University of Minnesota Small Grains Specialist        

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Prairie Grains is the
official publication of
the Minnesota
Association of
Wheat Growers,
North Dakota Grain
Growers Association,
South Dakota Wheat,
Inc., and the
Minnesota Barley
Growers Association.

The decision to spray a fungicide to suppress the foliar disease complex and/or Fusarium head blight is fast approaching. In grower meetings last winter, I talked a lot about this issue and urged wheat producers in the Red River Valley to budget for a fungicide application. I referred to the 'Foliar Fungicide Decision Aid for Wheat Leaf Spot' as a guide to help make a decision.

The guide on the next page was developed by Dr. Marcia McMullen of North Dakota State University, a hybrid of sorts that incorporates elements of foliar fungicide decision aid models developed by plant pathologists Dr. Roger Jones of the University of Minnesota, and Dr. Bob Bowden of Kansas State University.

In its basic form, the decision guide considers market price, previous crop, climatic conditions and the development of tanspot, Septoria, and/or powdery mildew relative to the growth stage of the crop. Note that the decision guide does not take into account differences in susceptibility between cultivars for any or all of the involved fungi. This is a shortcoming in the model. Another is that it does not deal with scab. The model was developed specifically with leaf diseases in mind, not head blight.

Market Price

The first factor the decision guide uses is market price. The low wheat prices we are currently enduring basically require a bigger differential between treated and untreated crop before there will be a return on the additional investment of the fungicide. Remember that the return per acre is a function of both number of bushels produced and the quality of those bushels. Use of a fungicide will not only translate to an increase in yield, but in almost all cases will also improve the quality of the grain produced, improving the market grade.

Previous Crop

Previous crop, as the second factor, basically assigns a risk value for the amount of residue that could harbor the disease inoculum. Local inoculum is very important in the development of powdery mildew, tanspot and to a large degree for Septoria and scab.

Yield potential is a third factor in the decision. Higher yield potential is more profitable to protect with a fungicide. But the case might be made that boosting yield from 20 to 30 bushels to reduce losses may also be justified.

In the decision guide, there are three decision points: The first is at Feekes 6, when the wheat starts to joint. The second is at Feekes 8, when the flag leaf appears and the third is Feekes 10.5, when anthesis or flowering starts.

Powdery mildew can be a very explosive disease and requires immediate attention. If powdery mildew is present at Feekes 6, it almost always warrants a fungicide treatment. The best approach is a treatment of two fluid ounces of Tilt as this will provide season-long control for powdery mildew and suppress early onsets of tanspot.

For control of the foliar disease complex, Feekes 8 has been considered the optimum timing since the flag-leaf contributes up to 85% toward the final grain yield. However, for suppression of scab, Feekes 8 is too early.

Although locally systemic, even Tilt or Folicur will not reach the spike when applied at Feekes 8 and thus will give no suppression of scab. The optimum timing for suppression of scab is at Feekes 10.5.1 or early flowering. Obviously this creates a dilemma since optimum control for the leaf diseases is earlier at Feekes 8. What to do? Does a single application of a fungicide at Feekes 10.5.1 provide enough control for the leaf disease complex to delay the spray? Do we need to split applications between Feekes 8 and Feekes 10.5.1?

All fungicides that we have access to are primarily protectant. In addition, Tilt and Folicur have some curative potential in the early stages of an infection. This means that leaf tissue that is affected by any of the fungi will not totally recover, and that this leaf area will not contribute to its full potential to the grain yield. This would be an argument to spray at both Feekes 8 ane Feekes 10.5.1 to get the full benefit of the applied fungicides.

Economic reality will likely be prohibitive in this dilemma. If we work under the assumption that we only have one shot between Feekes 8 and Feekes 10.5.1, the latter to suppress scab seems to be the preferred route. But how much yield can you lose to these other foliar diseases when delaying spraying from Feekes 8 to Feekes 10.5.1?

The University of Minnesota has little research data to give a definitive answer. In some cases, delaying the spray caused very little yield losses. In other cases, delaying the spray caused substantially more yield losses. It basically depends on how fast the foliar fungi develop between Feekes 8 and Feekes 10.5.1.

How can we come to some kind of recommendation in light of this dilemma?

1) If the weather was wet and humid and average temperatures were in the mid 60s to mid 70s during the daytime for the three weeks leading up to flag leaf emergence, seriously consider a fungicide application at Feekes 8.0. Especially if disease symptoms are present on the lower leaves and the decision guide indicates a score above 16 points. The best control will be given by a 4-fluid ounce rate of Tilt. The reduced rate of Tilt will not give adequate control of tanspot or Septoria. Reevaluate the situation at Feekes 10.5.

2) If the weather was wet* and humid and average temperatures are in the 70s to low 80s for the two weeks leading up to heading, seriously consider a fungicide application to control leaf diseases and suppress scab. Especially if the decision guide for the foliar disease complex indicate a value above 13 points. The best control will be given by either the Benlate/mancozab combination, the 4-fluid ounce rate of Folicur or the 4-fluid ounce rate of Tilt (if not used previously at either Feekes 6 or Feekes 8).

* The definition of wet weather is above normal precipitation (twice or more) and many days with high humidity and light winds which leave the crop wet for extended periods of time. n

Copyright Prairie
Grains Magazine
June 1998