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 watermelon varieties for tolerance to powdery mildew and Phytophthora fruit rot, 2014. Plant Disease Management Reports. 10:V082.
Interpretive Summary: N/A
Technical Abstract: This experiment was conducted at the U.S. Vegetable Laboratory farm in Charleston, SC. The soil was Yonges loamy fine sand. This study was undertaken to determine the performance of seeded and seedless commercial watermelon varieties for tolerance to powdery mildew (PM) and Phytophthora fruit rot as both of these diseases are 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 11 Apr. Seedlings were transplanted on 14 May onto raised beds with 40-in centers. Beds were spaced 15 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 regularly turned every week to keep the plants from growing into neighboring plots. Plants of PI 269677 and the seeded cultivar Sugar Baby were used as susceptible controls for PM and Phytophthora fruit rot. A USDA, U.S. Vegetable Laboratory, developed germplasm line, USVL531-MDR, 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 rows were planted with PI 269677 as an inoculum source for PM. PM occurs naturally at the USVL farm. However to enhance disease development, a suspension of 104 conidia/ml in water containing 0.02% tween 20 was sprayed on all plants on 6 Jun. Plant foliage for each variety plot was rated for powdery mildew on 20, 27 June, 2, 10 July 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). The underside (abaxial surface) 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. 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 14 May to the final rating on 10 Jul was 6.21 inches with 21 rainy days. The average temperature was 77 °F during this period. For the Phytophthora fruit rot study, three mature symptomless fruits were harvested from each plot on 9 Jul and placed on wire shelves in a walk-in humid chamber. Each fruit was inoculated in the center by placing a 7-mm agar plug from a 4-day-old actively growing isolate of P. capsici. 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 covering ½ the lesion area, 4=heavy sporulation covering ¾ of the lesion area and 5 = abundant spora