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Fruit and Vegetables |
If you work with cucurbits, you know about the disease downy mildew. Although the disease occurs every year, this year it has been especially severe on cucumber, causing approximately 40 percent yield loss. Watermelon is the only commercial cucurbit type that typically does not get this destructive disease. However, this year downy mildew has been detected on watermelon in at least three counties (Sampson, Lenoir and Bertie). While this is not a new report for North Carolina, it is fairly rare. The disease can be difficult to diagnose on watermelon, especially in the early stages. On cucumber, cantaloupe, squash and pumpkin, downy mildew causes angular, chlorotic, then water-soaked lesions, oftentimes with heavy sporulation on the leaf underside. In contrast, on watermelon, downy mildew causes a black spot and very little sporulation on the leaf underside. However, with a 20x hand lens one can see the fruiting structures of the fungus (sporangia and sporangiophores) on the leaf underside, especially in the morning when there is high moisture. One symptom that seems to be quite characteristic of the disease consists of lesions with irregularly shaped tan areas at the perimeter. This is not the most common symptom of the disease, but can usually be found if many leaves are inspected.
Downy mildew of watermelon should be controlled the same way it is controlled in other cucurbits. Several products are available and their relative efficacy can be determined by referring to Table 6-19 in the North Carolina Agricultural Chemicals Manual. A good program would alternate Bravo (or other chlorothalonil product) with Ridomil Gold Bravo, Cabrio, Tanos or Gavel on a 7-day interval. Fungicide evaluations for this disease are currently in progress and will provide additional data for next season.
Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Last modified on September 7, 2004 by Stephen J. Toth, Jr.
