Powdery mildew threatens South Carolina cucurbits

Powdery mildew is showing up on watermelon and cantaloupe plants in south Georgia and is expected to move into South Carolina, according to Anthony Keinath, Clemson University plant pathologist.

“Dry weather favors powdery mildew over other leaf diseases, because powdery mildew spores contain water that allows them to germinate on dry leaves,” said Keinath.

“All cucurbit growers should take action at this time,” he said.

Keinath recommends:

• All growers should have on hand and be ready to spray Nova, Procure, or Quintec on their watermelon, cantaloupe, and summer squash acreage.

• Growers should check 10 plants per field and 5 leaves per plant for a total of 50 leaves. Powdery mildew will most likely be found on the bottom half of the plant. Be sure to check both the upper and lower side of each leaf.

• Growers who wish to minimize the risk of yield loss to powdery mildew should make one preventative fungicide application and then scout fields regularly.

• Pristine has not prevented powdery mildew from developing in Georgia. It is possible that the powdery mildew present this year is not sensitive to strobilurin fungicides. Nova or Procure, which have the same active ingredient, should be rotated with Quintec if powdery mildew is found.

• Home gardeners can spray chlorothalonil (Daconil), sulfur, copper fungicide, or SunSpray horticultural oil to control powdery mildew.

Powdery mildew is a fungal disease that affects a wide range of plants. It is one of the easier diseases to spot, as its symptoms are quite distinctive. Infected watermelon and cantaloupe plants display white powder-like spots on the leaves. Older leaves are most affected, with mildew forming on both sides.

On summer squash, leaves and stems are affected.

Powdery mildew on the stems is more damaging than mildew on the leaves. As the disease progresses, the spots get larger and thicker as massive numbers of spores form, and the mildew spreads up and down the length of the plant. If not treated, the fruit will be smaller than normal and the plant will die.

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