Soybean South


ASR: Fungicide Application Timing

A timely fungicide application will protect the soybean crop,
help keep inoculum levels low if applied by a number of growers in a
region and aid in control of other diseases as well.

By Bob Kemerait

EDITOR’S NOTE: Following is an excerpt from Bob Kemerait’s “Soybean Disease and Nematode Control” section from the 2009 Georgia Soybean Production Guide. To view the entire text, go to

In 2009, growers will need to remain vigilant with regard to management of Asian soybean rust (ASR). However, effective management of rust may also lead to better control of other diseases, such a frogeye leaf spot and anthracnose.

Timing for application of fungicides to manage soybean rust is critical. It is unlikely that growers in Georgia can afford to spray fungicides on soybean without the imminent threat of Asian soybean rust or a disease such as frogeye leaf spot.

However, we have learned that soybean rust can be a very unforgiving disease if fungicide applications are delayed too long once it threatens. Where applications were delayed in our fungicide trials, significant reductions in yields often occurred.

Based on field studies conducted in Georgia, it appears that early reproductive growth (for example early bloom (R1-R2) through early pod (R3) stages) is an important time for rust management. To date, we have never detected rust in plots or fields prior to early bloom and typically began to find rust as the soybean crop reached early pod set and beyond. However, based upon a variety trial in the fall of 2005, we know that soybean rust can infect soybeans prior to bloom.

Lessons from the field
Listed below are thoughts about the timing of fungicide applications for management of Asian soybean rust.

1. Timing fungicide applications ahead of introduction of Asian soybean rust into a field is critical in the successful management of the disease.

2. From field observations, it appears that early reproductive growth is a critical period in the management of soybean rust. From both seasons, it appears that a well-timed fungicide application with an appropriate fungicide during this period is critical for maximum rust control if the disease is threatening.

3. If rust has not been detected in the local region (as assessed with sentinel plots and careful scouting), it is recommended that soybean growers delay application of a fungicide for control of soybean rust until the threat from the disease is more imminent, unless the grower is protecting against some other disease, such as frogeye leaf spot.

4. If soybean rust has been detected in the local area, or is thought to be likely, growers are advised to initiate fungicide applications once the crop reaches first bloom.

5. A second fungicide application should be considered within two to four weeks after the first application unless the crop has reached harvest maturity or the weather has been unfavorable for disease spread.

6. From field studies, it is clear that the first fungicide application is more important than the second. In 2006, a single, well-timed application of our best fungicides was at times as effective as two fungicide applications and sometimes better than two applications of a lesser effective fungicide.

Soybean producers should not miss the opportunity to achieve excellent control of soybean rust by using a less effective product in the first application if rust threatens.

Bob Kemerait is a plant pathologist with the University of Georgia (UGA) Cooperative Extension Service. Contact him at (229) 386-7495 or