Loose and Semiloose Smuts

Washington State University Bulletin

SP0004 -- 1993

Diseases of Washington Crops.

Otis C. Maloy and Debra Ann Inglis

 

Small Grains Home * Wheat Diseases * Barley Diseases

Cause

Hosts

Symptoms and Signs

Disease Cycle

Control

1997 PNW Pesticide

Recommendations

Other interesting Loose and Semiloose Smut sites

Loose smut and black or semiloose smuts have not been major problems in Washington except when seed is imported from the Midwest where cool, moist conditions during flowering promote loose smut infection. However, more loose smut has appeared recently in locally produced seed, either because weather conditions have favored infection or because some newer cultivars are more susceptible.

Cause
Different fungi, all of which are basidiomycetes, cause these diseases. Ustilago avenae causes semiloose smut on oats. Black or semiloose smut of barley is caused by U. nigra, U. nuda and U. tritici cause loose smut on barley and wheat, respectively.

Hosts
Hosts are oats, barley, and wheat in order of relative disease occurrence.

Symptoms and Signs
Diseased heads emerge from the boot a few days earlier than healthy heads and are replaced with black, powdery masses of smut teliospores (Photo 8). These masses are first enclosed by remnants of the cereal pericarp. However, this covering soon disintegrates to release the teliospores. In barley, infected plants may be slightly taller than healthy plants.

Disease Cycle
The fungus over-winters as dormant mycelium in infected grain. Infected seeds germinate normally, but the mycelium grows along with the growing point of the plant until it reaches the head. In the kernels it transforms into masses of teliospores. The teliospores are windborne to the open flowers of a respective host plant. Cultivars that have more open flowers or a longer flowering period are more susceptible.

The teliospores germinate but produce no basidiospores. Instead, dikaryotic (binuclear) hyphae form and penetrate through the ovary wall or possibly the stigma to establish in the embryo (seed infection). Temperatures of 61 to 72F and moisture from dew or light rains favor both teliospore germination and infection. Semiloose smut differs in its disease cycle, in that seeds carry the teliospores and infection takes place after seed germination (seedling infection).

Control
Chemical. Before the development of systemic fungicides, the only effective treatment consisted of heating the grain seed in water at 130F for 10 minutes before planting. Now carboxin seed treatment is the most important control method. Its use has eliminated this cumbersome procedure.

Cultural. Plant only seed certified to be smut free.

Resistance. Some cultivars are more resistant or are less susceptible than others, but it is not always convenient or profitable to grow them.

References
Loria, R., M.V. Wiese, and A.L. Jones. 1982. Effect of free moisture, head development, and embryo accessibility on infection of wheat by Ustilago tritici. Phytopathology 72: 1270-1272.

Thomas, P.L. 1981. Distinguishing between the loose smuts of barley. Plant Disease 65:834.

Other Interesting Loose Smut sites:
UC Pest Management Guidelines, SMALL GRAINS LOOSE SMUT -- descripttion and pictures.