation on and thus little curative significant effect of inoculum density on the seedlings, effect of metalaxyl on this variety (Table 5). The inoculum Propagule densities were highest on. density x metalaxyl interaction was significant because On, metalaxyl treatment increased seedling and metalaxyl had no effect on noninoculated controls root dry weight and decreased root rot but did not affect whereas it often had significant effects on inoculated other variables (Table 5). On, metalaxyl applications plants. increased scion and root weights and decreased root rot. On, seedling dry weight and root weight de On, there was no effect of metalaxyl on any variable, creased and root rot increased as inoculum density in Effect of rootstock on propagule densities in the field. In the creased (Table ). On, root weight decreased and root Avon Park and St. Cloud rootstock experiments, prop Table 5. Effect of metalaxyl treatment frequency on the growth, root rot, and final propagule densities on citrus rootstocks inoculated with Phytophthora parasitica. Metalaxyl (applications/yr)z Variabley Rootstock 8 Seedling dry wt ns.6ns Scion dry wt ns ns Root dry wt ns Root rot (%) ns Final propagule density/cm ns 7. 6.ns zdata included in this table includes only inoculated plants to discern the effect of fungicide treatment only in the presence of the pathogen. Sums of squares for plants inoculated with and propagules/cm were partitioned using orthogonal contrasts. yseedling dry wt of tops removed after budding; all other parameters measured when experiment terminated. x = sweet orange; = sour orange; and =. Coefficient of determination for the linear regression with number of fungicide application; +,,, = significant at P ^., P <.5, P ^.; ns = not significant. 76 Proc. Fla. State HorL Soc. : 99.

5 _ agule densities were highest on, PSL, and VL (Table 6). TO and especially supported only low populations in both experiments and CM supported intermediate pop ulations. Discussion Similar results were obtained in the greenhouse, screenhouse, and field evaluations of rootstock susceptibil ity to Phytophthora root rot. was quite susceptible in all tests where it was evaluated. TO and were highly resis tant. appeared more susceptible than TO in the greenhouse test (Table ), but the latter supported higher populations in one field experiment (Table 6). Inoculated did not respond to fungicide treatment (Tables 5). Results with other rootstocks were less consistent with many appearing as susceptible as. VL had a lower root rot rating than some other stocks in a greenhouse test (Table ), but supported high populations in one field test (Table 6). CM was intermediate in susceptibility in most tests, but in the field, scion cultivars on this stock are more likely to show leaf chlorosis as a result of fibrous root loss (Timmer, unpublished observations). In almost all cases, and PSL appear to be about as susceptible as to fibrous root rot. Results varied somewhat in the different tests, but with the exception of and TO, most commercial rootstocks must be considered susceptible to fibrous root rot. It will be difficult to determine with precision the susceptibility of potential new rootstocks to fibrous root rot; however, of primary importance in rootstock selection is the tolerance of the candidate to bark infection which results in collar rot and frequently in tree decline and loss. Rootstocks such as, CC, and CM which are resistant to bark infection and susceptible to root rot have been grown successfully in Florida and elsewhere (Castle et al., 989). Some yield loss may be incurred on these stocks (Sandier et al., 989; Tim mer et al., 989), but generally tree losses are not observed. Certainly rootstocks with other desirable traits should not be discarded because of their susceptibility to fibrous root rot. On the other hand, the general availability of rootstocks with a wide range of desirable traits and with the resistance of and TO to Phytophthora spp. would practically eliminate yield losses due to fibrous root rot. Rootstock is also a major consideration in decisions on