Memorial Day weekend is often the first time that reports of leaf diseases reach my desk. I call tan spot my ‘canary-in-a-coal-mine’ for small grains diseases, in part, because it requires a shorter leaf wetness period from either rain or dew and lower temperatures to start initial infections compared to either leaf rust or Septoria spp.
Any wheat on wheat and winter wheat that was seeded early last fall should probably be scouted for early-season tan spot. Do not simply add a fungicide to your herbicide program because it is cheap and convenient ‘insurance’. If no disease is present at the time, you are not doing any good and any new growth will not be protected anyway.
Very early infections from the tan spot pathogen show up like chocolate-colored specks that very quickly develop a yellow halo around them. The yellow halo is the plant's reaction to the toxin that the fungus produces. Seedlings are very sensitive to this toxin and whole leaves will turn yellow, creating an appearance of a nitrogen deficiency.
Half a labeled rate of many of the fungicides in groups 3 and 11 that listed on pages 31 through 41 and labeled for early season disease control in wheat in the North Dakota Field Crop Plant Disease Management Guide or that are listed in the North Central Regional Committee on Management of Small Grain Diseases' Fungicide Efficacy for Control of Wheat Diseases bulletin will provide excellent control of this early season onset. This will delay the exponential phase of the disease progress by several weeks should conditions for the disease progression remain favorable. This, in turn, will often allow you to postpone a second fungicide application at Feekes 9 to Feekes 10.51
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