Tan spot, primarily a foliar disease of wheat, can commonly be found in the Red River Valley during the spring and early summer. It has the potential to become a severe disease problem if wheat is planted into wheat residue (wheat-on-wheat) and the weather remains wet for an extended period.

This spring has been all of that – wet. Chris Motteberg, Plant pathology research scientist reports that small grain fields located north of Stephen are particularly hard hit by the wet weather. Stands are sparse and areas in the fields have drown out. Larry Smith, NWROC Head agrees that the condition of wheat stands appear to improve as one travels south from Stephen. Wheat stands near Argyle appear much more uniform, compared to those further north.

Tan spot is well established and disease severity is high in wheat fields that have been planted into wheat residue. Spring wheat varieties have differing levels of resistance to the disease. Varieties that have been known for good levels of tan spot resistance are even showing the strain with the continuing wet weather and high levels of local disease inoculum (Figure 1).

The fungus that causes tan spot (Pyrenophora triticirepentis) infects plants and causes disease over a wide range of temperatures as long as adequate moisture is present on leaves for an extended period (12-24 hours). Fungal survival structures over-winter on aboveground wheat stubble (Figure 2). This provides a local source of inoculum as the fungus release spores when temperatures warm in the spring.

Tan spot and Septoria/Stagonospora leaf spotting diseases produce similar leaf symptoms. Tan spot produces oval-shaped, tan lesions that enlarge with age. Lesions are often surrounded by a yellow halo, while dark areas eventually form on tissues that were infected initially (Figure 3). Severe disease development causes the leaf spots to expand until most, if not all, of the infected plant leaf is killed.

Disease management includes crop rotation to prevent early-season disease development. If the disease is established in the early season and extended wet weather is forecast, fungicide application provides good control. Seven systemic fungicide products are labeled for use during the early season in Minnesota.