Dwarf Smut

Washington State University Bulletin

SP0004 -- 1993

Diseases of Washington Crops.

Otis C. Maloy and Debra Ann Inglis

Small Grains Home

Cause

Hosts

Symptoms and Signs

Disease Cycle

Control

1997 PNW Pesticide

Recommendations

Dwarf smut or dwarf bunt, called TCK in the popular press, has become a serious disease to Washington wheat growers because of the threat of economic embargoes and subsequent effects on the export of Washington wheat.

Cause
Dwarf smut is caused by Tilletia controversa Kuhn (hence the TCK). The fungus, a basidiomycete, produces reticulate teliospores similar to those of the common smut fungus. The reticulations are deeper and larger than on common smut teliospores (hence fewer pits per spore), and are covered with a gelatinous sheath. Many pathogenic races occur.

Hosts
Wheat is the major economic host, but barley, rye, and many grasses also become infected.

Symptoms and Signs
Dwarf smut resembles common smut, except that plants are more of a bluish green, and severe stunting occurs in some cultivars (Photo 6). Because shoot infection takes place at the soil surface after tillering, individual tillers may be affected while others on the same plant remain healthy. The smut balls emit a foul, fishlike odor when crushed.

Disease Cycle
The disease cycle differs somewhat from that of common smut. At harvest, smut balls release teliospores which reside in or on the soil or are carried on the seed. But, they can persist in the soil for as long as 10 years if the smut balls remain intact. The teliospores germinate only after exposure to low temperatures (37-46F is optimum) and diffuse light. Under optimal conditions, germination may extend over a period of 3 to 10 weeks. In the field, prolonged snow cover on unfrozen ground favors dwarf smut development, presumably because it provides the high moisture, cool temperatures, and diffuse light needed for teliospore germination. The disease mainly is a problem in localized areas of the northwestern United States where these conditions recur.

Teliospore germination, fusion of basidiospores, and production of secondary basidiospores also occur with dwarf smut, in stages similar to those of common smut. However, because teliospore germination only occurs at the soil surface, infection occurs through the developing tillers (shoot infection). Infections usually begin in December and extend into early April. Most infections occur in January and February, after wheat seedlings begin to tiller. Spring grains are rarely infected.

Control
Chemical. Because infection takes place some distance away from the seed after seeding, seed treatments have not been very effective. However, the systemic fungicide TBZ gives some control, especially when used with resistant cultivars.

Cultural. Cultural practices such as very early or very late seeding, seeding deeper than 2 1/2-3 inches, avoiding soil compaction and avoiding sites conducive for teliospore survival and germination may reduce dwarf smut incidence.

Exclusion. Quarantines, such as the embargo of Pacific Northwest wheat by The People's Republic of China, are used to prevent the introduction of the pathogen. However, teliospores can be detected in railroad cars and in oceangoing vessels and have been reported viable after ingestion through animals, so they are probably widely disseminated despite regulatory measures. Importing grain with low levels of teliospores probably presents only minimal risk in causing disease outbreaks; studies indicate that as many as 20,000 teliospores per seed are necessary to cause significant infection.

Resistance. Planting resistant varieties is the primary method used to control dwarf smut. Because new pathogenic races develop and because some growers plant older, susceptible cultivars, other measures (chemical and cultural) are still needed.

Reference
Grey, W.E., D.E. Mathre, J.A. Hoffmann, R.L. Powelson, J.A. Fernandez. 1986. Importance of seedborne Tilletia controversa for infection of winter wheat and its relationship to international commerce. Plant Disease 70:122-125.