They might not be as prevalent or require quite as many in-season treatments, but all the foliar diseases that afflict peanuts in the Southeast threaten West Texas production.
“We have them all in West Texas,” says Jason Woodward, Extension plant pathologist at Lubbock. “Early leaf spot is the most prevalent,” Woodward told a group of peanut farmers at a production seminar recently in Littlefield. He said late leaf spot, web blight, pepper spot and leaf scorch, along with a handful of soil-borne diseases, also may cause problems for West Texas peanut farmers.
“All the foliar diseases display similar symptoms,” he said, “resulting in premature defoliation and reduced yield if not managed. Spanish and Valencia peanuts are more susceptible than runners and Virginias.”
Woodward said early leaf spot displays a yellow halo around lesions on the leaf surface. Spores appear on the top of the leaf and are transparent. Late leaf spot has dark brown to black lesions, without a halo, and spores form on the bottom of the leaf.
“Late leaf spot is more aggressive than early leaf spot but management options are the same; however, growers may need to be more timely with applications for late leaf spot.”
Web blotch displays black, necrotic, web-like lesions on the upper leaf surface and does not readily produce spores. “Spanish and Valencia peanuts are more susceptible than Virginia and runner types,” Woodward said.
Pepper spot and leaf scorch are caused by the same fungus, but display different symptoms. Pepper spot shows small, black flecks that look like black pepper. “Premature defoliation is possible and symptoms may resemble late leaf spot.”
Leaf scorch typically exhibits a V-shaped lesion, and is less severe than pepper spot.
“The good news is that some of the fungicides growers would use for leaf spot also work for web blotch and pepper spot. We have several products that fit nicely in a preventive management program,” Woodward said. “Some of these also work on soilborne fungi.”
He listed the following fungicides and the foliar diseases they control.
Woodward said timing is critical for adequate control. “Fungicide timing is weather dependent and is different on the Caprock than off. He typically recommends three applications a season for irrigated peanuts on the Caprock. “If it's dry, one or two treatments may be adequate. If it's a wet season, growers may need three or four.”
He recommends the initial treatment in mid to early August. With pepper spot he recommends a 14- to 21-day interval within the label restrictions regarding harvest.
“Fungicides have different modes of action and we recommend rotating chemical classes to avoid resistance. We have products with broad spectrum activity.” Woodward said applications on Spanish and Valencia peanuts, off the Caprock, should begin 35 to 40 days after planting. For runners and Virginias, 55 days after planting may be adequate.”
West Texas peanut farmers also have soilborne diseases including Southern blight, limb rot (Rhizoctonia), Sclerotinia blight, Botrytis blight, Rhizoctonia pod rot, Pythium pod rot, black hull, and Verticillium wilt.
Southern blight appears as a white fungal growth and produces brown, round structures. Fungicide options include Abound, Artisan, Moncut, Provost, Folicur and several others.
“This fungus is viable in the soil for three to five years so rotation is critical,” Woodward said.
Limb rot is characterized by necrotic, brown leaves and bull's-eye-shaped lesions. Fungicide options include Abound, Artisan, Moncut, Folicur, Provost, and others.
Sclerotinia blight may exhibit a white, cottony growth, bleaching and shredding of stems and small, angular black sclerotia. Woodward says Endura and Omega are control options. “A soil fumigant such as metam sodium would be a last resort.”
Botrytis blight “resembles Sclerotinia with large, black, irregular-shaped sclerotia. Gray mold may be present under optimum conditions. We have no fungicides labeled for Botrytis blight in peanuts,” Woodward said. “We are researching possibilities.”
Rhizoctonia pod rot is the number one soilborne disease in West Texas, he said. It shows brown discoloration of the pods. “This is more of a dry rot and shows a skeleton-like appearance.” Fungicides options include Abound, Moncut, Artisan and Provost.
Pythium pod rot resembles Rhizoctonia, but has more of wet appearance. Management options are limited to Abound and Ridomil Gold. “Pythium and Rhizoctonia often show up in the same fields,” Woodward said.
Black hull is another fungal pod rot, which produces shallow lesions. “Scratching the lesions shows healthy white pods underneath,” Woodward said. “It's primarily an aesthetic problem, and can be a serious problem for producers growing Valencias. We have no fungicides available for management of black hull.”
Verticillium wilt is increasing in West Texas fields, he said. Symptoms include necrosis or chlorosis on the leaves, an overall decline of plants and some vascular discoloration. “We have no fungicides labeled for peanuts and we need more information on how cultivars perform.”
In 2007, differences in Verticillium wilt incidence were observed among 10 cultivars evaluated, and continued testing will take place in 2008.
Woodward said 2007 fungicide trials for Sclerotinia blight control showed preventive treatments provided superior levels of control compared to applications made after symptoms appeared. He said Endura and Omega showed a significant reduction in disease injury compared to non-treated check plots.
He said timely fungicide applications are critical in combating peanut diseases. “If we get in late, we could have a train wreck. When we get behind, we can't catch up. We were lucky with fungicide application timings in our 2007 trials.”
Woodward said that crop rotation is a critical in the management of Sclerotinia blight.