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Title: Evaluation of fungicide rotations for management of Phytophthora fruit rot of watermelon, 2015

item Kousik, Chandrasekar - Shaker
item Ikerd, Jennifer

Submitted to: Plant Disease Management Reports
Publication Type: Research Notes
Publication Acceptance Date: 2/10/2016
Publication Date: 5/1/2016
Citation: Kousik, C.S., Ikerd, J.L. 2016. Evaluation of fungicide rotations for management of Phytophthora fruit rot of watermelon, 2015. Plant Disease Management Reports. 10:V083.

Interpretive Summary: N/A

Technical Abstract: The experiment was conducted at the U.S. Vegetable Laboratory farm in Charleston, SC. The soil was Yonges loamy fine sand. For the past 6 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 seedless watermelon cultivar ‘Wonder’ growing in 50-cell jiffy trays were transplanted on 17 Jun onto 18-in wide raised beds. Plants of the diploid variety Mickey Lee were planted after every third Wonder plant (75:25 ratio) to serve as the pollenizer. Beds were spaced 15-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. Fungicide 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 fungicide application was made on 16 Jul when most of the watermelon fruit were about 2.5-in. in diameter. Subsequent applications of all fungicide treatments were made weekly on 23, 29 Jul and 7, 13 Aug for a total of five applications. MELCAST did not indicate a spray on 23 Jul and none was applied to those plots. Non-treated plots were considered controls. Mefenoxam sensitive and insensitive isolates of P. capsici were grown together 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 28 Jul. Total number of fruit and the number of rotten fruit in each plot were monitored throughout the season and final ratings recorded on 19 Aug were used to calculate fruit rot incidence (%). Percent fruit rot data from the field were arcsine transformed and analyzed using the proc general linear models (GLM) procedure of statistical applied systems (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 16 Jul to final rating on 19 Aug was 4.36 inches with 13 rainy days and average temperature was 80.83 °F. Significant fruit rot (63%) was observed in the non-treated control plots. Significant differences (P=<0.0001) were observed among the fungicide treatments. The standard fungicide treatment of Ridomil Gold was not as effective as some of the other treatments because of the presence of insensitive strains in the field. MELCAST did not indicate a spray on 23 Jul and none was applied. Despite one less spray, the MELCAST scheduled treatments were not significantly different from some of the other effective fungicide rotation schemes in this trial. Zampro alternated with Orondis (DPX-QGU42, Oxitharpropalin (OXTP), oxathiapiprolin), or Revus with Presidio were the most effective in managing fruit rot in this trial. The rotation scheme of two sprays of OXTP and one of Presidio was as effective as some of the other schemes, despite two less sprays, however, it was not as effective this year as observed in 2014. No phytotoxicity was observed on the plant foliage in this experiment.