A healthy plant is the first step toward optimizing yields and preventing disease in soybeans.
"To circumvent losses associated with diseases, an effective plan should be in place prior to the onset of disease epidemics," said Dr. Boyd Padgett, LSU AgCenter plant pathologist.
Padgett said an effective disease management strategy incorporates disease identification, cultural practices, genetic resistance and fungicides.
Proper disease identification is crucial for effective management. "This will determine which cultural practices should be implemented, variety selection and the choice of fungicide application and timing," Padgett said.
"Cercospora blight or purple seed stain is the No. 1 soybean disease in Louisiana," Padgett said. The disease is caused by the fungus Cercospora kikuchii that can infect seedlings, resulting in plant death or dormancy.
Foliar symptoms of this disease usually are not evident until soybeans are in the mid- to late reproductive growth stages. Initial symptoms appear as small, chocolate-brown lesions on the petioles near the base of the leaflets, Padgett said.
Advanced stages of this disease result in premature defoliation, discolored pods and reduced seed quality. The seed phase of this disease is evidenced by purple-stained seed at harvest, Padgett said.
Seedling infection is favored during moderate temperatures (70 to 80 degrees F) and extended periods of leaf wetness (eight to 16 hours). But petiole and leaf symptoms during late reproductive growth stages, including defoliation, are more prevalent in hot, dry conditions.
The pathogen may be seedborne and survives on plant debris in the soil, Padgett said. The fungus also has been isolated from some weeds.
Practices that promote air movement within the canopy will reduce the leaf wetness period and lessen the risk from some foliar diseases, he said.
When possible, genetic resistance should be the foundation of any disease management strategy, Padgett said. "Always use high-yielding, disease-resistant varieties, when available."
LSU AgCenter scientists evaluate commercial varieties on several research stations located across the state. Disease reactions for each variety can be found on the LSU AgCenter soybean website at www.lsuagcenter.com/en/crops_livestock/crops/soybeans/Variety+Trials++Recommendations.
When genetic resistance is not available, fungicides can be used for managing diseases, Padgett said. Fungicides are monitored for efficacy in experiments conducted on research stations and in producer fields every year.
"None of this would happen without the help of producers," Padgett said. "We work with them one on one."
Padgett also said the LSU AgCenter collaborates with other land-grant universities in Arkansas and Mississippi.
The effects of single versus multiple applications and application timings on Cercospora blight are evaluated in these studies.
There are several fungicide classes from which to choose. The diseases present will dictate which class is needed, Padgett said. Three classes of chemistries are available to producers: strobilurins, benzimidazoles and triazoles.
In LSU AgCenter tests, the strobilurins have broad-spectrum activity, Padgett said. This class is effective against aerial blight and pod diseases, slightly effective against Cercospora blight and suppressive against soybean rust, he said.
Topsin M and TM-85, both benzimidazoles, are moderately effective against Cercospora blight and pod diseases but not effective against soybean rust or aerial blight.
Triazole fungicides are used for managing soybean rust, and there is some evidence they may be effective against Cercospora blight, frogeye leaf spot and some pod diseases, Padgett said, adding that more research is needed to make this determination.
Other factors to consider when making a fungicide application are timing and sprayer setup. Most applications should be made when soybeans are between the R3 (pod initiation) and R5 (seed initiation) growth stages, Padgett said.
Early applications have provided better efficacy against Cercospora foliar blight, and later applications have provided better efficacy against pod diseases. Ideally, sprays should be made at rates of 15 gallons of solution per acre by ground and 5 gallons of solution per acre by air, Padgett said. –Mary Ann Van Osdell
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