Progress 10/01/05 to 09/30/06
Outputs
Studies were conducted during the time period to investigate the cause of unknown disease killing watermelon plants in south Florida. Watermelon production has been greatly impacted in South Florida since 2003 when symptoms were first observed of a severe watermelon vine decline (WVD) that killed plants as the crop approached first harvest. Since 2003, watermelon plants have been affected in each fall (September-December) and spring (January-May) season. Symptoms of WVD occur as fruit are approaching harvest or soon thereafter. Foliage initially turns a slight yellow, then scorched and brown, followed by wilting and collapse of the entire vine. Abiotic and biotic causes of WVD were examined. Nutritional testing indicated soil and plant tissue nutritional deficiencies in most samples collected from affected fields. However, results varied from site to site, indicating that deficiencies may only have contributed to weakening the plants, thus increasing their
susceptibility to disease. Examination of cultural practices did not uncover common links to fungicides, insecticides, herbicides, irrigation, cultivars, transplant production, fumigation, or other practices. Likewise, environmental conditions (rainfall, temperature) were variable from season to season, and while it does not appear that there is an environmental cause of this disease, the environment may play a role in WVD. Several fungi and bacteria have been recovered from affected vines and fruit which caused mortality of watermelon seedlings and decay of fruits in controlled environmental studies. However, neither fungal nor bacterial pathogens appeared to be the primary cause of WVD. Field studies conducted in the spring and fall 2006 at SWFREC showed that watermelon plants grown adjacent to SqVYV-infected squash plants developed vine decline symptoms, including fruit rot, and eventually died. Plants which were grown in the same field, but protected by screened walk-in cages to
prevent infestation by whitefly, did not develop symptoms of vine decline. Insecticide applications did not prevent vine decline on treated plants in the spring but slowed the rate of development of vine decline in the fall. Field trials were conducted in the spring of each year to test chemical, systemic acquired resistance activators, and biological control organisms to control or suppress fungal diseases on watermelon. Compounds were identified that supressed downy mildew and gummy stem disease significantly compared to untreated control and to to other treatments.
Impacts
We are examining management options in the control of vine decline on watermelon. Growers will benefit from the efficacy and timing data from the chemical and non-chemical spray evaluation trials which will allow them to make managment decisions for greater management of foliar diseases of watermelon and other cucurbits.
Publications
- Roberts, P.D., Adkins, S., Bruton, B.D.. 2006. Fungi and bacteria associated with watermelon vine decline in Florida. Phytopathology S188.
- Roberts, P. D. Systma R., and Bolick L.A.. 2006. Fungicide comparison for control of foliar diseases in watermelon, spring 2005. Fungicide and Nematicide Reports.
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Progress 10/01/04 to 09/30/05
Outputs
Since 2003, a watermelon vine decline of unknown etiology has caused widespread crop loss in South Florida. As fruit approach maturity, the foliage turns yellow, then scorched and brown, followed by collapse of the entire vine. A uniform tan to light brown discoloration of the xylem was observed. Although there are no external symptoms, fruit frequently exhibited greasy-brown blotches in the rind. Standard isolation techniques yielded numerous fungi and bacteria from crown, root, and fruit tissue. The fungi most frequently isolated were Fusarium oxysporum, Since 2003, a watermelon vine decline of unknown etiology has caused widespread crop loss in South Florida. As fruit approach maturity, the foliage turns yellow, then scorched and brown, followed by collapse of the entire vine. A uniform tan to light brown discoloration of the xylem was observed. Although there are no external symptoms, fruit frequently exhibited greasy-brown blotches in the rind. Standard isolation
techniques yielded numerous fungi and bacteria from crown, root, and fruit tissue. The fungi most frequently isolated were: Fusarium oxysporum, F. semitectum, Plectosporium tabacinum, Rhizoctonia solani, Pythium spp. and Didymella bryoniae. Race 0, 1, and 2 of Fusarium oxysporum f. sp. niveum are often isolated from symptomatic plants, but vine decline symptoms are distinct from Fusarium wilt. Some bacterial strains caused tissue maceration and/or necrosis or brown blotches similar to field symptoms on fruit. The role of fungi, bacteria, viruses and other factors continue to be investigated. Trials were conducted to evaluate products for the control of the foliar fungal pathogens downy mildew and gummy stem blight. Plants were examined at 2-week intervals for disease symptoms and rated according to a modified Horsfall-Barrett disease rating scale. Gummy stem blight and downy mildew infestation occurred naturally. Disease pressure for both downy mildew and gummy stem was severe; the
control plants were dead by the end of the season. Downy mildew and gummy stem blight infestations were from natural inoculation. The program containing Bravo Weather Stik, Topsin , Penncozeb and Pristine had plants with the lowest AUDPC for both diseases. Bravo Weather Stik, Penncozeb and Quadris Opti also conferred good control of both diseases. Other material combinations also performed very well, reducing the AUDPC significantly compared to the control. No differences in yield were detected. Only one spray program had significantly reduced non-marketable yield compared to the control.
