Managing Bacterial Fruit Blotch of Watermelon in the Greenhouse

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

As watermelon transplant production begins in greenhouses across Delmarva, there are a few practices to keep in mind to best manage BFB of watermelon and avoid its introduction into production fields. BFB is caused by the bacterium Acidovorax avenae subsp. citrulli. Symptoms of BFB on seedlings are water-soaked areas of the lower surface of the cotyledons and inconspicuous lesions on leaves. BFB lesions will become necrotic and develop yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die.

If the bacterium is present, conditions in greenhouse transplant houses are highly favorable for the development of BFB symptoms and the spread of disease. Good practices for greenhouse transplant production are to disinfect surfaces before planting (benches, walls, walkways, etc.). The seed source should have tested negative for the pathogen with a minimum assay number of 10,000 seeds. Clean transplant trays must be used (disinfect trays if they will be reused) and new soil. Destroy any volunteer seedlings and keep the area in and around the greenhouse weed free. Avoid overhead watering if at all possible, or water in the middle of the day so that the plants dry thoroughly before evening. The bacterium can spread on mist and aerosols, so keep relative humidity as low as possible through proper watering and good air circulation in the greenhouse. Separate different seedlots, to reduce lot-to-lot spread. If BFB is suspected, collect a sample and submit it to your extension educator, specialist, or UD or UMD plant diagnostician. Accurate diagnosis is especially important because there are other seedling diseases that may look similar to BFB in the greenhouse. Angular leaf spot, which is common in cantaloupes, also produces water soaked lesions. The fungal diseases gummy stem blight, Alternaria leaf blight, anthracnose, and Fusarium wilt can also be introduced into the greenhouse on watermelon seed or through inoculum from a previous crop.

 The first symptom of BFB in the greenhouse is water-soaking on the undersurface of cotyledonsThe first symptom of BFB in the greenhouse is water-soaking on the undersurface of cotyledons

Once you suspect BFB, but before it is confirmed, separate the potentially infected trays. If BFB is confirmed, destroy all trays with symptomatic plants, and the trays that were immediately adjacent to them (this should include those that were adjoining when symptoms were first observed). Remove trays that were near the destroyed trays to a separate – isolated – area for observation. Monitor these isolated seedlings daily and destroy trays where symptoms develop. The remaining trays should be sprayed with a labeled fungicide and the applications continued until the plants are transplanted to the field.

BFB in watermelon transplantsBFB in watermelon transplants

 Angular leaf spot on cantaloupe (a “look alike” disease)Angular leaf spot on cantaloupe (a “look alike” disease)