Fungicides of all types will provide greater control if applied before rust spores are deposited on the soybean leaves. Once rust symptoms are visible, fungicides with protectant activity will not be effective, and curative types will have limited activity. Early detection and proper identification of soybean rust will be key to successful management of soybean rust.

Reports from Brazil indicate that infections exceeding 20 to 30% of the soybean canopy cannot be controlled with fungicide applications. At that point fungicides are no longer able protect plants sufficiently from additional infections, or yield reduction is already so great that fungicide application cannot recover treatment cost.

When it comes to timing of application, there are two obvious mistakes, both of which can be costly. Soybean rust spreads quickly and poor timing of a fungicide spray will significantly increase the risk of failure. Spray too early and the effects of the fungicide may dissipate by the time spores arrive and infect plants. Conversely, delayed applications will result in applications after initial infection has occurred and the disease may have progressed beyond the point where effective control is possible with fungicides.

Timing of fungicide applications will be different than for herbicides
Considering that most soybeans are Roundup Ready, the first application of glyphosate should be applied to these soybeans when they are in the V2 or V3 growth stage. This early timing is required to control weeds before they compete with the soybeans. It is also the best timing to get the greatest herbicide activity because the weeds are still small.

However, this timing is too early for fungicide treatments to control soybean rust. Based on the experience in other countries, the earliest fungicide treatments are often made around the R1 soybean growth stage. Because of Wisconsin's northern latitude, the rust may not arrive until even later in the season.

If a fungicide-herbicide tank mix was sprayed at the correct timing for the rust, weed control could be reduced because of the large size of the weeds. In addition, you should not delay your herbicide application in anticipation of a herbicide-fungicide application because the rust may not even develop in Wisconsin each year.

One situation when a herbicide-fungicide tank mixture might be appropriately timed for both pests is when a second herbicide application is being made to control late emerging weeds. If this is the case, drift and label precautions still need to be considered.