Location: Vegetable ResearchTitle: Evaluation of fungicide rotations for management of Phytophthora fruit rot of watermelon, 2013
Submitted to: Plant Disease Management Reports
Publication Type: Research Notes
Publication Acceptance Date: 2/5/2014
Publication Date: 3/28/2014
Citation: Kousik, C.S., Ikerd, J.L., Harrison Jr, H.F. 2014. Evaluation of fungicide rotations for management of Phytophthora fruit rot of watermelon, 2013. Plant Disease Management Reports. 8:V256.
Interpretive Summary: N/A
Technical Abstract: The experiment was conducted at the U.S. Vegetable Laboratory farm in Charleston, SC in summer of 2013. The soil was Yonges loamy fine sand and the field has been infested with Phytophthora capsici for the previous 2 years. Five-week-old seedlings of the seedless watermelon cultivar Vanessa growing in 50-cell jiffy trays were transplanted 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. Plants were irrigated, as needed, using drip irrigation. 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 18 Jul when most of the watermelon fruit were 2.5-inches in diameter. Subsequent, applications of all fungicide treatments were made on 25 Jul, 1, 8, and 19 Aug for a total of five applications. Plots were inoculated with a mixture of mefenoxam sensitive and insensitive isolates by scattering equal amounts of infested rice grains in the plots. Total numbers of fruit and the numbers of rotted fruit in each plot were recorded on 23 Aug and used to calculate fruit rot incidence. On 24 Aug, five days after the last spray treatments, symptomless fruit were harvested from all the plots and placed on wire shelves in a sealed room. 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 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. Four 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 the area, within the lesion, with pathogen growth and sporangia was also measured. The intensity of sporulation was recorded on a 0-5 scale. The length and width of the each fruit was also recorded to determine the area of each fruit covered by lesion. All data were analyzed using SAS and means were separated using Fisher’s Protected LSD test (a=0.05). Significant fruit rot (63%) was observed in the field in non-treated control plots indicating that the trial was effective. All the fungicide treatments and or rotations significantly reduced fruit rot in the field compared to the non-treated control (Table 1). Similarly all the fungicide treatments significantly reduced post-harvest fruit rot (measured as % fruit area with rot) in the humid chamber compared to the non-treated control (Table 2). Rotation of Presidio and V-10208 was the most effective in reducing post-harvest fruit rot. All fungicide treatments also significantly reduced Phytophthora fruit rot lesion development and sporulation compared to the non-treated control.