Flag Smut

Washington State University Bulletin

SP0004 -- 1993

Diseases of Washington Crops.

Otis C. Maloy and Debra Ann Inglis

 

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Cause

Hosts

Symptoms and Signs

Disease Cycle

Control

1997 PNW Pesticide

Recommendations

Flag smut was not a problem in Washington until the release of the club wheat cultivar, 'Paha,' which is extremely susceptible. For example, in test plots in Klickitat County, the cultivars 'Gaines' and 'Nugaines,' had less than 5% infection, whereas 'Paha' had about 95% infection. Elimination of whatever genotype resulted in this high susceptibility will probably reduce the importance of flag smut.

Cause
Flag smut is caused by Urocystis agropyri (synonym U. tritici), a basidiomycete. The fungus forms small spore balls consisting of 1 to 4 teliospores surrounded by a layer of smaller sterile cells. The teliospores are red-brown, smooth and round. They germinate to produce a short hypha (promycelium) with 3 to 4 basidiospores (sporidia) near the tip.

Hosts
Wheat is the only host of the strain attacking it. Other strains of the fungus attack a number of grasses.

Symptoms and Signs
Leaves are twisted laterally, giving the appearance of hormone-type herbicide injury, and heading may be prevented. Diseased plants are stunted and produce excessive numbers of tillers. Usually systemic, the infection normally affects all tillers. Occasionally, only isolated tillers are affected. At or near heading, long, gray-black streaks develop between the veins on the leaf blades and sheaths (Photo 7). These streaks, the smut sori, have a greasy appearance. The sori split open to release the teliospore masses, resulting in ragged, tattered leaves.

Disease Cycle
Teliospores survive in the soil or on stored seed for up to 4 years. They germinate in the soil at temperatures ranging from 40 to 86F. Optimum germination occurs at 64-77F. Seedling infection takes place soon after seed germination since the basidiospores fuse and the infectious hypha infects the coleoptile prior to emergence. Low soil moisture (10-15% of field capacity) and temperatures between 50 and 68F favor infection. The fungus over-winters as mycelium in seedlings, then systemically invades and sporulates within the upper portions of the plant.

Control
Chemical. Seed treatment with the systemic fungicide carboxin is highly effective.

Cultural. Shallow planting is better than deep planting to reduce infection, probably because the seedling is in a susceptible stage for a shorter period of time. A 1- or 2-year break in wheat cropping may be beneficial.

Resistance. Use resistant cultivars or at least avoid very susceptible cultivars such as 'Paha' and 'Luke.'

References
Line, R.F. 1972. Chemical control of flag smut of wheat. Plant Dis. Rep. 56:636-640.

Nelson, B.D., Jr., and R. Duran. 1984. Cytology and morpho- logical development of basidia, dikaryons, and infective structures of Urocystis agropyri from wheat. Phytopathology 74:299-304.