Loose Smut of Wheat

Loose Smut of Wheat

Loose smut is one of the most common seedborne diseases of wheat. It can be found in wheat growing areas throughout the world. Fortunately, yield losses caused by the disease are less than 1%. In severe cases yield loss may be as high as 27%. The causal organism of the disease is Ustilago tritici (Pers.) Rostr. All cultivated wheats as well as rye, triticale, and barley are cereal hosts of the pathogen. Grass hosts include some species of the Aegilops, Agropyron, Elymus, Haynaldia, and Hordeum genera. A similar loose smut (Ustilago avenae) occurs on oats. Ustilago tritici overwinters as dormant mycelium in the embryo of infected seed. Seed carrying the fungus appears normal and its ability to germinate is unaffected. The milling and feed quality of the seed is also unaffected. Mycelium of Ustilago tritici is activated by seed germination. The fungus grows intracellularly to the growing point (shoot apex) of the seedling. It continues to grow in this manner through the culm and into the developing spikelet and seed primordia. All developing spikelet tissue except the rachis is invaded. The mycelium then fragments into thick walled brown teliospores. Formation of the sorus (mass of teliospores) occurs before the head emerges from the boot. Smutted heads emerge sooner than healthy heads. Teliospores are held together by a thin membrane of host tissue which easily ruptures. Spores are wind blown or rain splashed to the flowers of healthy heads. Teliospores germinate forming promycelium (basidium) which in turn produces four uninucleate hyphae. Compatible hyphae fuse forming dikaryotic infectious hyphae. No basidiospores (primary sporidia or secondary sporidia) are formed. The infectious hyphae penetrate the ovary wall or stigma and dormant mycelium is established in the embryo. The infection period is restricted to one week beginning at flowering. Environmental conditions that favor infection are humid conditions and moderate temperatures (60-70^oF).

Symptoms

It is very difficult to detect infected plants in the field until heading. At this time, infected heads emerge earlier than normal heads. The entire inflorescence is commonly affected and appears as a mass of olive-black spores, initially covered by a thin gray membrane. Once the membrane ruptures, the head appears powdery. Spores are dislodged, leaving only the rachis in tact. In some cases remnants of glumes and awns may be present on the exposed rachis. Smutted heads are shorter than healthy heads due to a reduction in the length of the rachis and peduncle. All or a portion of the heads on an infected plant may exhibit these symptoms. Other symptoms may be detected if the field is examined closely. While infected heads are shorter, the rest of the plant is slightly taller than healthy plants. Prior to heading affected plants have dark green erect leaves. Chlorotic streaks may also be visible on the leaves.

Plant Health Management

Fungicide Seed Treatment.

Systemic fungicides are affective in controlling loose smut. Compounds labeled for control include Benomyl (Benlate), carboxin + thiram (Vitavax CT) and difenoconazole (Dividend) 1997.

Use cleaned/certified seed collected from smut-free fields.

Scout seed production fields

during flowering for the presence of smutted heads.

Hot Water Seed Treatment.

This management practice has been used in the past but requires controlled water baths and decreases germination so it is not used on a commercial basis.

Solar Heat Seed Treatment.

This control measure has been used in India and Pakistan. It involves soaking the seed in water then exposing it to the sun for drying. It is not well adapted to commercial use in many countries.

References

Agrios, G.M. 1978. Plant pathology, 2nd ed. Academic Press. New York, NY. 703 pp.

Mathur, S.B. and Cunfer B.M., eds. 1993. Seed-borne diseases and seed health testing of wheat. Jordbrugsforlaget. Frederiksberg, Denmark. 168 pp.

Watkins, J.E. and Prentice, L.J. 1997. Diseases affecting grain and seed quality in wheat. Cooperative Extension Service, Institute of Agriculture and Natural Resources, University of Nebraska - Lincoln. EC97-1874.

Wiese, M.V. ed. 1987. Compendium of wheat diseases, 2nd ed. APS Press. St. Paul, MN. 112 pp.

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Caveat

This description is presented for information only and no endorsement is intended 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 researched and drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by: Dr. David Wysong, Department of Plant Pathology, University of Nebraska-Lincoln

Questions, Comments, Complaints and Complements?

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

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