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3 reasons not to tank mix rust fungicides with herbicides

Agriculture.com Staff 07/06/2010 @ 5:21pm

Some soybean growers are wondering whether it would be a good idea to save application costs by tank mixing rust-controlling fungicides with herbicides this growing season. University of Wisconsin Weed Scientist Chris Boerboom and Plant Pathologist Craig Grau shared their thoughts in a recent article in the Wisconsin Crop Manager newsletter, reprinted here by permission.

By Chris Boerboom and Craig Grau University of Wisconsin Soybean rust has been confirmed in the southern US. While it is unknown when the rust will be introduced into different regions of the US, 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.

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.

This timing is likely to be too early for fungicide treatments to control soybean rust, however. Based on the experience in other countries, the earliest fungicide treatments are often made around the R1 soybean growth stage. If a fungicide-herbicide tank mix were to be sprayed at the correct timing for 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 develop in your area each year. Herbicide-fungicide applications might be timed right for one pest, but not the other. Worst, 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.

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. Their application may be more similar to the needs of fungicides.

Compared with herbicides, fungicides require smaller droplets and higher spray volumes to achieve maximum coverage of the soybean canopy. 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 reduce the risk of drift. Medium or coarse-sized droplets (250 - 450 VMD) are recommended for herbicides, which are less prone to drift.

That means 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 were made at the lower spray volume (e.g. 10 - 15 gpa) used for many herbicides, spray coverage with the fungicide may be compromised as well.

Most herbicide and fungicide labels are vague regarding the recommendations for 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 soybean rust. Until that time, it may be wise to avoid tank mixtures. The greatest risk with a tank mixture is likely to be somewhat reduced performance of glyphosate if it is applied in the higher spray volumes that are recommended for fungicides. The reduced performance might also be associated with the fact that weeds are likely to be larger at the time of the tank mix application.

From a legal standpoint, if neither label prohibits 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.

In addition to this article, the January 17, 2005 issue of Wisconsin Crop Manager includes an information sheet about soybean rust created by Brian Hudelson, UW-Madison Plant Pathology and Bryan Jensen, UW-IPM Program. It offers full color photos of signs of rust, as well as a form to fill out and instructions for sending a suspected rust sample to the University of Wisconsin Plant Disease Diagnostics Clinic.

Some soybean growers are wondering whether it would be a good idea to save application costs by tank mixing rust-controlling fungicides with herbicides this growing season. University of Wisconsin Weed Scientist Chris Boerboom and Plant Pathologist Craig Grau shared their thoughts in a recent article in the Wisconsin Crop Manager newsletter, reprinted here by permission.

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