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P.D. Roberts, R. J. McGovern, A. Hert, C.S. Vavrina, and R.R. Urs Unprecedented outbreaks of Phytophthora blight, caused by Phytophthora capsici, occurred during the spring 1998 on a wide range of vegetable crops in Florida including tomato, cantaloupe, cucumber, eggplant, pepper, squash, and watermelon. The disease incidence, or percentage of diseased plants, in Southwest Florida was estimated at 40% in cucumbers, 25% in watermelons, and 15% each in cantaloupes and peppers. Losses in spring vegetables in 1998 in Manatee country were also high and ranged up to 31% in tomato, 65% in cantaloupe, 42% in bell and jalapeno pepper, 100% in squash, and 36% in watermelon in surveyed fields. The epidemic of Phytopthora blight on these vegetable crops was related to unusually wet and warm weather in the fall and spring of 1997-98 (McGovern et al. 1998). In 1999, substantial losses due to Phytophthora blight were reported on squash, watermelon, and pepper in South, Southwest, and the East Coast of Florida. However, these losses were confined to fewer sites than the 1998 outbreak. Less rainfall and higher temperatures in 1999 were not as conducive to Phytophthora outbreaks. However, the occurrence of outbreaks over such a widespread geographic area in 1999 indicates that P. capsici has probably already established in these areas and, given the right environmental conditions and a susceptible host, more disease problems by this pathogen can be anticipated in the future. Most of the information on the survival and spread of P. capsici is from work involving the host plant pepper (Larkin et al. 1995; Bowers and Mitchell, 1990; Bowers et al. 1990b; Ristaino, 1991). Tomato is a known host (Satour and Butler, 1967) although no information is available regarding the susceptibility of commercial tomato plants, the survival of P. capsici in tomato fields, and effective chemicals for control P. capsici. Screening for resistance of 13 tomato cultivars and species did not identify any resistance to P. capsici (Satour and Butler, 1967). The fungus is reported to survive in soil at different moisture regimes and various temperatures for at least 8 weeks in soil types of California and Maryland (Bowers et al.1990; Satour and Butler, 1967). We initiated experiments to investigate the fungus' ability to survive in the soil type and agricultural system of south Florida. Control of the disease by chemicals is difficult due to the rapid rate of development of the disease, especially under ideal environmental conditions, and potential insensitivity of fungal isolates to fungicides. A recent report by researchers found that over 50% of tested isolates of P. capsici from plants in fields in New Jersey and North Carolina were insensitive to mefenoxam (Ridomil Gold) and metalayxl (Ridomil) in laboratory screening tests (Parra and Ristiano 1998). In the outbreak in 1993 in Florida, isolates of P. capsici varied in levels of sensitivity and insensitivity to metalayxl (McGovern et al. 1993). This research was undertaken to provide information on the interaction of P. capsici and tomato. Inoculation of different tomato cultivars at different ages were performed to determine the reaction of different cultivars to infection by P. capsici and the age that tomatoes are most susceptible to infection. Soil survival studies were initiated to determine the ability of the fungus to survive in the soil type common in Southwest and West Central Florida. Screening of P. capsici isolates in vitro and in planta was performed to evaluate sensitivity to various compounds for effficacy in controlling of this disease. MATERIALS AND METHODS Evaluation of the effect of tomato cultivar and age to infection by P. capsici. Five cultivars of tomato at different ages were evaluated to examine whether plant age and cultivar affected disease development. The tomato cultivars used were: FL47, Sanibel, Sunny, FTE30, and Agriset. Transplants were produced by sowing seeds in 242- cell styrofoam trays containing Metro Mix 500. Plants were transplanted to 6" pots at age 6 weeks and fertilized with 6 g Osmocote slow release fertilizer. Fifteen plants of each cultivar