Watermelon plants showing signs of powdery mildew

Photo provided Watermelon plants found last Monday showing symptoms of powdery mildew [Click to enlarge]

Southeast Missouri watermelon growers have a good-looking crop, for the most part. But a few problems have been showing up recently, according to Tim Baker, Southeast Regional Horticulture Specialist for University of Missouri Extension.

"A grower called me and described watermelon plants with yellow and dying leaves around the crown of the plant," says Baker. "When I took a look at the field, the first thing that I thought of was powdery mildew." Early stages of powdery mildew in watermelons do not show the "powdery" appearance that you see on other plants. The leaves may simply appear to be dying, without lesions typical of other foliar diseases. While this could be a nutritional problem, there are signs to look for to see if it's powdery mildew.

Under magnification, you can often see a network of mycelium if it's powdery mildew.

In some cases, you may see a few leaves that show the powdery-like symptoms normally associated with powdery mildew. In watermelons, this usually is not seen until later stages of the disease.

After identifying the disease, Baker sent some digital photos to Dr. Benny Bruton, Plant Pathologist with the USDA Agricultural Research Service in Lane, Oklahoma. "Dr. Bruton was surprised to see powdery mildew this early in the season, but said that there was not doubt as to the diagnosis," says Baker.

Powdery mildew historically was not a major problem in watermelons until recent years.

But last year, the disease was confirmed in several Southeast Missouri watermelon fields.

Fortunately, there are fungicides available which can do a good job of controlling the disease.

Mr. Baker has seen a few other problems surfacing in area watermelon fields already this year.

"Last week, two watermelon growers called me describing a silvery-whitish cast to the crowns of the melons, that ran along the row," says Baker. "When I inspected the fields, I could see no signs of any disease that would cause a problem like this." Upon closer inspection with a hand lens, Baker found some very tiny insect larvae in relatively high numbers. These turned out to be immature thrips.

A final problem which has been surfacing in Southeast Missouri watermelon fields is spider mite damage. "This has been more widespread than powdery mildew or thrips," notes Baker. "While they have not progressed to the point where they are taking out fields, they are definitely around, sometimes in high numbers." Spider mite populations can multiply very quickly under high temperatures and dry weather conditions. "Since we have definitely had dry weather, and some periods of high temperatures, spider mite outbreaks are not totally unexpected," says Baker.

Spider mites are easily controlled using miticides such as Kelthane or Acramite. "Be sure to use a miticide," says Baker, "since these will kill only mites, and will not harm beneficial insects such as honey bees or predators which will help keep the mite populations down." Baker has put photographs of powdery mildew, thrips damage, and spider mite damage on his web site. The web site may be found at: http://outreach.missouri.edu/dunklin/hor... Baker says that he is always ready to do on-site inspections, to help growers with diseases or other problems in their crops. He encourages growers to call him at the Dunklin County Extension Center whenever they need help with their crop. The Center's phone number is 573-888-4722.

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