Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; firstname.lastname@example.org
The fungal diseases gummy stem blight, Alternaria leaf blight, and anthracnose can be introduced into the greenhouse on watermelon seed or through inoculum from a previous crop. To minimize the occurrence of these diseases, the greenhouse should be disinfected before planting (benches, walls, walkways, etc.). The seed source should have tested negative for the pathogen with a minimum assay number of 1,000 seeds. Use clean transplant trays (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. Keep relative humidity as low as possible through proper watering and good air circulation in the greenhouse.
As the seedlings develop, inspect them carefully. Infected seedlings will have small brown lesions on the leaves and/or water-soaked lesions on the stem. Diseases that are transmitted on seed often are randomly located throughout the greenhouse. Initial infections will occur as ‘foci’ or clusters of diseased plants.
Gummy stem blight infected transplants occur as clusters in an area around the initial infected seedling (foci).
If the seedlings appear diseased, destroy all trays with symptomatic plants. Remove adjoining trays to a separate area for observation. Monitor these seedlings daily and destroy trays where symptoms develop. Do not ship any trays containing plants with disease symptoms. After symptomatic and adjoining trays are discarded, spray the remaining trays with a labeled fungicide and continue applications until plants are shipped.
Bacterial fruit blotch (BFB) of watermelon is caused by a bacterium that also is seedborne. Initial symptoms of BFB are water-soaked areas on the lower surface of the cotyledons. BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die. Angular leaf spot, which also is a bacterial disease, occurred in Delmarva’s greenhouses several years ago. Symptoms are small, dark brown, irregular lesions on cotyledons or leaves. ALB is favored by cool wet weather. Usually conditions after transplanting to the field do not favor ALB disease development.
Small inconspicuous lesion of bacterial fruit blotch
Fusarium wilt also can be seedborne. Although I have not seen Fusarium wilt infected transplants in local commercial greenhouses, it has occurred in other states. Symptoms are wilted seedlings that may remain green or become chlorotic (yellow). No lesions are observed along the stem or petiole but the vascular system is discolored and tan, pink or brown. This disease is of special concern because new strains or races can be introduced into an area on seedlings grown from infested seed.