whether to apply fungicides to control fibrous root loss. Sweet orange is highly susceptible to fibrous root rot and to belowground bark infection. Fungicide applications on rootstock have reduced Phytophthora populations and increased fibrous root densities (Sandier et al., 989; Tim mer et al., 989). However, fungicide applications have failed to reverse tree decline due to bark infections on scaffold roots (Timmer et al., 988a; Timmer et al., 989; Timmer, field observations). Fungicide applications to groves on rootstock may be useful where fibrous root rot is the only problem or to prevent development of bark infection, but should not be made in an attempt to reverse tree declines. Groves on and TO do not support high populations of P. parasitica, do not suffer from fibrous root rot, and should not require fungicide application for disease con trol. The greatest benefits of fungicide application should be derived on rootstocks which are tolerant to bark infec Proc. Fla. State Hort. Soc. : 99. Table 6. Mean propagule densities of Phytophthora parasitica in 2 rootstock experiments with 2yrold 'Valencia' sweet orange. Rootstock Sweet orange Palestine sweet lime Volkamer lemon Cleopatra mandarin Trifoliate orange Avon Park2.6 ax. ab 8.5 be 7. be 5. c Propagules/cm St. Cloud w 2. ax. a 7.9 b 6. b 8. c zmean of 5 sampling dates from Mar. 988 to Feb ymean of 5 sampling dates from Dec. 987 to Dec xmean separation in columns by Duncan's multiple range test, P <.5. w Rootstock not included in test. tion but susceptible to fibrous root rot. Methods for sample collection and assay of propagule densities in citrus or chards using selective media have been developed (Tim mer et al., 988b). Propagule densities of 5 per cm are considered low, 55 moderate, and above 5 high (Tim mer et al., 988a). Groves on have been encountered with average populations of 2 propagules per cm, extensive fibrous root rot, and wilt in the presence of adequate soil moisture (Timmer, unpublished data). The highest Phytophthora populations have been encountered in bedded groves with seepage irrigation. Where condi tions favor development of high Phytophthora populations on, VL, PSL, CC, CM, and other stocks of similar sus ceptibility, fungicide application may prove beneficial. Literature Cited Agostini, J. P., L. W. Timmer, W. S. Castle, and D. J. Mitchell. 99. Effect of citrus rootstocks on soil populations of Phytophthora parasitica. Plant Dis. 7:296. Castle, W. S., D. P. H. Tucker, A. H. Krezdorn, and C. O. Youtsey Rootstocks from Florida citrus. Univ. Florida Coop. Ext. Publ. SP. 7 pp. Carpenter, J. R. and J. R. Furr Evaluation of tolerance to root rot caused by Phytophthora parasitica in seedlings of citrus and related gen era. Phytopathology 52: Graham, J. H. 99. Evaluation of tolerance of citrus rootstocks to Phytophthora root rot in chlamydosporeinfested soil. Plant Dis. 7:776. Graham, J. H. and L. W. Timmer. 99. Phytophthora diseases of citrus, p In: A. N. Mukhopadhyay, U. S. Singh, H. S. Chaube, and J. Kumar (eds.). Plant diseases of international importance, Vol. III. PrenticeHall, NJ. Grimm, G. D. and D. J. Hutchison Evaluation of Citrus spp., rela tives and hybrids for resistance to Phytophthora parasitica Dastur. Proc. Int. Soc. Citriculture : Kannwischer, M. E. and D. J. Mitchell The influence of a fungicide on the epidemiology of black shank of tobacco. Phytopathology 68: Sandier, H. A., L. W. Timmer, J. H. Graham, and S. E. Zitko Effect of fungicide applications on populations of Phytophthora parasitica and on feeder root densities and fruit yields of citrus trees. Plant Dis. 7:9296. Smith, G. S., D. J. Hutchison, and C. T. Henderson Screening sweet orange cultivars for relative susceptibility to Phytophthora foot rot. Proc. Fla. State Hort. Soc. :666. Timmer, L. W., J. H. Graham, H. A. Sandier, and S. E. Zitko. 988a. Populations of Phytophthora parasitica in bearing citrus orchards in Florida and response to fungicide applications. Citrus Ind. 69( ):, 5. Timmer, L. W., H. A. Sandier, J. H. Graham, and S. E. Zitko. 988b. Sampling citrus orchards in Florida to estimate populations of Phytophthora parasitica. Phytopathology 78:99. 77