were inoculated at ages 6, 8, 10, and 12-wks. Five control plants were inoculated with water at each age. Production of zoospores for inoculation of plants was by the method of Mitchell and Rayside (1986). Phytophthora capsici 99-46 isolated from tomato was used for inoculations. Zoospore concentration was adjusted to 1 x 105 /ml and 3 ml was applied to soil at plant crown. The symptoms were evaluated by measuring the lesion lengths (brown lesion extending up stem) and the percentage of girdled plants. Measurements were recorded at 10-14 days after inoculation. Survival of Phytophthora capsici in Immokalee fine sand with different moisture contents and at a field site in West Central Florida. Immokalee fine sand was collected from a field site at Southwest Florida Research and Education Center, Immokalee, FL. Immokalee find sand is 97% sand, 2.5 % silt, 0.3% clay, with an organic content of 1.96%, and pH 4.2 at a soil depth of 0-13 cm. Soil was dried in an oven set at 103 F for 10 days to remove all moisture content. .The soil was autoclaved to kill any microbes that could interfere with the survival or recovery of P. capsici from the soil. Inoculum of P. capsici was produced by growing the fungus on V-8 agar plates. After sporangial formation, ten grams of autoclaved soil was sprinkled onto each plate and the sporangia, mycelium and soil mixture was scraped from the plate and pooled. Soil was added until the final weight was 10.4 kg soil and sporangial mixture. A 400 g sample was placed into a sterile container and sterile water was added to make final concentrations of 0, 75, 90, and 100% soil moisture. Two replicates of each moisture content were placed into growth chambers set at 20, 24, and 30 C. At 10 day intervals, 10 g was plated to selective media using the agar overlay method. Colonies of P. capsici were counted after 3-days of incubation. A 70 ft-long plastic-mulch-covered bed formed of EauGallie fine sand in Bradenton, FL was examined for the survival of P. capsici in the field. The bed was previously used during the summer and fall of 1998 for a series of experiments which studied the efficacy of fungicides against the pathogen. Phytophthora blight was widespread and uniformly distributed during these experiments, the last of which was terminated on 9 Sept., 1998. On 11 Feb., 1999, ten soil samples were removed from plant hole at a depth of 4-6 in., and at equal distances over the length of the bed. Isolation of P. capsici from the soil samples used the procedures developed by Mitchell and Kannwischer-Mitchell with two modifications: 1.) the soil samples were saturated with sterile deionized water and allowed to stand and drain overnight, and 2.) a higher soil:water ratio (1:1, w/w) was used for sample dilution. Soil was plated on Phtophthora-selective medium (Jeffers and Martin, 1986). Phytophthora capsici was identified morphologically and by means of bioasssay in which yellow squash were inoculated with putative isolates. Screening of isolates for sensitivity to mefenoxam (Ridomil Gold). A total of 50 Phytophthora capsici isolates from field samples were collected from a range of hosts including tomato, cantaloupe, cucumber, eggplant, pepper, squash, watermelon, and zucchini (Table 1). Mefenoxam was added to 20 ml Corn Meal Agar (CMA) for final concentrations of 10ppm and 100ppm. Each CMA petridish was divided into four quadrants and a 5.0 mm plug taken from a three-day old culture grown on CMA was placed at the center of each quadrant. Each treatment was duplicated. Radial growth was measured after three days of incubation at 28C. Insensitivity was determined by comparing the percentage of growth at both the 10 and 100ppm levels to the control (Goodwin et al. 1996). Isolates that grew less than 40% of the control at both concentrations were considered sensitive. The isolates that grew greater than 40% of the control only at the 10 ppm level were considered to be moderately sensitive. The isolates that grew greater than 40% of the control at 100ppm were considered insensitive. RESULTS Effect of tomato cultivar and age. All five of the tomato cultivars tested were susceptible by P. capsici as evidenced by girdling and stems lesions