Watermelon Fruit Blotch

Nancy Gregory, Plant Diagnostician; ngregory@udel.edu and Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Bacterial fruit blotch on watermelon has shown up in Delaware for the first time this year. This disease is caused by the bacterium Acidovorax avenae subsp. citrulli, which is most commonly seed-borne. Fruit blotch is favored by warm, humid conditions. The typical symptoms include a dark olive green irregular stain on the fruit. It appears water-soaked, but can be dry. The blotch will spread in size, but usually doesn’t extend down into the flesh of the fruit. Older lesions may split open, and then fruit rot can occur from the entry of other bacteria and fungi. Once fruit matures, the waxy rind prevents infection, so infections seen now probably occurred at fruit set or in early fruit development. Fields ready for harvest should be kept dry, and severely affected fruit or those with splits or wounds to the rind should be culled. Contaminated seed or seedlings are the primary cause of infection, but the bacteria can survive on crop debris and weed hosts. On Delmarva, we have observed instances where the pathogen overwintered and survived on watermelon “volunteer” plants grown in alternate, non-host years. Fields with known infections should be rotated away from cucurbits, plowed, and weeds and volunteer watermelon plants controlled. Greenhouse sanitation and clean seedlings for the next year are important. Copper or Tanos (8.0 – 10.0 oz/A) applied every seven days will suppress disease progress.

Watermelon fruit blotchWatermelon fruit blotch