6 Timmer, L. W., H. A. Sandier, J. H. Graham, and S. E. Zitko Phytophthora feeder root rot of bearing citrus: fungicide effects on soil populations of Phytophthora parasitica and citrus tree productivity. Proc. Fla. State Hort. Soc. 2:59. Tsao, P. H. 97. Chlamydospore formation in sporangiumfree liquid cultures oi Phytophthora parasitica. Phytopathology 6:2. Whiteside, J. O. 97. Zoosporeinoculation techniques for determining the relative susceptibility of citrus rootstocks to foot rot. Plant Dis. Rptr. 58:777. Proc. Fla. State Hort. Soc. : FREQUENCY AND DISTRIBUTION OF CITRUS BLIGHT IN A TEST OF NEW HYBRID ROOTSTOCKS H. K. WUTHER United States Department of Agriculture, ARS 22 Camden Road, Orlando, FL 28 F. W. BlSTLINE Horticultural Research Department CocaCola Foods P. O. Box 68 Plymouth, FL 2768 Additional index words, citrus blight susceptibility, distribu tion, soil Ca and Mg. Abstract. The number of trees affected by citrus blight in a 'Valencia' orange rootstock test planted in 98 was recorded in April 99. The test was part of a commercial grove on the lower ridge near Sebring and consisted of four tree replica tions of trees on 9 rootstocks, 7 of them new, unnamed hyb rids. Trees on of the 9 rootstocks were included in the blight survey, a total of 76 trees, 6 trees on each rootstock. Eleven trees with visual symptoms of blight had been re moved before the survey. The remaining trees were inspected visually, and 66 trees were tested by water injection with a syringe and analysis of the trunk wood for zinc and potas sium. All trees with visual symptoms were tested; when all The technical help of Ms. Susan Chalk in collecting the data is grate fully acknowledged. trees in a tree plot were healthy, one randomly chosen tree was tested. There were more blighted trees in replications and 2 than in replications and, where soil Ca and Mg were lower. Trees on FF669, a hybrid of Christiansen trifoliate orange X Cleopatra, were most severely affected by blight (7/6 trees), followed by trees on FF5, Cleopatra X Carrizo and rough lemon 7665 (5/6 trees); FF65, Cleopatra X Swingle trifoliate orange, and citrangequat CPB 82 (/6 trees); FF57, Cleopatra X Swingle trifoliate orange (/6); FF88, Rangpur X Swingle trifoliate orange (2/6); and Carrizo and C568, Rangpur X Troyer (/6). No blighted trees were found on Flying Dragon trifoliate orange and. Citrus blight, a tree decline of unknown cause, con tinues to be Florida's and Brazil's most serious production problem (9,). Opinions are divided about the cause of the decline (,,5), and resistant rootstocks are the only effective defense. The blight tolerance of commonly used citrus rootstocks is fairly well known (6,,6). Rough and Volkamer lemon (C. limon Burm. f.) and Rangpur lime (C. reticulata hybrid) are very susceptible, Sunki mandarin (C. reticulata Blanco), sweet orange (C. sinensis L. Osbeck), sour orange (C. aurantium L.), and (C. paradisi Macf. X Poncirus trifoliata L. Raf.) are the most resistant rootstocks (5,6). Cleopatra mandarin (C. reticulata Blanco) was thought to be resistant, but recent observations show it to be susceptible, especially when the trees get older Table. Blight incidence on an elevenyearold planting of'valencia' orange trees on rootstocks. Water absorption in syringe injection, Zn and K in the outer trunk wood, trees removed before survey, number of trees tested, and total number of trees lost or affected by citrus blight, April 99.z Rootstocks Water absorption, i ml/min Blight Healthy ppm Zn Blight Healthy Blight A Healthy Trees removed before survey Trees tested affected by affected bv blight Christiansen TF x Cleopatra, FF66 Cleopatra x Carrizo, FF5 Rough lemon, 7665 Cleopatra x Swingle TF, FF65 Citrangequat, CPB82 Cleopatra x Swingle TF, FF57 Rangpur x Swingle TF, FF88 Carrizo Rangpur x Troyer, C568 Flying Dragon TF, FF a 5 b 5 b be be cd 2 de ef ef f f Means Statistical significance NS.5 ztotal trees tested: 66 (2 blight positive, 5 healthy) Total trees planted on each rootstock: 6 78 Proc. Fla. State Hort. Soc. : 99.