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Disease Cycle and Epidemiology

Disease Cycle

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These two smut fungi survive between growing seasons as teliospores on the surface of healthy seed or in the soil (Figure 4). They can remain viable in either location for a number of years, perhaps ten years or more, particularly if the spores remain dry on the seed surface.

When environmental conditions are favorable, each teliospore germinates, producing sporidia (basidiospores) (Figure 7). After compatible sporidia anastomose (fuse) to form the H-shaped structure, the resulting dikaryotic, infectious hypha penetrates a seedling. As the plant grows, the fungal hyphae also grow, keeping pace with the apical meristerm. Eventually, the hyphae replace the cells of the seed, and the individual cells of the smut fungus become teliospores.

During harvest, the smut spores are released from infected heads as the heads pass through the combine (Figure 5) used to harvest the grain. They contaminate other seed being harvested or are spread by the wind to the soil surface, later to be incorporated into the soil mass during cultivation.


Figure 4	Figure 7	Figure 5

Epidemiology

Cool temperatures (5-15°C/41-59°F) favor the germination of the teliospores. Usually the soil moisture, which favors seed germination, also favors spore germination. Therefore, for winter wheat which is planted in the fall, infection is favored when the seed is planted later when soil temperatures are cooler. Planting winter wheat early late when the soil temperature is above 20°C (68°F) results in very low infection (see Disease Management). In contrast, early seeding of spring wheat when soil temperatures are cool favors infection.

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