CHARLESTON – Eating cold, juicy, sweet watermelon is how many people in South Carolina survive hot summer days. But a new virus has been detected that could put a dent in watermelon and other cucurbit crop yields across the state.
South Carolina watermelon growers and growers of other cucurbit crops should regularly check their crops for the cucurbit leaf crumple virus. No treatment exists for this virus – scouting and prevention are key for managing it.
“Growers must be vigilant and check their cucurbit crops weekly to prevent this virus from taking over their crops,” said Tony Keinath, a vegetable pathologist stationed at Clemson’s Coastal Research and Education Center.
Although the virus was not seen in 2018, Keinath cautions growers to remain vigilant.
Cucurbits are plants including: melons, pumpkins, squash and cucumber, that belong to the gourd family. Cucurbit leaf crumple virus usually starts in small patches in a field with new leaves showing yellowing. In early stages of infection, the yellowing appears between the infected leaf’s veins. As the disease progresses, the yellowing of the leaves will spread to the leaf margin. Crumpling of the leaves is a sure sign the plant is infected with the leaf crumple virus. Eventually, infected plants become stunted and their leaves become unusually twisted, thick, crumpled and yellowed. Watermelon plants may seem to recover from being infected with the virus, but it is not uncommon for virus symptoms to rebound.
On other cucurbits, such as yellow squash and zucchini, symptoms include thickened and distorted leaves, which become curled and crumpled. Zucchini fruit do not show obvious symptoms, but the fruit of yellow squash become streaked with green, making them unmarketable.
This virus is spread by whiteflies. Rebecca Schmidt-Jeffris, an entomologist, said that, while whiteflies are a sporadic problem in South Carolina, especially in the Lowcountry, an outbreak of the insect in 2017 was unusually severe and widespread.
“There are no established thresholds for whiteflies in southeastern cucurbits,” Schmidt-Jeffris said. “Crops should be scouted weekly and if the number of observed whiteflies increases, a chemical treatment should be made.”
Whiteflies can be monitored via yellow sticky cards placed in fields and by checking leaves with a hand lens. Adults should be easily visible as small white insects. The juveniles (nymphs) are transparent to pale yellow and have bright red eyes. Crops experiencing substantial whitefly damage will become sticky due to the honeydew (sugary excrement) whiteflies produce. Molds can form on leaves that are covered with honeydew. The vein area on leaves where whiteflies have been feeding will turn silver, especially in squash.
Recommendations for whitefly treatment are available in the Southeastern U.S. Vegetable Crop Handbook.
“Carefully read label directions to determine what stage of whitefly the product works best against, as some only affect juveniles,” Schmidt-Jeffris said. “Make sure to rotate between mode of action (MOA) groups to prevent the development of pesticide resistance. This information can be found in the Handbook and on the pesticide label.”
To lessen the risk of attracting whiteflies to a crop, Schmidt-Jeffris said to choose a virus-resistant variety when available and confirm that any transplants are free of whiteflies and other pests before planting. She also suggests growers avoid planting near fields that are known to have whiteflies or plants with virus symptoms and use reflective mulch to decrease whitefly abundance and virus transmission.
A neonicotinoid insecticide applied via drip irrigation should be used to prevent whiteflies in areas where these insects are a regular problem. If soil applications of a neonicotinoid are made, Schmidt-Jeffris advises growers not to follow it with a foliar application of a neonicotinoid (even if it is a different product), because this increases the risk of pesticide resistance development.
To help control the leaf crumple virus on cucurbit crops in South Carolina, Keinath said growers must be prepared to scout for whiteflies early in the fall season, beginning a week after transplanting.
“Growers must be ready to treat crops for whiteflies as soon as they are found,” he said.
Growers of small crops and organic crops can cover cucurbits with row covers to exclude whiteflies for several weeks after transplanting until the plants begin to flower. Growers also should watch for whiteflies at the end of the spring season and promptly destroy crops after the last harvest so that whiteflies do not build up in advance of fall transplanting.
On a final note, Keinath advises growers not to plant cucurbits in the same fields for two consecutive years to prevent carry over and build-up of fungi and nematodes in soil that attack cucurbits. Any crop not in the cucurbit family, such as the nightshade (tomato) family, or bean family is a good rotation crop for cucurbits, he said.