In southern Minnesota, many early seeded wheat fields are approaching the 4-5 leaf stage. Weather factors, varietal disease resistance levels, and the type of crop residue nearby, or in the field are important in determining the potential for disease development.

Tan spot is a leaf spot caused by a fungal pathogen of wheat called Pyrenophora teres-repentis (Figures 1 and 2). The fungus survives each winter on above-ground wheat residue, producing abundant spores that can infect a subsequent wheat crop. Tan spot epidemics are common in the RRV during the wheat crop’s early growth stages.           

Producers are encouraged to scout for tan spot if:

1. a susceptible wheat variety is used (e.g., Alsen, Granite, Hanna, Marshall, Oxen, Ulen, Walworth);
2. wheat is planted into a field with wheat residue;
3. the weather is wet for extended periods.

RESEARCH

During 2005, information was collected from the RRV On-Farm Disease Mgmt. Trials. These trials were located near Oklee (Ray & Barbara Swenson), Perley (Brian & Theresa Hest), and Fergus Falls (Tom & Deb Jennen). The Perley site was planted into sunflower residue, while the other two sites were planted into soybean residue. Just for the sake of comparison, last year’s spring weather was cool and wet – much like this year (see Table 1).

The disease management trials included a number of fungicide application treatments on six spring wheat varieties (Alsen, Granite, Knudson, Oxen, Reeder, and Walworth). The tan spot management treatment (Tilt 2 fl oz./A applied at the 4-5 leaf stage) versus the no fungicide treatment was not expected to result in much of an economic benefit since all three locations were planted into broad-leaf crop residue. Table 2 shows the yield and economic differences of applying fungicide compared to the no fungicide treatment at each location.

The fungicide application resulted in the best overall economic outcome in Perley, a site where disease pressure was high, even though the trial was planted into sunflower residue and the site received less rain than the other two sites. The Fergus Falls location still had a positive economic treatment mean ($0.41/A), but at the Oklee site the cost of fungicide wasn’t offset by the grain yield and quality factors at $3.76/bu (Nov. 2005). Reeder benefited from the application at all trial locations, whereas Granite and Knudson benefited in yield responses, but not economically.

Overall, the tan spot recommendation is sound. Even so, there may be some locations and varieties that benefit economically from a half rate of fungicide at the 4-5 leaf stage. Another year’s data will help to identify varietal trends.

This research was supported, in part, by the Minnesota Wheat Research and Promotion Council. Grain samples were graded by Mid-Valley Grain Cooperative in Crookston to determine applicable grain grades. A pseudo-sale of the grain occurred after DON, or vomitoxin discounts, were suspended during November of 2005. Discounts and/or premiums were applied based on grade, test weight, damage, and protein.

Note: Additional data and statistical analyses are published in 2006 On-Farm Cropping Trials for NW and WC Minnesota available from your local or regional extension educator.

Charla Hollingsworth
Extension plant pathologist