Tan spot causing yellowing of wheat planted into wheat stubble

Figure 1. Yellowing of wheat in a grower’s field near Auburn caused mainly by tan spot.  Symptoms of wheat soilborne mosaic virus were also observed in the field (April 18). Figure 2.  Pseudothecia of Pyrenophora tritici-repentis, causal agent of tan spot, on wheat stubble in a grower’s field near Auburn (April 18). Figure 3.  Initial symptoms of tan spot on wheat leaves collected from a grower’s field near Auburn (April 19). Figure 4.  Tan spot lesions causing yellowing of wheat leaves collected from a grower’s field near Auburn (April 19).

Recent observations

On April 13, numerous tan spot lesions were observed on Jagalene wheat drilled into wheat stubble at the South Central Agricultural Laboratory near Clay Center. Due to heavy infections, leaves were turning yellow or brown. Lower leaves were most severely affected. On April 18, yellowing of wheat in a grower’s field near Auburn (Figure 1) was determined to be caused mainly by tan spot.

Cause

Tan spot is caused by the fungus Pyrenophora tritici-repentis. The fungus survives the winter on wheat stubble as tiny, black sexual fruiting structures called pseudothecia (Figure 2). Pseudothecia mature on wheat stubble in the fall and winter. Starting in early spring and continuing throughout the growing season, they release ascospores which cause initial (primary) infections on wheat.

Symptoms

Initial symptoms on leaves and sheaths are small tan to brown spots surrounded by a yellow halo (Figure 3). The spots can be so numerous as to cover entire leaves. As the lesions enlarge, they coalesce, causing leaves to yellow (Figure 4) and giving the field an overall yellow cast (Figure 1). Asexual spores called conidia are produced in the lesions and cause secondary infections.

Factors favorable to disease development

Infections are favored by prolonged wet periods lasting 6 to 48 hours, occur throughout the growing season, and are most numerous near wheat stubble. Rains and dew promote release of spores and expansion of lesions. Spores are spread by wind. The optimum temperature range for disease development is 68o to 82oF. Because the causal fungus overwinters on wheat stubble, tan spot is favored by continuous wheat culture and tillage practices that leave crop residue on the soil surface.

Management

Due to the need to conserve soil and water in Nebraska, it is common practice to leave crop residue on the soil surface. A three-year crop rotation system such as ecofarming (ecofallow) in which a non-host crop is planted into wheat stubble and wheat is planted into residue of a non-host crop can be used to manage tan spot. Fungicide applications can be used to control the disease. They should be timed to protect the flag leaf. Scouting is essential for determining whether and when to apply a fungicide. To realize a return on investment in a fungicide application, the yield potential should be at least 45 bushels per acre for dryland wheat and 85 bushels per acre for irrigated wheat. Planting resistant cultivars can help to reduce damage caused by tan spot.

Stephen N. Wegulo
Extension Plant Pathologist, Lincoln
Jennifer L. Chaky
Extension Educator, Lincoln
Gary W. Lesoing
Extension Educator, Auburn

Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.

Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.

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