Tan Spot of Wheat

Tan Spot of Wheat

Tan spot is a very common foliar disease of wheat and has been increasing in importance with the widespread use of conservation tillage. The disease is also referred to as yellow leaf spot and is caused by Pyrenophora tritici-repentis (Died.)Drechs. (anamorph: Dreschlera tritici-repentis (Died.) Shoemaker). The fungus also infects rye and many native grasses including bromegrass and wheatgrass. P. tritci-repentis overwinters as structures called pseudothecia on wheat stubble that is standing, lying on the ground or partially buried. The fungus can also survive on weathering straw in bales. Infection and disease development are favored by wet (rain, mist, or fog) weather. A wet period of 12 to 24 hours with free moisture on the leaves is necessary for infection. Primary infection occurs in the spring (April in Nebraska) when ascospores are released. The spores are most commonly wind blown to lower leaves. Conidia are produced in leaf lesions and serve as secondary inoculum. Both primary and secondary inoculum are produced throughout the growing season when wet weather persists. Inoculum of both types is wind dispersed and the disease progresses from lower to upper leaves. The fungus invades straw tissue late in the growing season, producing pseudothecia. Tan spot commonly occurs in conjunction with leaf rust and Septoria leaf blotch. Yield loss from the disease is generally low but can be of economic importance if the flag leaf becomes infected; as is the case with all foliar wheat diseases.

Symptoms

Initial symptoms occur as lens or diamond shaped tan to brown spots with a distinct yellow border. Spots remain small on actively growing leaves. As the leaves mature, the spots enlarge and coalesce. A small dark brown spot remains in the center of the lesions but the rest of the affected area is tan and the yellow border is less obvious. A heavily infected field appears to have a yellow cast. Pseudothecia become evident on the straw by late summer (mid-August). They appear as numerous small, black, raised bodies and are readily visible.

Plant Health Management

Residue Management

Outright destruction of residue is not an option in a conservation tillage system. In this case ecofarming incorporating a winter wheat-grain sorghum/corn-fallow rotation helps breakdown the inoculum bearing stubble without limiting the valuable erosion control and soil moisture retention properties provided by conservation tillage.

Foliar Fungicides

The use of foliar fungicides may be cost effective if the expected yield in a given field is expected to be 45 bu/acre or greater and the disease appears to be progressing rapidly up the plant. Fungicides labeled for control of tan spot include mancozeb, propiconazole (Tilt) or triadimefon (Bayleton) plus mancozeb.

References

Nyvall, R.F. 1989. Field crop diseases handbook, 2^nd ed. Van Nostrand Reinhold, New York, NY. 817 pp.

Watkins, J.E., Klein, R.N, and E.D. Kerr. 1993. Tan spot disease of wheat. Cooperative Extension, Institute of Agriculture and University of Nebraska-Lincoln, Lincoln, NE. NebGuide G79-429-A.

Willis, W.G. 1984. Wheat diseases. Cooperative Extension Service, Kansas State Univ. Manhattan,KS. Extension Publication S-23.

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Caveat

This description is presented for information only and no endorsement is intented for products listed, nor criticism meant for products not mentioned. Always consult the product label before purchasing and using any pesticide.

Material contained on the Links from the page are the responsibility of the linked page's author(s).

This page was drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by:

Dr. John Watkins

Dr. W.G. Willis,

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This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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