Submitted to: Plant Disease Management Reports
Publication Type: Research Notes
Publication Acceptance Date: 2/1/2015
Publication Date: 5/1/2015
Citation: Kousik, C.S., Ikerd, J.L. 2015. Evaluation of fungicide rotations and MELCAST for management of Phytophthora fruit rot of watermelon, 2014. Plant Disease Management Reports. 8:V077. (Research Report)
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, and for the past 5 years, the field has been infested with Phytophthora capsici. The experimental design was a randomized complete block with four replications. Four-week-old seedlings of the seedless watermelon cultivar ‘Wonder’ growing in 50-cell jiffy trays were transplanted on 11 June 2014 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; and 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 and the 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 17 July 2014, when most of the watermelon fruit were about 2.5-in. in diameter. Subsequent applications of all fungicide treatments were made weekly on 24, 31 July and 7, 14 August 2014 for a total of five applications. 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 29 July 2014. Total numbers of fruit and the numbers of rotted fruit in each plot were monitored throughout the season and final ratings recorded on 18 August 2014 was used to calculate fruit rot incidence. Percent fruit rot data from the field were arcsine transformed and analyzed using the proc general linear model procedure of SAS and means were separated using the Fisher’s Protected least significant difference test (a=0.05). Total rainfall from start of sprays to final rating was 11.41 inches with 17 rainy days and average temperature was 79.6 °F. Significant fruit rot (91%) was observed in the non-treated control plots. Significant differences (P=<0.0001) were observed among the fungicide treatments. All the fungicide rotation schemes significantly reduced fruit rot compared to the non-treated control. 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 indicated no spray on 31 July 2014, however it did suggest a spray on 1 Aug 2014. However, we could not get back into the field till 7 Aug 2014, due to difficult conditions for spray treatment. Despite one less spray, the MELCAST scheduled treatments were not significantly different from the other effective fungicide rotations for managing fruit rot. Zampro alternated with QGU-42, or Revus with Presidio were effective in managing fruit rot. The rotation scheme of two sprays of QGU-42 (17 Jul, 14 Aug) and one of Presidio (31 Jul) was not significantly different from other schemes, despite two less sprays. No phytotoxicity was observed on the plant foliage in this experiment.