Effect of Fungicide Applications Scheduled to Control Gummy Stem Blight on Yield and Quality of Watermelon Fruit.

Yield and quality reductions in watermelon infected with Didymella bryoniae may be attributed to reduced number or weight of fruit, sunburned fruit, fruit rot, or low sugar content due to gummy stem blight on foliage and black rot on fruit. Number, weight, soluble solids content, and external appearance of fruit were determined in four experiments conducted in fall 1996 and 1997 and spring 1997 and 1998. Severity of gummy stem blight was varied by applying no fungicide, mancozeb, or chlorothalonil according to different schedules. In the fall, when disease severity was high, total fruit weight, percent marketable fruit, and soluble solids content were lower and percent fruit with black rot was higher in nonsprayed than in sprayed treatments. Fungicide applications did not affect total fruit weight, soluble solids content, or black rot in the spring, when disease severity was moderate to low. Percent sunburned fruit was greater in treatments sprayed every 14 days than in those sprayed weekly. In fall experiments, the number of healthy, unblemished fruit increased linearly as the number of fungicide applications was increased from zero to nine per season. Yield losses in watermelon to gummy stem blight and black rot resulted primarily from a reduction in total fruit weight and an increase in number of diseased and sunburned fruit.