Impacts
It is expected that identification of the causal agent of the vine decline will lead to management options for growers. Growers will benefit from the efficacy and timing data from the chemical and non-chemical spray evaluation trials which will allow them to make managment decisions for greater management of foliar diseases of watermelon and other cucurbits.
Publications
- Roberts, P.D., Urs, R.R., French-Monar, R.D., Hoffine, M.S., Seijo,T.E., and McGovern, R.J. 2005. Survival and recovery of Phytophthora capsici and oomycetes in tailwater and soil from vegetable fields in Florida. Annals of Applied Biology. 2005. 146:351-359.
- Roberts, P. D. and Kucharek, T. 2005. Florida Plant Disease Management Guide: Watermelon. University of Florida. EDIS PDMG-V3-55. 4 p.
- Roberts, Pam, Muchovej, Rosa M., Gilreath, Phyllis, McAvoy, Gene, Baker, Carlye A., and Adkins, S. 2005. Watermelon Research Update Issue: Mature-vine decline and fruit rot of watermelon. Citrus and Vegetable Magazine. 69(6):W2.
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Progress 05/17/99 to 09/30/05
Outputs
OUTPUTS: Significant progress was made in understanding the biology of significant bacterial disease and control of fungal diseases of cucurbits. Unusual blight symptoms (pitted, watersoaked lesions on fruit and leaf lesions) were observed on field-grown cantaloupe (Cucumis melo) in Florida in 2002. The disease reduced yields in affected fields and a Pseudomonas species were consistently recovered from the symptomatic fruit and leaves. Fluorescent pseudomonads were isolated from cucurbits with bacterial spot and cantaloupe blight (leaf lesions and fruit rot on cantaloupe) in commercial fields in Florida as well as from transplant seedlings in Florida and compared to strains from cucurbits isolated in Georgia. The strains were characterized and compared by bioassay and by physiological, biochemical, and molecular tests, including LOPAT and fatty acid methyl ester (FAME) analysis, rep-PCR (ERIC), hrpZ gene grouping, and the presence of the syrB gene. The causal organism was identified as Pseudomonas syringae pv. syringae. Based on PCR detection of the syrB gene, analysis of the ITS, and rep-PCR, the Florida and Georgia strains were not clonal. Variability of the strains detected by pathogenicity and molecular methods indicate that distinct strains of P. syringae pv. syringae were associated with different outbreaks on various cucurbit hosts at different times. For the fungal pathogen control, the results of several field trials indentified compounds (fungicides and biological agents) that were effective in reducing the disease severity of two fungal diseases over the course of a growing season. my program made significant contributions to the detection of Phytophthora capsici in soil and on previously unknown weed hosts. He determined that the length of survival in soil of this pathogen is much longer (27 weeks vs. 13 weeks) than previously detected. We recovered P. capsici and oomycetic pathogens from retention ponds and tailwater confirming irrigation water as a potential source of contamination from pathogens. Reports were disseminated to growers, agrichemical representatives, IR4, and a manuscript has been submitted to a refeered journal for publication. PARTICIPANTS: Nothing significant to report during this reporting period. TARGET AUDIENCES: Nothing significant to report during this reporting period. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
The findings were reported at field days, disseminated to growers via reports and on-line publications that have been used to manage these diseases. Recommendations based upon these findings were developed and disseminated to grower clients through the plant disease clinic at the Southwest Florida Research and Education Center, Florida Extension Disease Identification Clinic which processes more than 300 samples from clients per year. Because of this reserach, growers were provided with information to effectively manage fungal diseases and identify bacterial and fungal pathogens in their crops.
