Disease Management Guidelines

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.

What is "Risk"? Some Examples:

• A low risk situation occurred in 2006, when dry conditions in the Gulf Coast states prevented the build-up of soybean rust spores in the early season, reducing the impact of the disease in more northern states and Canada.
• A situation of moderate risk in New Jersey would occur if soybean rust is detected in a neighboring state (such as Maryland. Delaware, or Pennsylvania) but not here. In another scenario, New Jersey would be at moderate risk for soybean rust if the disease builds to significant levels in one or more southern states, the long range forecast is for storm fronts to pass through those areas and up into New Jersey within the next two weeks, and the weather in New Jersey is predicted to be suitable for soybean rust development.
• New Jersey soybean growers would be at high risk for soybean rust if the disease is detected in a sentinel plot, commercial soybean field, or kudzu patch in New Jersey and weather conditions are favorable for disease development.

Relationship of Crop Stage to Disease:

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. Check the Fungicides page for more information on chemical control for soybean rust.

Rust Absent

Crop stage - Early to mid-vegetative:

• Fungicide strategy: no application. Sprays are not economical at this time.

Crop stage - R1 through R5:

Soybean rust risk LOW:

• Scenario: soybean rust not detected in New Jersey or in adjacent areas of contiguous states based on reliable reports of rust in sentinel plots (local and distant) and disease forecasts.
• Fungicide strategy: do not spray.

Soybean rust risk MODERATE:

• Scenario: soybean rust confirmed in adjacent areas of Delaware and Pennsylvania; OR possibility of rust passing up into New Jersey with storm fronts from southern states.
• Fungicide strategy: spray with a strobilurin or triazole fungicide or a combination product; followup with a triazole or combination product may be needed.

Soybean rust risk HIGH:

• Scenario: soybean rust detected in field to be sprayed (< 10% incidence) or confirmed in other fields in the same region.
• Fungicide strategy: spray with a triazole fungicide or combination product; followup with a triazole may be needed. The yield benefit from sprays in fields with soybean pustules that are easy to detect in the mid- to upper canopy (< 10% incidence) is uncertain.

Crop stage - R6 (full seed) or later:

• Fungicide strategy: no application.

Rust Present

Barely detectable in lower canopy (incidence less than 10%):

• First application: combination product or triazole
• Second application: triazole or combination product

Easy to detect in mid- to upper canopy (incidence greater than 10%):

• Yield benefit from fungicide is uncertain: check with Rutgers Cooperative Extension personnel for specific guidelines