UW Soybean Plant Health
Chris Boerboom, Extension Weed Scientist and Craig Grau, Extension Plant Pathologist
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We recommend that herbicides and fungicides be applied separately for the best control of weeds and rust. Although tank-mixing fungicides with herbicides may reduce application costs, there are three good reasons why a herbicide-fungicide tank mixture is not the best idea: improper timing, drift, and labeling precautions. Trying to make a tank mixture work for weed control and soybean rust control may result in poor performance of both pesticides.
Improper timingConsidering 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, which would cause yield loss. 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.
Drift
The goals for spray coverage with herbicides and fungicides differ. For translocating herbicides like glyphosate, uniform spray distribution is required, but complete coverage of each leaf surface is not. Contact herbicides require more thorough leaf coverage than translocating herbicides and their application may be more similar to the needs of fungicides.
Fungicides require smaller droplets and higher spray volumes to achieve maximum coverage of the soybean canopy than herbicides. This may require either different spray nozzles or higher spray pressure to generate the smaller droplets. Some recommendations suggest that fine to medium-sized droplets (150 - 350 VMD) are appropriate for fungicides. (VMD = volume mean diameter and is measured in microns).
However, we want larger droplets during herbicide applications to reduced the risk of drift. Medium or coarse-sized droplets (250 - 450 VMD) are recommended for herbicides, which are less prone to drift. If the application of a herbicide-fungicide tank mixture is optimized with larger droplets to reduce herbicide drift, the performance of the fungicide may suffer. If smaller droplets are used to optimize the fungicide performance, herbicide drift is more likely.
For the best performance, fungicide applications by ground rigs should be made in a minimum of 15 gpa of water and 20 gpa may be preferred. If a herbicide-fungicide application was applied at the lower spray volumes (e.g. 10 - 15 gpa) typically used for many herbicides, spray coverage with the fungicide may be comprised as well.
Label precautions
Most herbicide and fungicide labels are vague regarding the recommendations of herbicide-fungicide tank mixtures. Many soybean herbicide labels make no mention of fungicide tank mixtures while others make generic statements that tank mixtures with other pesticides may reduce efficacy.
Headline® allows tank mixing with herbicides, but tank mixtures with herbicides are not addressed on most other fungicide labels. Chemical companies may make more specific recommendations in regard to the risk of incompatibility or reduced performance with tank mixtures as more experience is gained with the soybean rust. Until that time, it may be wise to avoid tank mixtures.
The greatest risk with a tank mixture is likely to be reduced performance of glyphosate if it is applied in the higher spray volumes that are recommended for fungicides. This reduced performance might also be associated with the fact that weeds are likely to be larger at the time of this tank mix application.
From a legal standpoint, if both labels do not prohibit a tank mixture, it is legal for the applicator to mix and apply the tank mixture. If desired, you could try the tank mixture in a small field test, but the responsibility for performance would be your own.
Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: University of Wisconsin-Madison, Departments of Agronomy, Entomology, and Plant Pathology at www.plantpath.wisc.edu/soyhealth.
Last update 4/5/06