Publications
- French-Monar, R.D., Jones, J.B., Hanlon, E.A., and Roberts, P.D. 2005. Soil Monitoring of viable inoculum of Phytophthora capsici localized in soil under vegetable production conditions in Southwest Florida Fitopatologia 40(3): 178-187. Roberts, P.D., Urs, R.R., French-Monar, R.D., Hoffine, M.S., Seijo,T.E., and McGovern, R.J. 2005. Survival and recovery of Phytophthora capsici and oomycetes in tailwater and soil from vegetable fields in Florida. Annals of Applied Biology. 146:351-359. Urs, R.R., Mayhew, D.E., and Roberts, P.D. 2004. Hypovirulence of Didymella bryoniae associated with dsRNA. J. Phytopathology 152:55-59. Roberts, P.D., England Z. C., and Bolick, L. A. 2002. Control of downy mildew and gummy stem blight of watermelon with foliar sprays. Fungicide and Nematicide Tests 58:V034.
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Progress 10/01/03 to 09/30/04
Outputs
A severe watermelon vine decline occurred just before or soon after the first harvest in spring 2003 and 2004 in southwest Florida and in fall 2003 in west central Florida. Frequently, the interior fruit rind appeared greasy and discolored (brown) and fruit were not marketable. Disease progress was very rapid. In some fields, vine decline incidence increased from 10% to greater than 80% within a week. Entire fields were lost. Some growers experienced losses in excess of 50% of their crop. Possible environmental and biological causes of this vine decline were explored including nutrient analyses of soil and plant tissue, grower cultural practices and pathogens. Various fungi and bacteria were recovered from symptomatic crown, root, foliar, and fruit tissue. Several fungal isolates caused watermelon seedling death following inoculation in the laboratory but their role as primary factors or opportunistic secondary factors in vine decline remains under investigation.
Watermelons with decline symptoms were also examined for the presence of viruses and virus-like agents. Control of foliar diseases on watermelon with foliar sprays, Spring 2004. A field trial was conducated to evaluate the efficacy and timing of application of various type compounds to control foliar diseases of watermelon. The study was conducted at the Southwest Florida Research and Education Center located in Immokalee, FL. The soil was Immokalee fine sand (97:2.5:0.3% sand-silt-clay, pH 4.2, O.M. 1.96%). Raised beds were made on 2 Feb after fumigation with Telone C-35 applied in-bed at 35 gal/treated A. Beds were fertilized with a bottom mix of 5-17-8 at 800 lb/A and top mix applied in two bands on the bed top of 19-0-19 at 1000 lb/A. Beds were 32 in-wide with 12 ft-centers and covered with black polyethylene film. Watermelon transplants cv. `Mardi Gras' donated by Johnson Plant Inc., Immokalee, FL, were transplanted to the field on 1 Mar in a complete randomized block treatment
design with four replications. Plant spacing was 36 in between plants. Plots consisted of 10 plants each and spaced with 10-ft buffers between plots. Fields were watered by seep irrigation. Pesticides for insects and weed control were applied as needed at labeled rates and consistent with commercial production of watermelon in Southwest Florida. All compounds were applied @ 2 mph with a high clearance sprayer at 200 psi. with a side boom equipped with 2 nozzles at 33 gal/A. Plants were examined at 2-week intervals for disease symptoms and rated according to a modified Horsfall-Barrett disease rating scale. Plants were inoculated with gummy stem blight on 23 Apr and downy mildew infestation occurred naturally. Fruit were harvested on 24 May. Fruit were rated as marketable or non-marketable (small, misshapen or diseased). Disease pressure from the two diseases was severe and untreated plants were dead at the end of the trial. All treatments were highly effective to reduce downy mildew
and gummy stem severity except one treatment containing Ranman plus Dithane and Penncozeb, OmegaGrow Plus, and Seaside.
Impacts
It is expected that identification of the causal agent of the vine decline will lead to management options for growers. Growers will benefit from the efficacy and timing data which will allow them to make managment decisions for greater management of foliar diseases of watermelon and other cucurbits.
Publications
- Roberts, P.D., Systma, R., Bolick, E.A. 2004. Control of foliar diseases on watermelon with foliar sprays, Spring 2004. SWFREC Experimental Station Reports. SWFREC-IMM-2004-009.
- Urs., R.R., Mayhew, D.E., and Roberts, P.D. 2004. Hypovirulence of Didymella bryoniae associated with dsRNA. J. Phytopathology 152:55-59.
- Roberts, P.D., Muchovej, R.M., Gilreath, P.S., and McAvoy, G. Baker, C.A., Adkins, S. 2004. Mature vine Decline and fruit rot of watermelon. Citrus and Vegetable Magazine. p. 12
- Adkins, S.,Baker, C.A., Achor, D., Roberts, P.D.,Muchovej, R.M., Gilreath, P.S., and McAvoy, G. 2004. Viral Involvement in Vine Decline of Watermelon in Florida. (abstract).
