ISU Weed Science Online -

Questions about tank-mixing herbicides and fungicides
by Micheal D. K. Owen
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January 27, 2005 - Recent discussions and questions posed at the Crop Advantage Series meetings addressed the issue of tank-mixing fungicides and herbicides. The questions are related to the concern for anticipated soybean rust problems and the current wide spread adoption of glyphosate resistant soybean varieties and the concomitant use of glyphosate. The thought is that the herbicide and fungicide should be applied as a tank-mixture thus gaining some time management efficiency. However there are a number of keys questions that must be considered. Notably, whether or not the tank mixture of the specific herbicide and fungicide is actually labeled must be considered. According to FIFRA, unless a tank mixture is restricted on a label, the chemicals can be mixed if the use is in accordance with "normal application practice". It is important to consider that unless the mixture is specifically described by the label, the liability of the decision to tank mix products rests on the knowledgeable expert who made the recommendation. Other considerations are application procedure/technique and application timing. As a general statement, application procedure and timing of a fungicide are different than those needed for a herbicide application. Thus the plan to tank mix fungicides and herbicides may not be the best strategy. Below are articles that address this issue in greater detail. The first article is from the University of Wisconsin (copied below) and the second is from Purdue University (use hyperlink to the PDF file).

Tank Mixing Herbicides and Soybean Rust Fungicides (from the Wisconsin Crop Manager, volume 12, number 1 and used with permission)

by Chris Boerboom, Ext. Weed Scientist and Craig Grau, Ext. Plant Pathologist

Soybean rust has been confirmed in the southern US. While it is unknown when the rust will be introduced into Wisconsin, fungicides will be applied to control the rust when it appears. At that time, there may be interest in tank mixing fungicides with herbicides to reduce application costs. However, there are three good reasons why a herbicide-fungicide tank mixture may not be 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. Instead, we recommend that herbicides and fungicides are applied separately for the best control of weeds and rust.

Improper timing. 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, 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 likely to be 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. Herbicide-fungicide applications might be timed right for one pest, but not the other or the application might be timed wrong for both.

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. Likely, the greatest risk with a tank mixture is somewhat 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 basically be your own.

The other article of importance was written by Greg Shaner, Bill Johnson and Glenn Nice from Purdue University. It can be accessed at:

The article includes much of the discussion describing the issues addressed in the University of Wisconsin article, but also includes a table that lists herbicides and respective restrictions for tank mixing fungicides. New information will be added as it is developed.