Evaluation of watermelon varieties for tolerance to powdery mildew, 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: 2/5/2017
Publication Date: 3/12/2018
Citation: Kousik, C.S., Ikerd, J.L., Mandal, M.K. 2018. Evaluation of watermelon varieties for tolerance to powdery mildew, 2016. Plant Disease Management Reports 12:V075. https://doi.org/348569.

Interpretive Summary:

Technical Abstract: The experiment was conducted at the U.S. Vegetable Laboratory farm in Charleston, SC (32° 48' 5'N 80° 3' 50'W). The soil was Yonges loamy fine sand. The goal of this study was to determine the performance of seeded and seedless commercial watermelon varieties for tolerance to powdery mildew (PM) as it is becoming a problem across the U.S. based on recent a survey. The experimental design was a randomized complete block with three replications for each variety. Commercial watermelon varieties were seeded in 50-cell jiffy trays on 31 Mar. Seedlings were transplanted on 11 May onto raised beds on 40-inch (in) centers. Beds were spaced 15-feet (ft) apart and covered with white plastic mulch. Plants were irrigated weekly using subsurface drip irrigation using a drip tape placed 1-in below the top of the plastic mulched beds. Each variety plot was a single row of 5 plants spaced 18-in apart with 9-ft spacing between plots. Vines of the watermelon plants were turned every week so as to keep the plants from growing into the adjacent plots. Plants of USVL677-PMS (derived from PI 269677) were used as the susceptible controls for PM. A USDA, U.S. Vegetable Laboratory developed germplasm line, USVL531-MDR that is resistant to PM was used as the resistant control. After bedding but before planting, the row middles were sprayed with Roundup Pro (1 pt/A), Dual Magnum (1 pt/A) and Sandea (1 oz/A) for weed management. Weeds between beds were controlled during the season with spot application of Roundup. The ends of all the rows were planted with USVL677-PMS as an inoculum source. PM occurs naturally at this location. Plant foliage for each variety plot was rated for powdery mildew on 21 and 28 Jun, 12 and 20 Jul using a 0-10 rating scale similar to the Horsfall and Barrett rating scale of increasing disease severity (0=no visible symptoms of disease observed, 1=trace <1-3% on foliage, 2=3-6%, 3=6-12%, 5=25-50%, 7=75-87%, and 10=97-100% area of leaf covered with PM). During each rating period disease severity was recorded on lower leaves in the canopy. The underside of five lower leaves for each plot was observed to provide rating for each plot. The ratings were converted to the mid percentage points for analysis. The percentage values for individual ratings on 28 Jun and 20 Jul were arcsine transformed for analysis and means were separated using Fisher’s protected LSD (a=0.05). The percentage values are presented in the table. Area under disease progress curves (AUDPC) was calculated for each plot and means were separated using Fisher’s protected LSD (a=0.05). Total rainfall from transplanting on 11 May to the final rating on 20 Jul was 12.47 inches with 22 rainy days. The average temperature was 73 °F during this period. A significant difference (P=0.0001) in the response of watermelon varieties to PM over time was observed. The appearance of PM on these varieties was confirmed by the presence of conidia of the pathogen on the abaxial surface of the leaves. Significant disease development was observed on the susceptible USVL677-PMS and some of the commercial varieties. USVL developed germplasm line USVL531-MDR and the diploid pollenizers Lion, SP5 and SP6 were the most resistant to PM.