Evaluation of fungicide rotations and melcast for management of phytophthora fruit rot of watermelon, 2016

Authors: Kousik, Chandrasekar - Shaker; Ikerd, Jennifer; MIHIR, MANDAL - Oak Ridge Institute For Science And Education (ORISE)

Submitted to: Plant Disease Management Reports
Publication Type: Research Notes
Publication Acceptance Date: 2/5/2018
Publication Date: 3/12/2018
Citation: Kousik, C.S., Ikerd, J.L., Mandal, M.K. 2018. Evaluation of fungicide rotations and Melcast for management of Phytophthora fruit rot of watermelon, 2016. Plant Disease Management Reports 12:V076. https://doi.org/348568.

Interpretive Summary:

Technical Abstract: The experiment was conducted at the U.S. Vegetable Laboratory farm in Charleston, SC (32° 48' 12'N 80° 4' 13'W). The soil was Yonges loamy fine sand. For the past 7 years, the field has been infested with Phytophthora capsici. The experimental design was a randomized complete block with four replications. Five-week-old seedlings of the mini seedless watermelon cultivar ‘Extazy’ growing in 50-cell jiffy trays were transplanted on 6 Jul onto 18-inch (in) wide raised beds. Plants of the diploid variety Mickey Lee were planted after every third Ecstazy plant (75:25 ratio) to serve as the pollenizer. Beds were spaced 15-feet (ft) apart and covered with silver plastic mulch. Plots were a single row of 12 plants spaced 18-in apart with 12-ft spacing between plots. Plants were irrigated as needed using drip irrigation. After bedding, but before planting, the row middles 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 applications of Roundup and hand cultivation. A week (13 Jul) after transplanting for three treatments the plants were drenched with Ridomil Gold 480 SL (1 pt/A) in 100 GPA water. Foliar fungicide spray treatments were applied using a CO2 pressurized 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 foliar fungicide application was made on 12 Aug when most of the watermelon fruit were about 2.5-in in diameter. Subsequent applications of all fungicide treatments were made on 19 and 26 Aug and 1 and 9 Sep for a total of five foliar applications. Non-treated plots were considered as controls. Three spray treatments were scheduled based on Melcast (http://melcast.ceris.purdue.edu/) at 30, 35 and 40 Environmental favorability index (EFI) based on daily weather data from Charleston. 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 to enhance disease development on 30 Aug. Total number of fruit and the number of rotten fruit in each plot were monitored throughout the season and final ratings recorded on 15 Sep were used to calculate fruit rot incidence (%). Percent fruit rot data from the field were arcsine transformed and analyzed using the proc GLM procedure of SAS (Version 9.4) and means were separated using the Fisher’s Protected LSD test (a=0.05). Total rainfall from start of sprays on 12 Aug to final rating on 15 Sep was 10.5 inches with 10 rainy days and average temperature was 72.8 °F. Total rainfall from the first drench treatment on 13 July to final rating was 12.3 inches with 17 rainy days. Significant fruit rot (91.9%) was observed in the non-treated control plots. Significant differences (P=<0.0001) were observed among the fungicide treatments. All fungicide treatments performed better than the non-treated control. Melcast scheduled spray treatments were not as effective as the regular fungicide rotation schemes which were all highly effective. There were no significant differences among the three Melcast EFI that were evaluated. The 35 and 40 EFI Melcast scheduled treatments had two less sprays and the 30 EFI had one less spray application compared to the weekly scheduled applications. However all weekly scheduled treatments significantly reduced Phytophthora fruit rot compared to Melcast scheduled treatments. The last scheduled foliar spray could not be applied due to heavy rains and standing water after hurricane Hermine. Despite the hurricane and standing water, the weekly scheduled treatments reduced fruit rot by over 70% compared to non-treated control when ratings were recorded on 15 Sep. No phytotoxicity was observed on the pla