Submitted to: Plant Disease Management Reports
Publication Type: Research Notes
Publication Acceptance Date: 1/30/2012
Publication Date: 6/1/2012
Citation: Kousik, C.S., Ikerd, J.L., Thies, J.A., Harrison Jr, H.F. 2012. Evaluation of Actigard and fungicides for management of Phytophthora fruit rot of watermelon, 2011.. Plant Disease Management Reports. 6:V012.
Interpretive Summary: N/A
Technical Abstract: The experiment was conducted at the U.S. Vegetable Laboratory, Charleston, SC. The soil was Yonges loamy fine sand. The field has been infested with Phytophthora capsici for the previous 2 years. The experimental design was a randomized complete block with 4 replications. Four-week old seedlings of a susceptible seedless watermelon cultivar Vanessa were grown in 50-cell jiffy trays and transplanted on 6 July 2011 onto raised beds with 40-in. centers. Plants of the diploid variety Mickey Lee were planted after every third Vanessa plant to serve as the pollenizer. Beds were spaced 21-ft apart and covered with white plastic mulch. Plots were a single row of 12 plants spaced 18-in. apart with 15-ft spacing between plots. Plants were irrigated as needed using drip irrigation. After bedding, but before planting, the middle rows were sprayed with Roundup Pro (1 pt/A) and Strategy (2 pt/A) for weed management. Weeds between beds were controlled during the season with spot application of Roundup and cultivation. Fungicide treatments were applied using a CO2 backpack sprayer equipped with 3-nozzles (flat fan, Teejet 8002VS) spaced 19-in apart on a hand-held boom calibrated to deliver 31 gal/A. The first fungicide application was made on 5 August 2011, when most of the watermelon fruit were about 3 inches in diameter. Subsequent applications of all fungicide treatments were made on 16, 23, and 30 August 2011. Mefenoxam sensitive and insensitive isolates of P. capsici were grown separately on rice grains soaked in V8 juice in Mason jars. Plots were inoculated with a mixture of isolates by scattering equal amounts of infested rice grains in the plots on 18 August 2011. Total fruit and number of rotted fruits per plot were recorded on 30 August 2011 and used to calculate fruit rot incidence. Percent fruit rot data from the plots were arcsine transformed and were analyzed using the proc GLM procedure of SAS and means were separated using the Fisher’s protected LSD (a=0.05). Total rainfall from the time of transplanting to final rating was 11.45-in. and rainfall from the start of spray treatments on 5 August 2011 to final rating on 30 August 2011 was 5.95-in. On 2 Sepember 2011, 4 days after the last spray treatments, symptomless fruit were harvested from all the plots and placed on wire shelves in a sealed room. Five fruits were harvested from each fungicide treated plot and 2-3 from the non-treated control plots. In addition, non-treated fruits of the same age grown in a P. capsici area were also used as controls. Each fruit was inoculated in the center by placing a 7-mm agar plug from a 3-day old actively growing isolate of P. capsici. The agar plug was placed on the surface of the fruit without injuring the fruit. The isolate used was insensitive to mefenoxam. After inoculation, high-relative humidity (>95%) was maintained in the room using a humidifier and the temperature was maintained at 80 ºF. Five days after inoculation, the diameter of the lesion on each fruit was measured. The agar plug was considered the center of the lesion for measuring the diameter. The diameter of area within the lesion with sporangia was also measured. The intensity of sporulation was recorded on a 0-5 scale, where 0 = no visible sporulation, 1 = sparse sporulation, few seen next to the agar plug, 2 = some sporulation covering less than ½ the lesion area, 3 = medium sporulation covering ½ the lesion area, 4 = heavy sporulation covering ¾ of the lesion area and 5 = abundant sporangia covering >80% of the entire lesion area. The length and width of the each fruit was also recorded to determine the area of each fruit covered by lesion. Data were analyzed using SAS and means were separated using Fisher’s protected LSD (a=0.05). Significant fruit rot was observed in the field on non-treated control plots indicating that the trial was e