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Progress 10/01/02 to 10/01/03
Outputs
A bacterial disease has been observed on transplants of cantaloupe and watermelon in Florida since 1999. Seedlings had symptoms typical of a bacterial disease. However, in spring 2003, for the first time, mature cantaloupe plants and fruit in the field in west central Florida were found with the bacterial disease. The bacteria causing the disease on cantaloupe and yellow squash in Georgia is identified as Pseudomonas syringae pv. syringae. Based on preliminary laboratory tests, the strains in Florida are also more closely identified as P. s. syringae. This disease appears to be distinct from the other bacterial diseases by the unique symptoms and bacteriological tests. Additional tests are underway to confirm relatedness of the P.s. syringae strains from these outbreaks to each other and to strains of P.s. lachrymans and P.s. syringae. The confirmation of the bacterial disease etiology was by pathogenicity and bacteriological tests. Confirmation of the causal bacteria
was by Koch postulates, in which the suspect pathogen is inoculated onto the plant, similar plant symptoms must develop, and the pathogen re-isolated to confirm the causal agent. Isolated strains caused a necrotic, hypersensitive response tobacco and were pathogenic on cucurbits and other hosts. Spray trials were conducted to test the efficacy of foliar spray compounds to control diseases on watermelon. transplants cv. Regency were transplanted to the field on 12 Mar in a complete randomized block treatment design with four replications. Plots consisted of 10 plants each and spaced with 6-ft buffers between plots. Plants were examined at 2-week intervals for disease symptoms and rated according to a modified Horsfall-Barrett disease rating scale. Gummy stem blight and downy mildew infestation occurred naturally. Fruit were harvested on 21 May. Fruit were rated as marketable or non-marketable. Disease pressure for downy mildew and gummy stem blight was severe. Plants in untreated
control plots had more than 60% combined disease severity for gummy stem blight and downy mildew. All treatments were effective in reducing the disease severity of gummy stem blight but six treatments had ratings of less than 10%. Reduction in the amount of damage due to downy mildew was significant in seven treatments. The least amount of damage due to either disease was on plants treated with experimental compounds, TD2448 and KP481 which experience practically no damage and had the high the highest mean yield. Treatments with several different types of compounds generally reduced disease severity greater than a single compound or two, probably due to presence of both gummy stem and downy mildew diseases which are generally not easily controlled by the same compounds. Treatments containing chlorothalonil, Topsin M 70WP, mancozeb, and/or strobulin formulation in rotation were generally very effective in controlling both diseases.
Impacts
It is expected that identification of the causal agent of the Pseudomonas to species and pathovar will indicate whether this is truly a new disease or an usual outbreak of a known pathogen. Management decisions may be then suggestions if the pathogen is known. If this is a new disease, then further monitoring and experiments are needed to determine its relationship to other known bacteria and what its potential ecological impact may be.
Publications
- Roberts, P. D., England, Z. C., and Bolick, E. A. 2002. Control of foliar diseases on watermelon with foliar sprays, Spring 2002. SWFREC - Immokalee: SWFREC-IMM-2002-9 2002. Pp. 1-2.
- Roberts, P. D. and Kucharek, T. 2003. Florida Plant Disease Management Guide: Watermelon. University of Florida. EDIS PDMG-V3-55. p. 1-4.
- Roberts, P. D. and Kucharek, T. 2003. Florida Plant Disease Management Guide: Squash. University of Florida. EDIS PGMG-V3-49. p. 1-3.
- Roberts, P. D. and Kucharek, T. 2003 Florida Plant Disease Management Guide: Cantaloupe. University of Florida. EDIS PGMG-V3-34. p. 1-3
- Roberts, P. D. and Kucharek, T. 2003 Florida Plant Disease Management Guide: Cucumber. University of Florida. EDIS PDMG-V3-38. p. 1-4.
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Progress 10/01/01 to 10/01/02
Outputs
Phytophthora capsici is a widespread and destructive pathogen that causes root and crown rot, fruit rot, and foliar blight on many crops and worldwide. Vegetable hosts of the pathogen include squash, watermelon, zucchini, cucumber and pepper and tomato. We established a culture collection of 33 isolates of P. capsici from South Florida that includes isolates from 1997, 1998 and 2001. The isolates were obtained infected tomato, pepper, squash, and watermelon plants or fruit. The isolates were characterized by mating type (8 A1 and 25 A2 mating types) and evaluated for their ability to cause disease on each of their respective and the other three hosts. Some isolates were highly virulent on their primary hosts and less virulent on the other ones. Other isolates proved to be highly cross-pathogenic on several hosts. P. capsici isolates were tested for sensitivity to mefenoxam (Ridomil Gold EC), the active enantioner of metalaxyl, the old chemical formulation.
