Gummy Stem Blight on Watermelon

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Gummy stem blight on watermelon is widespread on Delmarva now in fields that have been repeatedly cropped to watermelons and where the disease has been introduced on plant material. Those of you who have followed MELCAST (the gummy stem blight disease forecaster for watermelon) know that there have been many days where weather was excellent for disease spread. If you want to sign up to receive MELCAST forecasts please call Mrs. Jeri Cook and request to get on the list (410) 742-8788. MELCAST is delivered by email, fax or online http://mdvegdisease.umd.edu/forecasting/index.cfm. The following are guidelines that can be used to manage gummy stem blight in watermelons.

Application Timing
Apply the first fungicide spray when the vines meet in the row (when the longest runner is about 1 ½ ft long). If disease was observed on transplants prior to planting, you may need to apply the first fungicide earlier. Subsequent applications should be scheduled according to MELCAST or on a weekly basis. Using MELCAST should reduce sprays but also insure that the fungicide gets applied when it is necessary.

Application Guidelines
The fungus that causes gummy stem blight needs moisture to infect. Therefore, apply fungicides before a rain. If the fungicide application has a reasonable chance of drying it should be applied before rainfall so there is a protective barrier on the plant when the pathogen germinates. Modern fungicides are formulated to stick to leaves during rain, so reapplication immediately after a rain is not usually necessary.

Apply an Effective Fungicide
Don’t waste money and time on fungicides applied to treat diseases for which they are not effective. Know if a fungicide is effective before you apply it. The cause of gummy stem blight, Didymella bryoniae, has developed resistance to the strobilurin fungicides (eg. Quadris) and to benzimidazoles (eg. Topsin M). Resistance to Pristine has been reported in Georgia. Because of the prevalence of southern grown transplants on the Delmarva, growers should watch for possible resistance to Pristine here.

Recommended Fungicides
Under low to moderate disease pressure apply chlorothalonil (Bravo at 2-3 pt 6F/A) every 7 days. Under high disease pressure, apply chlorothalonil alternated with Pristine plus chlorothalonil. Other fungicide programs that performed well on gummy stem blight in 2008 are Folicur applied weeks 1, 2, 4, 5, and 7, alternated with chlorothalonil applied weeks 3 and 6; and Switch applied weeks 1, 2, 4, 5, and 7, alternated with chlorothalonil applied weeks 3 and 6.

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