Powdery mildew on watermelon leaves.
Powdery mildew on watermelon leaves.

Clemson University Extension specialist Anthony Keinath advises South Carolina watermelon growers to spray fungicide to limit loss from downy mildew, a water mold that destroys plant foliage.

Downy mildew typically doesn’t hit South Carolina watermelons until late June or July, but an outbreak was confirmed in a field in Bamberg County on Friday. Previously, the earliest confirmed case of downy mildew on watermelon in a growing season was June 18. That was in 2013.

The sighting is particularly concerning as watermelons are so young and the rainy season that tends to spread disease is approaching.

“The yield-loss potential is great on watermelon, cucumber, cantaloupe and pumpkin, and it’s because the disease destroys the foliage very quickly,” Keinath said.

Each year, South Carolina growers harvest around 7,300 acres of watermelons valued at around $27.6 million, according to the National Agricultural Statistics Service.

Downy mildew also was confirmed on pickling cucumber in Clarendon County, which is a normal occurrence at this point in the growing season.

Keinath advises all South Carolina growers of cucurbits — a family of plants that includes various melons, squashes and gourds — to apply preventative fungicides. Appropriate fungicides for downy mildew can be found in the Watermelon Spray Guide for 2015.

Downy mildew appears on watermelon as dark brown oval- or irregular-shaped spots on leaves. The nocturnal pathogen, which is dormant during the day but active as temperature cools at night, also causes watermelon leaves to curl inward.

Keinath is a professor of plant pathology at Clemson’s Coastal Research and Education Center in Charleston.