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This growing season was one of the worst for watermelon diseases that I have encountered in recent years. A number of watermelon growers will certainly agree with that, especially those in south Dunklin County, who were hard hit by frequent rain at the end of June and first part of July. Even the best growers using a top notch spray program couldn't keep ahead of the diseases, with weather like that.

One disease which has been appearing more and more frequently in Southeast Missouri watermelon fields is downy mildew. This disease can affect most cucurbit crops, including watermelon, cucumber, muskmelon, squash and pumpkin.

The causal organism for downy mildew is a fungus, Pseudoperonospora cubensis. The fungus does not overwinter in Missouri, but survives in active colonies in the Gulf Coast states. That means that the spores must travel from the south to Missouri each year, on the wind. Since the spores must travel to get here, we usually don't see downy mildew early in the growing season. However this year, I did see it in one field as early as June 28th.

This means that late melons are more likely to see a problem with downy mildew compared to beginning-of-the-season melons. The past few weeks, I have been traveling further north, looking at watermelon and pumpkin fields in Mississippi, Scott, and Stoddard Counties. Since downy mildew had already developed in Dunklin County watermelons, I fully expected to find it plaguing our watermelon and pumpkin growers to the north. I did find it, and the growers are now having to adjust their fungicide program to try to control this disease.

A fairly large number of fungicides include downy mildew on their labels, but one of the better materials is Ridomil. Dr. Benny Bruton, USDA-ARS Plant Pathologist in Lane, Oklahoma, was in our area for a couple of days a few weeks ago, and identified several outbreaks of downy mildew in area watermelon fields. Dr. Bruton said that in his experience, Ridomil is the best by far for downy mildew control in cucurbit crops.

There are a lot of versions of Ridomil, and you must be careful to choose the right one. Ridomil Gold Bravo is a good material, but not for watermelons with fruit on them, since Bravo can cause sunburn problems on watermelons. Ridomil Gold Bravo is good for pumpkins, however, and area pumpkin growers are now starting to use it for downy mildew control.

Ridomil Gold MZ is a good choice for watermelons, but it is not labeled for pumpkins. I had one grower who had both watermelons and pumpkins, and wanted to spray both with the same material. So I suggested Ridomil Gold EC, which is labeled for both crops.

Area pumpkin growers will want to start an aggressive spray program, if they haven't already. We are starting to see a lot of diseases in pumpkins. A good program would include rotating between Ridomil Gold Bravo for downy mildew and black rot, Flint for powdery mildew, and finally Pristine or Maneb for broad-spectrum disease control.


		Sarah Denkler, Horticulture Educator University of Missouri Extension P.O. Box 160, Kennett, MO 63857 573-888-4722 Denklers@missouri.edu Updated 04/18/08

Southeast Region