By Stephen Wegulo, Extension Plant Pathologist
Tan spot is caused by Pyrenophora tritici-repentis. The fungus can survive and reproduce on wheat straw. Early in the growing season, spores called ascospores are released from pseudothecia (tiny black, raised fruiting structures formed on wheat straw) and spread by wind or blowing rain. They are the primary source of infection of the lower, more mature leaves during tillering and early jointing. After the initial infection, the fungus spreads within a field or to neighboring fields by a different type of spore called a conidium that is produced in the tan spot lesions themselves.
Pseudothecia Tan spot symptoms usually appear in early April. On leaves, the disease first appears as small, tan to brown lens or diamond-shaped spots. Characteristic symptoms are elliptical to elongate spots that are tan colored, have a dark brown spot near the center and are surrounded by a yellow border. As the lesions increase in size they tend to coalesce, producing larger, irregular areas of dead tissue. When leaves are young and actively growing, the spots typically remain small. Where spots are abundant, leaves may yellow giving the field an overall yellow cast. The lower, more mature leaves are infected first and the pathogen spreads to the upper leaves as the disease progresses. As plants mature the fungus invades the straw where it produces pseudothecia in the fall. By mid-August, pseudothecia are visible on the stubble that remains after harvest and are diagnostic of tan spot. They feel like coarse sandpaper to the touch.
Favorable Environmental ConditionsTan spot is favored by wet weather. Frequent rain in May and June promotes wheat growth as well as disease development. Spores are spread by wind or blowing rain. The disease progresses most rapidly when many spores are present and rainy, misty or foggy weather lasting more than 24 hours allows spores to germinate and infect plants. New releases of spores occur with each wet period. Symptoms appear within five to seven days after infection. Free moisture on the leaf surface is the most critical environmental factor associated with infection.
Cultural PracticesA unique three-year conservation tillage rotation system called ecofarming (ecofallow) can be used to reduce the threat of tan spot. Ecofarming is a crop rotation system of controlling weeds and conserving soil moisture with minimum disturbance of crop residue. In this system, corn or sorghum is seeded directly into winter wheat stubble in a winter wheat-grain sorghum or corn-fallow rotation. In the ecofarming system a crop is planted directly into the residue of a different crop, rather than into the residue of the same crop. This crop rotation-fallow system effectively breaks disease cycles which involve host specific pathogens such the tan spot fungus that survive in crop residue.
Chemical / Biological ControlFoliar fungicides can be applied to control tan spot if the disease becomes serious during the crucial post-boot stage of wheat development.Crop scouting should begin early in the growing season and should continue on a regular basis until disease is detected or a decision to spray is made based on the observed disease level.
For additional information, see the UNL Extension NebGuide, Tan Spot Disease of Wheat (G429).