Insensitivity was determined by comparing the percentage of growth at both the 10 and 100ppm levels to the control. 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 percent of the control at 10 ppm mefenoxam. Isolates collected in 1998 also were mostly sensitive (29 of 42) to mefenoxam at both concentrations however levels of insensitivity varied. Five isolates were considered to be insensitive to either concentration and 11 were considered to be moderately insensitive. Isolates that were found to be insensitive to mefenoxam were from squash (3 isolates), pepper and tomato (1 each). P. capsici isolates were recovered from irrigation water by a lemon leaves bait technique. The recovered P. capsici isolates were identified according to their microscopic morphological features, mating with authentic A1 and A2 mating types on V-8 agar and their pathogenic
potential on pepper seedlings. Recycled water can serve as a principal source as well as an efficient dissemination vehicle of inoculum, which can lead to disease epidemics in vegetables. Field trials were conducted in Spring 2002 to test 15 chemical, systemic acquired resistance activators, and biological control organisms to control or suppress disease on watermelon. Disease pressure for downy mildew and gummy stem blight was severe. Sprays were discontinued when disease severity reached 100% (plants dead) in the control and some test plots. Only two treatments significantly decreased the AUDPC of gummy stem blight by less than half of control plants. All compounds in these two treatments were used in other spray combinations that were not as effective. Similarly, several treatments controlled downy mildew at a significant level compared to the untreated control, however, many treatments contained the same compounds that did not have significance differences compared to the control.
Yields were generally unaffected with a few exceptions.
Impacts
Efficacy data is used for selection of products by growers used in disease control and data is available for for companies and grower organizations that seek registration of compounds on watermelon. Characterization of Phytophthora capsici and recovery in irrigation water will be used to describe its epidemiology and formulate control methods.
Publications
- French-Monar, R.D., Mitchell, D. J., Roberts, P.D., and Jones, J. B. 2002. Improved detection of Phytophthora capsici in a Florida pepper field. Phytopathology 92: S27
- Tohamy, A. M., Urs, R.R., and Roberts, P.D. 2002. Characterization of Phytophthora capsici in south Florida. Phytopathology 92: S81
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Progress 10/01/00 to 10/01/01
Outputs
Two field experiments examing more than 20 biological, systemic acquired resistance inducers, new chemistry fungicides and labeled fungicides were initiated in March and September 2001 to examine the efficacy of these various products to control foliar diseases of watermelon. The spring trial was sucessfully completed. Data on disease supression and yield was analyzed and several compounds were found to be effective to reduce disease severity and either increase or have no negative effect on yield. The fall trial was not completed due to flooding of the field in October which resulted in 100% plant loss.
Impacts
Information was disseminated to watermelon producers to use in selecting products to apply for disease control and data is now available for companies or organizaions (such as the Florida Fruit and Vegetable Association or IR-4) that seek registration of these compounds on watermelon in Florida.
Publications
- No publications reported this period
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Progress 10/01/99 to 09/30/00
Outputs
Three field trials were conducted at SWFREC, Immokalee on cucurbit crops in the spring and fall 2000 that utilized an integrated approach to control diseases including systemic aquired resistance inducers, biological control agents, and chemical control methods. Plots of watermelon, squash, and cantaloupe were used to test new and labeled chemicals for control of foliar fungal diseases. At least 20 chemicals were tested that reduced disease incidence by more than 50% in most trials. Hypovirulence in Didymella bryoniae, causal agent of gummy stem blight, was associated with the detection of dsRNA and is under further investigation. The prevalence of several diseases of cucurbits was recorded for southwest Florida including the identification of the pathogens that cause gummy stem blight, powdery mildew, downy mildew and watermelon fruit blotch.
Impacts
The relative efficacy of over 20 compounds was evaluated for control of diseases of cucurbits including gummy stem blight and downy mildew. Chemicals were identified that acheived good control of these important diseases of cucurbits.
Publications
- Roberts, P.D. 2000. Evaluate efficacy of test compounds for control of diseases on Watermelon. SWFREC-IMM-20009-04.
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