Foliar Health Basics: Soybean Rust Control
Soybean rust, caused by the fungus Phacopsora pachyrhizi, has been a serious disease in Asia for many years. It appeared in Africa in 1997 and South America in 2001. Hurricanes carried fungal spores to North America by 2004. While soybean rust can wipe out fields, work in Brazil and the United States shows proper use of fungicides can control the disease.
Soybean rust spores cannot survive freezing temperatures, but spores are highly mobile on the wind and can travel great distances from southern states northward into the Midwest. Spores were found in 196 U.S. counties in 2015, although the disease has had limited effects.
Severe infection has produced total crop defoliation in Brazil. While losses have been minimal in the U.S., the explosive potential of soybean rust has U.S. plant pathologists on high alert. Growers should pay attention to regional scouting reports, scout their fields and be prepared to take action if soybean rust spores could reach their fields while soybean plants are vulnerable.
Soybean rust overwinters on kudzu and more than 90 other plant hosts, including clovers, vetches and trefoil. Warming temperatures and storms can multiply and move spores quickly into distant soybean fields. Rain, heavy dews and high humidity and temperatures from 60 to 82 degrees F favor infection of soybean leaves. Spore-producing pustules are produced as few as 7 to 10 days after infection.
The first signs of soybean rust are small (2 to 5 millimeters) brick-red spots on leaves. Spots may turn black as they age. Spots usually appear in the lower leaf canopy on the undersides of leaves at or after flowering. Spots multiply quickly and grow into pustules that produce new spores. Soybean rust destroys leaf surface area, and then proceeds to cause leaf drop, sometimes resulting in total defoliation and crop loss.
The USDA maintains a map of soybean rust observations.
An application of DuPont™ Aproach® Prima fungicide made at the first sign of infection is most effective at reducing soybean rust severity to protect yield. Infections early in the season (before flowering) or from R1 to R6 are considered most threatening to yield. This Soybean Rust Yield Prediction Tool developed by the University of Kentucky offers treatment threshold guidelines. Fungicide treatment may protect yield by controlling multiple diseases at once and reducing plant stress.
Results from a 2013 Auburn University trial showed DuPont™ Aproach® Prima fungicide decreased the incidence of soybean rust from 75 percent in untreated plots to 12 percent, while Quilt Xcel reduced rust incidence to nearly 18 percent. That improved disease control increased soybean yield by 3.5 bushels per acre over Quilt Xcel plots and 7.9 bushels per acre over untreated plots. Applications were made at R3.
DuPont™ Aproach® Prima fungicide is not registered in all states. See your DuPont retailer or representative for availability in your state.
Quilt Xcel® (Syngenta).
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