at 6 wks of age (Figures 1, 2, and 3). Stem girdling generally preceded plant death. Tomato cultivar FL47 had the highest percentage of girdling (100%) compared to Sanibel (92%), Sunny (80%), FTE30 (73%) or Agriset (8%) at 6 weeks. Plants that were inoculated at 8 weeks after transplanting were also very susceptible to girdling and stem lesion development except Agriset. All other cultivars exhibited a decrease in symptoms at 10 and 12 weeks of 5-25% girdling or 0-5 mm stem lesion length. Agriset was symptomless when inoculated at ages 8, 10, or 12 weeks. Survival of Phytophthora capsici in Immokalee sand soil with different moisture content and at a field site in West Central Florida. Phytophthora capsici was not recovered from soil with 0% moisture at any of the three temperatures. At 75, 90, and 100% moisture, P. capsici was recovered on selective medium at all three temperatures at 10 and 20 days after inoculation (DAI) of soil. At 20 DAI, recovery of the fungus was still high at all three moistures and temperatures (Figure 4). Results from 30 and 40 DAI are pending. Phytophthora capsici was recovered at a low level (a single isolate was detected representing 2.0x10-2 colony-forming-unit/g soil) form the field 5 months following elimination of squash plants. This isolate produced typical symptoms of Phytophthora blight in squash. Interestingly, the area of the bed from which the P. capsici isolate originated was a "hot spot" of Phytophthora blight in a subsequent experiment conducted during 1999. Screening of isolates for sensitivity to mefenoxam (Ridomil Gold). Isolates from the 1993 outbreak were predominately sensitive (7 of 8) to mefenoxam. Only one isolate from 1993 was considered to be moderately insensitive and grew at greater than 40% of the control at 10 ppm mefenoxam. (Figure 5) Isolates collected in 1998 also were mostly sensitive (29 of 42) to mefenoxam at both concentrations however levels of insensitivity varied (Figure 6). Five isolates were considered to be insensitive to either concentration and 11 were considered to be moderately insensitive (Figure 7). Isolates that were found to be insensitive to mefenoxam were from squash (3 isolates), pepper and tomato (1 each). DISCUSSION The tomato cultivars tested in this study, except Agriset, were susceptible to P. capsici at 6, 8, 10, and 12 weeks of age although susceptibility decreased over time as measured by percent girdling and stem lesion length. Agriset had very little to no girdling or plant death due to the disease at age 6 wks and no symptoms were observed on older plants. From this initial data, it appears that tomato transplants are most susceptible to infections of P. capsici at 6 –8 wks of age but by 12 weeks of age are less prone to infection by P. capsici at the inoculation levels used in this study. This suggests that the physiological status of the plant at time of infection has influence over the susceptibility to infection by P. capsici. Applications of Ridomil Gold to the beds, to transplants in the greenhouse, or at planting to control damping off by Pythium sp. may also protect transplants. Chemical control for late blight caused by Phytopthora infestans may not offer any control of P. capsici because the pathogenic nature of P. capsici to root and stem infection and when it occurs in the field. Additional greenhouse and field tests are underway to confirm these results. The survival of P. capsici in soil is well documented in California and Maryland soils. The current studies are the first step in confirming that P. capsici can also survive in soil types and temperatures that are unique to Florida. Confirming the ability of P. capsici to infect tomato plants and its survival for extended periods of time is very important for strategy planning for management of second crops following tomatoes. These preliminary results and the reports of losses of tomatoes in the field suggest that tomato may act as a host to propagate inoculum that may infect subsequent crops. Other hosts, such as the cucurbits, are very susceptible to this pathogen and the best method of control would be fallow or grow non-host plants. Even though squash had been removed from the field site