Kansas State University | College of Agriculture | Department of Plant Pathology | Site Map | Home

Tan spot is caused by a parasitic fungus called Pyrenophora tritici-repentis. Some older literature uses the names Pyrenophora trichostoma or Helminthosporium tritici-repentis for the causal fungus. Tan spot can be a serious problem, especially in continuously cropped wheat. Average state-wide yield losses due to tan spot in Kansas have been estimated at almost 2% per year. In individual fields, losses of 25% or more have been documented. Tan spot often occurs with other foliar diseases like speckled leaf blotch, nodorum leaf blotch, or leaf rust. This complex of diseases can be confusing to diagnose, even for experts.

SYMPTOMS

The initial symptoms are small dark brown spots surrounded by a yellow halo. Mature lesions are large (1/8 to 1/2"), elliptical or diamond-shaped, and light tan with a yellow halo (Fig. 1). Tan spot symptoms are quite variable depending on the wheat variety. Some varieties have lesions with very large halos and some varieties show almost no halo. Frequently there is a small dark brown spot in the center of the tan spot. This gives the lesions the appearance of an eye. Eventually, older infected leaves begin to die from the tip back as lesions grow together and become less distinct.

In Kansas, tan spot symptoms usually first appear in late March. Heavy infection of a susceptible variety can turn a whole field yellow in a few days. As plants grow, new leaves soon mask the old infected leaves and fields improve in appearance. If weather is conducive, the disease moves to the middle leaves and eventually to the flag leaf.

There are several similar diseases that could be confused with tan spot. Speckled leaf blotch, caused by Septoria tritici, produces tan lesions that contain very small round black speckles that appear like grains of black pepper. The black speckles are fungal fruiting bodies. Careful examination usually allows a clear distinction between tan spot and speckled leaf blotch. Unfortunately, tan spot is easily confused with nodorum leaf blotch caused by Stagonospora nodorum. Under ideal conditions, nodorum leaf blotch produces very small light brown speckles in the lesions. They are much smaller and more difficult to see than with speckled leaf blotch caused by Septoria tritici. Unfortunately, nodorum leaf blotch often fails to form speckles and then it looks very similar to tan spot. Accurate diagnosis often requires special lab procedures. Finally, certain varieties like Triumph 64 and Cimarron exhibit brown lesions with a yellow halo on certain soil types.  This is due to chloride deficiency. The malady differs from tan spot because symptoms are very uniform and all leaves show symptoms simultaneously.

LIFE CYCLE

Although it has been reported in New York, seedborne infection by the tan spot fungus appears to be negligible in Kansas. The tan spot fungus oversummers and overwinters in old wheat residue. Small (approximately 1/64"), raised, black fungal fruiting bodies are often abundant on old residue. These fruiting bodies produce ascospores in early March that are shot a short distance (less than 6") from the fruiting body during spring rains. If the ascospores land on a young wheat leaf, they germinate and initiate primary infections. Infection requires a moist period ranging from 6 to 48 hours, depending on temperature. Other spores called conidia are produced on old, dead infected leaves and also on old straw. The conidia cause secondary cycles of infection during the growing season. The conidia can travel longer distances than the ascospores, but very few go farther than 25 yards. Therefore, severe infections are usually limited to continuous wheat fields with lots of wheat straw on the soil surface. Rye and several species of bromegrass and wheatgrass are alternative hosts of the tan spot fungus. Barley is resistant and oats are immune to the fungus. Alternative hosts do not seem to play a significant role in tan spot epidemics in Kansas.

CONTROL

The best control for tan spot is crop rotation. Even one year out of wheat drastically reduces the primary inoculum produced on old wheat straw. A full year of fallow is also effective. Burning or plowing to destroy or bury wheat straw are options on ground that is not highly erodible. However, recently developed resistant cultivars make residue destruction unnecessary. Karl, Karl 92, Jagger, 2137, Tomahawk, and many others have good resistance to tan spot. See "Wheat Variety Disease and Insect Ratings", MF-991 for details. The fourth option is foliar fungicide application. Fungicide sprays are directed at protecting the flag leaf for as long as possible. Unfortunately, tan spot is difficult to control well with just one application of currently labeled fungicides. Of the currently labeled products, Tilt is systemic and has good activity against tan spot. Mancozeb is also good, but it is not systemic and may be washed off by excess rain. Quadris is partially systemic and has superior effectiveness against tan spot.

revised 11 June, 2000

For more information about wheat, visit the Kansas State University Wheat Page

It is the policy of Kansas State University Agricultural Experiment Station and Cooperative Extension Service that all persons shall have equal opportunity and access to its educational programs, services, activities, and materials without regard to race, color, religion, national origin, sex, age, or disability.  Kansas State University is an equal opportunity organization.   These materials may be available in alternative formats.

Web updated 9/01/06
Web comments to jpierz@ksu.edu