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Tan spot of winter wheat
Tan spot is caused by a fungus (Pyrenophora tritici-repentis). It overwinters on wheat residue and it's spores are spread by wind and splashing water.
Tan spot is prevalent in some fields of winter wheat in central South Dakota. Late spring is the time to start scouting for tan spot in winter wheat.
Symptoms
Tan spot infections appear as a tan to brown, round to slightly elongate leaf spot, surrounded by a yellow halo. The tan center of the leaf spot may appear somewhat diamond shaped (Figure 1). Often, when many infection sites are clustered together on seedling plants the majority of the leaf may appear to be yellowed, but the severity is generally not as severe as it might seem to be at first glance. Field symptoms of tan spot are yellow streaks that often follow residue strips (Figure 2).
How Tan Spot Develops
The risk of leaf disease is greatest in fields that were planted to wheat last year. Either winter or spring wheat will support the pathogen. The tan spot fungus overwinters on crop residue. The environment in central South Dakota can be very favorable for tan spot development with during rainy periods in the spring when the fungus produces spores. As little as six hours of moisture on the wheat leaves are needed for tan spot infections to occur. Those spores can lead to new infections on susceptible crops such as spring or winter wheat. No-till fields recropped to wheat or no-till fields with heavy stubble remaining from two years ago pose a risk of tan spot.
Wheat varieties and tan spot
Some varieties of winter wheat such as Arapahoe, Vista, TAM 107, Scout 66, or Alliance are more susceptible that varieties such as Nekota, Jagger, and 2137.
Fungicide Options
An application of 2 fluid ounces of Tilt per acre or 1 pound of a mancozeb product (Dithane, Penncozeb, Manzate, Manex, etc.) with 2-3 gallons of water will provide good protection to new foliage. Fungicide may be applied to the crop at about the 5-leaf stage. A spreader-sticker will improve the duration of the control. If the 2 fl. oz. rate of Tilt is used in the seedling stage, only 2 fl. oz. of the Tilt will be available for flag leaf application.
If a herbicide is applied at that time the fungicide can be tank mixed with most post emergence herbicides.
Check the label of your herbicide to be sure there are no restrictions. A fungicide application at this growth stage is intended to slow the progress of the disease and prevent new infections for a period of time. The crop should be monitored through the season and if a second fungicide application is needed it should be applied at the flag leaf stage.
Summary of risk factors and guidelines for fungicide use
Not all fields are at the same risk of disease. A seedling treatment to a wheat crop is warranted *ONLY* if
1. Yield potential is high (>45 bu/A)
2. Wheat is seeded into wheat stubble (especially reduced-tillage)
3. Environment is favorable for disease
4. If there is a threat of leaf rust, especially in Eastern SD (flag leaf application)
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Plant Science Department, South Dakota State University |