Evaluation of watermelon varieties for tolerance to powdery mildew and Phytophthora fruit rot, 2015

Authors: Kousik, Chandrasekar - Shaker; Ikerd, Jennifer; NABDAK, MIHIR - 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. and Mandal, M.K. 2018. Evaluation of watermelon varieties for tolerance to powdery mildew and Phytophthora fruit rot, 2015. Plant Disease Management Reports 12:V076.

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 some seedless commercial watermelon varieties for tolerance to powdery mildew (PM) and Phytophthora fruit rot as both these diseases are becoming a problem in the southeast U.S. 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 1 Apr. Seedlings were transplanted on 5 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 neighboring plots. Plants of USVL677-PMS (derived from PI 269677) and the seeded cultivar Sugar Baby were used as susceptible controls for PM and Phytophthora fruit rot respectively. A USDA, U.S. Vegetable Laboratory developed germplasm line, USVL531-MDR that is resistant to PM and Phytophthora fruit rot 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 12, 18 and 26 Jun, 2 and 8 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 disease severity ratings for each plot. The ratings were converted to the mid percentage points for analysis. 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 5 May to the final rating on 8 Jul was 11.9 inches with 20 rainy days. The average temperature was 79 °F during this period. Three mature symptomless fruits were harvested from each plot on 8 Jul and placed on wire shelves in a walk-in humid chamber. Fruit were surface sterilized with 0.6% bleach and then washed with tap water. Each fruit was inoculated in the center by placing a 7-mm agar plug from a 6-day-old actively growing isolate of P. capsici on 8 Jul. The agar plug was placed on the center of the fruit with the mycelial side touching the fruit. Plugs were placed without injuring the fruit. After inoculation, high relative humidity (>95%) was maintained in the room using a humidifier. The temperature in the room was maintained at 80 °F. The room was continuously illuminated using fluorescent lamps to enhance pathogen sporulation. Five days after inoculation the lesion diameter on each fruit was measured. The agar plug was considered the center of the lesion. Similarly the diameter of area within the lesion with pathogen growth and sporangia was measured (Pathogen growth diameter). The intensity of sporulation was recorded on a 0-5 scale, where 0 = no visible sporulation, 1 = sparse sporulation, few seen next to the agar plug, 2 = some sporulation covering less than ½ the lesion area, 3 = medium sporulation coverin