Posts Tagged ‘alternaria leaf blight’

MELCAST Forecasting for Bummy Stem Blight on Watermelon and Alternaria Leaf Blight on Cantaloupe

Friday, June 4th, 2010

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

Forecasts for fungicide applications in watermelon and cantaloupes for the 2010 season are set to begin on Friday, June 4. If you have not received forecasts in the past and would like to receive them, please call Mrs. Jeri Cook (410-742-8788) and give her your email address. Locations will remain the same as last year. Maryland forecasts are for Woodbine, Waldorf, Galestown and Hebron. Delaware forecasts are for Coverdale Crossroads, and both northeast and southwest of Laurel. To use MELCAST, select weather data for the site closest to your farm.

Instructions on use of MELCAST are available at

Watermelon Seedling Diseases in the Greenhouse

Thursday, April 3rd, 2008

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

 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

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.

bacterial fruit blotch

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.