DOWNY GO THE MELONS!

In BYGL 2011-16 (07/21/11), Erik Draper reported that DOWNY MILDEW had begun its annual rampage against cucumbers in Geauga County. This week Erik shared with BYGLers that this same fungus had also begun to infect and overwhelm fields of muskmelons. Pseudoperonospora cubensis, an extremely virulent fungal pathogen, rapidly infects and kills the foliage of plants in the Cucurbitaceae family. In most gardens, cucumbers and pickles are the main crop affected by this disease; however, the next most susceptible vine crop after cucumber is cantaloupe, followed by pumpkins and winter squashes and least susceptible is watermelon. The real problem with cantaloupes is the rapid death of the protective foliage which shades the developing and ripening fruit. The exposure of fruit to direct sunlight results in rapid loss of nearly mature fruit due to overheating, while sunscald severely reduces the quality of any developing fruit.

On muskmelons or cantaloupes, downy mildew is much more difficult to accurately identify; contrastingly, the leaf lesions appear nothing like the easily identifiable, checkerboard-like appearance of downy mildew on cucumber leaves. Leaf infections appear initially as yellow spots or flecks, with the chlorotic spots quickly turning necrotic and then the dead tissues begin to coalesce. At first, just the older leaves begin to appear affected; however, these leaves quickly turn yellow, followed by brown causing the leaf blade to roll upward as it dies and dries out. As the older leaves begin to collapse, the disease symptoms progressively move onto younger leaves, constantly moving out toward the ends of the vine. The leaf petiole and the vine remain untouched and green, but eventually, the entire plant collapses due to the rapid loss of the leaves. In moist, humid conditions, on the underside of yellow leaf lesions, there may appear a fungal layer of white to purplish to almost black in color.

This foliar disease can be managed but it requires a strict adherence to a fungicide application program. Unfortunately for homeowners, when downy mildew symptoms are easily recognized on the plants, it is usually too late to do anything about this disease. However, for commercial growers, if symptoms are detected early, using specific fungicides makes it possible to delay the plant's demise long enough to get the crop picked. The same fungicides which work on cucumbers are registered for use on muskmelons too. Commercial fungicide recommendations for Ohio can be found at http://vegnet.osu.edu/news/currentvn1809.htm