in West Central Florida 5 months prior to detection of P. capsici, both broad-leaf and grassy weeds had colonized the planting holes. We previously identified volunteer peppers and residual tomatoes as sources of the pathogen (McGovern et al., 1998). Infection of, or association with weeds by P. capsici may be important an over-seasoning strategy for the pathogen, and such a survival mechanism should be a high priority for future research. Insensitivity to fungicides (Ridomil, Ridomil Gold) has been reported in other Phytophthora sp. and the result is that the fungicide has limited effectiveness alone in controlling the disease (Kadish and Cohen, 1985; Goodwin et al. 1996). Several of the P. capsici isolates from the 1998 outbreak and one isolate from the 1993 exhibited insensitivity to mefenoxam, the compound that is currently used on many of the vegetable crops. The prevalence of these fungicide-insensitivity isolates and the lack of effective labeled, alternative fungicides will also adversely impact disease control strategies using chemicals. The Phytophthora blight epidemic that occurred in 1998 was geographically widespread in Florida and caused severe crop losses in many economically important vegetable crops. This disease has the potential to become a major limiting factor for production of most major vegetable crops in Florida unless good management strategies are established. MANAGEMENT Use an integrated approach for management of Phytophthora blight including the following actions:
LITERATURE CITED Bowers, J.H., and Mitchell, D.J. 1990. Effect of soil-water matric potential and periodic flooding on mortality of pepper caused by Phytophthora capsici. Phytopathology 80:1447-1450. Bowers, J.H., Papvizas, G.C., and Johnston, S.A. 1990a. Effect of soil temperature and soil-water matric potential on the survival of Phytophthora capsici in natural soil. Plant Dis. 74:771-777. Bowers, J.H., Sonoda, R.M., and Mitchell, D.J. 1990b. Patho coefficient analysis of the effect of rainfall variables on the epidemiology of Phytophthora blight of pepper caused by Phytophthora capsici. Phytopathology 80:1439-1446. Goodwin, S.B., Sujkowski, L.S., and Fry, W.E. 1996. Widespread distribution of probable origin of resistance to metalaxyl in clonal genotype of Phytophthora infestans in the United States and Western Canada. Phytopathology 86:793-800. Kadish, D., and Cohen, Y. 1985. Estimation of metalaxyl resistance in Phytophthora infestans. Phytopathology 78:915-919. Jeffers, S.N., and Martin, S.B. 1986. Comparison of two media selective for Phytophthora and Pythium species. Plant Dis. 70:1038-1043. Larkin, R.P., Gumpertz, M.L., and Ristaino, J.B. 1995. Geostatistical analyisis of Phytophthora epidemic development in commercial bell pepper fields. Phytophathology 85:191-203. McGovern, R.J., Jones, J.P., Mitchell, D.J., Pluim, R.A., and Gilreath, P.R. 1993. Severe outbreak of Phytophthora blight and fruit rot of cucurbits in Florida. Phytopathology 83:1388. McGovern, R..J., Roberts, P.D., Kucharek, T.A., and Gilreath, P.R. 1998. Phtytophthora capsici: New problems from an old enemy. pages 9-16 in 1998 Florida Tomato Institute Proceedings. Mitchell, D.J., and Kannwischer-Mitchell, M.E. 1992. Phytophthora. Pages 31-38 In:L.L. Singleton, J.D. Mihail and C.M. Rush (eds.). Methods for Research on Soilborne Phytopathogenic Fungi. APS Press, St. Paul, MN. Mitchell, D.J. and Rayside, P.A. 1986. Isolating, Identifying, and Producing Inoculum of Pythium spp. pages 67-70 in Methods for Evluating Pesticides for Control of Plant Pathogens. K.D. Hickey, Eds. APS Press. Parra, G. and Ristaino, J. 1998. Insensitivity to Ridomil Gold (mefenoxam) among field isolates of Phytophthora capsici causing Phytophthora blight on bell peppers in North Carolina and New Jersey. Plant Dis. 82:711. Ristaino, J.B. 1991. Influence of rainfall, drip irrigation, and inoculum density on the development of Phytophthora root and crown rot epidemics and yield in bell pepper. Phytopathology 81:922-929. Satour, M.M., and Bulter, E.E. 1967. A root and crown rot of tomato caused by Phtyophthora capsici and P. parasitica. Phytopathology 57:510-517. |
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