Fungicides Overview

What control options are available for controlling Asian soybean rust?

Fungicides are the only option for control of Asian soybean rust. Resistant varieties of soybeans currently are not available and are unlikely to be available for the next 10 years. The significance of yield loss this season will be determined by when a soybean fungicide is applied. Data from South America shows possible yield loss could range from 10 percent to nearly 100 percent.

Your best line of defense is to pay attention to weather patterns and watch for early reports of soybean rust across the country. To help track the disease with up-to-date maps, visit the USDA Soybean Rust Information Site.

If rust is reported within a one- to two-state distance upwind or approximately 300 miles away, a preventive spray should be applied if, or when, soybeans are in the R1 (flowering) to R6 (full seed) stage. Some Brazilian crops showed signs of soybean rust in 2003 to 2004 during the V2 (vegetative) stage, so scouting is the best line of defense.

When selecting a fungicide, there are three chemical groups approved for soybean rust – triazole, nitrile and strobilurin. For soybean rust control, management programs can consist of a preventive and/or curative approach, but a preventive approach is recommended.

A true preventive application is one made by applying a fungicide before infection has occurred, while a curative application is one which takes place after an infection occurs – even when no evidence of disease symptoms are present.

The significance of yield loss will be determined by when a fungicide is applied. Dow AgroSciences offers the following recommendations from its experiences in South America:

• Curative applications made with less than 10 percent visible disease incidence in lower canopy and 0 percent in mid-canopy are most likely to protect yield. Ideally, the application should be made prior to 5 percent visible disease incidence.
• Applications made with less than or equal to 20 percent visible disease incidence in lower canopy and less than or equal to 10 percent in mid-canopy are not likely to prevent yield loss.
• Applications made with more than 10 percent visible disease incidence in mid-canopy and any infection in upper canopy may be too late to prevent severe crop loss.

Click here to review how Laredo^® fungicide offers double-punch protection against soybean rust. Laredo is available through 2006 for control of soybean rust in soybeans through Section 18 exemptions.¹