Asian soybean rust can drastically reduce yields in areas where it commonly occurs, so monitoring for this disease will likely be necessary for New Jersey soybean growers. Crop insurance may require treatment to meet best management criteria. In some fields, no application may be the right decision. Spray decisions are based on soybean crop growth stage and the risk of disease development. Several factors are involved in the risk of soybean rust moving into an area: incidence and severity of rust in areas to the south, wind patterns that can move the spores of the soybean rust fungus over long distances, regional and local weather, and growth of the soybean crop. See the Soybean Rust Fungicide Decision Guidelines (19k PDF) for fungicide application guidelines.
Chemical sprays for soybean rust are based on the risk for the disease (discussed above) as well as the growth stage of the soybean crop. Current data indicate that fungicide applications are not needed in the early vegetative growth stages. Spraying just prior to crop flowering (R1), however, may be prudent if disease risk is high. This is especially true for late-planted crops and/or very late-maturing varieties that may develop a large canopy before flowering.
Soybean rust develops most rapidly during soybean reproductive growth stages. The first fungicide application should be made before rust has appeared on more than 10% of the leaflets in the canopy. At an incidence this low, each rusted leaflet may have only one or very few pustules, and the disease will be difficult to detect without careful scouting. An application of a fungicide when the level of disease is greater than 10% incidence may protect newly emerging leaves, but may not result in a yield benefit.
Spraying at late growth stages is not recommended due to lack of yield response. In addition, many fungicides have days-to-harvest (pre-harvest intervals) or growth stage restrictions. Refer to fungicide labels for specific directions and restrictions.
Crop stage - R1 through R5:
Soybean rust risk LOW:
Soybean rust risk MODERATE:
Soybean rust risk HIGH:
Crop stage - R6 (full seed) or later:
Barely detectable in lower canopy (incidence less than 10%):
Easy to detect in mid- to upper canopy (incidence greater than 10%):
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