Keep a watch out for foliar diseases this summer

In Delta Farm Press

A wet planting season pushed some growers to get soybeans and other crops in the ground.

And if excessive rainfall continues, farmers should be prepared for the threat of more mid- and late-summer disease pressure, says a University of Arkansas Extension plant pathologist.

Travis Faske, who closely monitors fungal diseases and nematodes from the UA Lonoke station east of Little Rock, says that for Arkansas and the rest of the South, a lot of diseases can invade soybean fields.

“But frogeye leaf spot seems to catch most people’s attention due to it being a foliar disease,” Faske tells Delta Farm Press. “It really starts picking up later in the summer.

“If a variety is susceptible to frogeye, farmers will need to treat those fields with a fungicide. If they plant varieties with some resistance to frogeye — a trend I hope continues — they may not need a fungicide.”

If not managed properly, frogeye can cause severe yield losses on a susceptible variety when conditions favor disease development. The leaf spot symptoms begin as dark brown, water-soaked spots and mature into lesions with tan or brown centers and a narrow reddish brown-to-purple margin, Faske says. Older lesions are translucent and have whitish centers.

“In severely infected plants, several lesions may merge into larger irregular-shaped spots,” he says. “When leaves are heavily infected, they may wither quickly and prematurely shed, a condition called blighting.”

UA Extension’s soybean production handbook notes that stem and pod symptoms are less common, but may appear late in the growing season with prolonged conditions that favor disease development. Stem lesions are elongated, whereas pod lesions are circular. Mature lesions are slightly sunken with light gray centers and brown borders. The fungus can grow through the pod into the seed. Infected seeds may be gray to brown in color.

New fungicide programs

“We have had more of a Midwestern-type summer with cooler temps the past two years,” Faske says. “But if we stay hot and dry this summer, we may see less frogeye.”

He adds that new fungicide programs may be needed for many fields to counter fungicide-resistant frogeye. “About 90 percent of our Arkansas acres have to deal with these new strains of frogeye,” he says. “Strobilurin fungicides are not effective. Farmers need to be using a premix that includes something with a triazole to manage the disease.”

There’s a chance that sudden death syndrome will also attack soybean fields. Faske says SDS is often seen in production systems familiar to Arkansas growers. “The disease is often in well-managed, high-yield potential, irrigated fields growing under optimal conditions,” he says. “Yield losses range from slight to 100 percent, depending on the time of infection, cultivar susceptibility and disease severity.”

Later in the summer, charcoal rot may strike fields. It’s a root and stem disease that commonly occurs in hot, dry conditions. “Charcoal rot is most severe when plants are stressed from lack of moisture or nutrients,” Faske says. “It’s also severe in excessive plant populations, or where soil compaction, other diseases or nematodes or improperly applied pesticides impair root development.”

He says the earliest symptoms are leaves that are smaller than normal sized leaves. They become chlorotic, then turn brown, but remain attached to the petiole giving the entire plant a dull greenish-yellow appearance. “In many cases, these plants wilt and die,” Faske says.

He notes that there were again a few sightings of soybean rust last fall. “Fortunately, it was too late in the growing season to create any problems,” Faske says. “When we get to the R6 stage, we’re past facing any problems. In the past 10 years we’ve detected soybean rust six times in Arkansas, but we recommended treatment only once. In the past, widespread fungicide use on soybeans across the South has likely prevented its movement northward. I don’t think it will be an issue, but it is something we are always monitoring.”

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