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Powdery mildew in watermelons

This growing season has surely seen its share of problems in the watermelon crop. In some fields I have seen terrible winds that damaged vines, poor pollination, fungal diseases that were difficult to control, spider mites, splitting melons, and other assorted ills. On the other hand, some fields appear to be trouble-free, and on their way to producing a good crop.

The latest problem is another fungal disease, powdery mildew. Early this past week, I received a call from a grower who reported that his melons were starting to decline. He was on a good spray program, and wasn't sure what was happening.

When I got to the field, I saw leaves which were starting to develop a pale, yellow cast. Although there were some lesions from various fungal diseases, they were not the main problem, as many of the leaves had no major symptoms at all. They were simply turning yellow, with some vines starting to die.

First, I wondered if the problem was physiological in nature, such as a nutritional problem. The grower reported that he had recently had foliar samples analyzed for nutrients, and everything looked fine. So that probably wasn't the problem.

We next dug up a few plants and looked at the roots. One plant had nematodes, but the others did not. Generally, the roots looked fine. I noted that some plants showed vascular discoloration when cutting a cross section, but other plants showing the problem did not. No evidence of fusarium wilt was found, and it really didn't look like that anyway.

I had been taking digital photos of the field, so I decided to send some of them to Dr. Benny Bruton, the plant pathologist who spoke at our annual Watermelon Grower's Meeting back in January. Dr. Bruton is with the USDA Agricultural Research Service in Lane, Oklahoma, and is a recognized authority on watermelon diseases. Dr. Bruton took a look at the photos and decided that it might be powdery mildew, but that he would need some samples to tell for sure.

In most plants, powdery mildew is very easy to diagnose, since it has a very powdery look when it infects the leaf. Dr. Bruton explained that it normally doesn't look like this at all on a watermelon leaf. You may see a little powdery substance, but more often it is simply a pale yellowing of the leaf. It needs to be examined under a microscope to tell for sure.

Historically, watermelons were not considered a high risk crop for powdery mildew. When researching additional information for this article, I ran across some Extension literature from other states which claimed that powdery mildew did not infect watermelons in their state. However, in recent years, new strains of this disease have arisen which have started causing problems in watermelon fields, especially in the Southeast.

The weather that we have been experiencing recently has been ideal for powdery mildew development. Although the fungus does not infect plants when the leaves are wet, it does develop rapidly under humid conditions. Our frequent rains have provided plenty of moisture to raise the humidity. In addition, the optimal temperatures to develop this disease are from 80 to 90 degrees, which we have been experiencing as well.

Another factor which has favored powdery mildew development is that some of the fungicides which do such a good job of controlling lesion-causing fungal diseases may not keep powdery mildew in check. Fortunately for the grower, there are several fungicides which are targeted to powdery mildew, and do an excellent job of controlling this disease.

After Dr. Bruton received the watermelon samples that I sent to him, he called me and said that he was confident that it was powdery mildew. He suggested that the grower spray immediately with Flint, a fungicide that can stop powdery mildew in its tracks. There are several good fungicides that are effective for controlling this disease, but Flint has been shown to be one of the most effective, with the ability to even "cure" the plant. He did caution that growers who use Flint should not exceed two sprays per season. There is some concern that overuse will select for resistant strains of powdery mildew, and Flint will become ineffective in controlling the disease.

If you would like to see what powdery mildew looks like in watermelons, I have put some of the photos that I took at this grower's field on my web site at: http://outreach.missouri.edu/dunklin/hort/problems.shtml. If you think that your field may have powdery mildew, or any other problem, please don't hesitate to give me a call at 573-888-4722.

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Timothy Baker is a horticulture specialist with University of Missouri Extension in Dunklin County.