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Disease Management

Seed Treatment

Perhaps one of the greatest success stories in plant disease control is found in the use of chemical seed treatments for control of stinking smut. Early in the history of seed treatment, compounds containing copper were used, e.g., copper sulfate. Later, mercury compounds were found to be even more effective, particularly in killing the seedborne teliospores. However, even the mercury compounds on the seed surface were not effective in eliminating infection from soilborne teliospores. The compound hexachlorobenzene (=HCB) was effective, and was widely used in the Pacific Northwest states where soilborne inoculum was prevalent. In recent years, however, even more effective compounds for seed treatment have been developed (Figure 8). Most of these are called "systemic" fungicides because they actually move systemically into and inside of the developing seedling to provide internal protection against the smut fungi. The first such compound was carboxin, which was sold initially in the early 1970s and is still used today. Other systemic fungicides have since been discovered, including difenoconazole and tebuconazole, which are even more effective in small doses than carboxin. To date, no significant problems have occurred with the development of fungicide resistance in the smut fungi. This is probably due to the fact that there is only one cycle of reproduction of the smut fungus each year, rather than multiple cycles as occurs in many of the foliar infecting fungal pathogens where fungicide resistance has been well documented.

Figure 8

Seed treatment is relatively inexpensive, very effective, and environmentally safe since very little of the active ingredient is applied per unit of land (Figure 9). For growers who do not want to use chemical seed treatments, the use of organic seed treatment does offer some control, i.e., use of skim milk powder.

Figure 9

Resistant Cultivars of Wheat

Stinking smut also can be controlled by the use of disease resistance. In the early part of the 20th Century when highly effective seed treatments were unavailable, a great effort was made by wheat breeders to produce smut-resistant cultivars. Today, that effort is almost nonexistent for two reasons. First, the smut fungus often mutated to form new strains, which could attack the new resistant cultivars. Hence, a cultivar's usefulness in regard to smut resistance was often short lived. Second, the effectiveness of the modern systemic seed treatments has been so great, and the wheat grower's adoption of the practice of treating their seed is so widespread in the "smut prone" areas of the United States, that wheat breeders can spend their efforts on other problems, e.g., yield enhancement, quality enhancement, and resistance to other pests and diseases. While most planted cultivars are susceptible to stinking smut, some are resistant, especially those developed with resistance to dwarf bunt (See Signficance section).

Seeding Date

The severity of stinking smut can also be manipulated to a certain extent by choosing a planting date when the soil temperature is above that for optimal teliospore germination, e.g., above 20°C (68°F). This usually means either early fall seeding for winter wheat, or late spring seeding for spring wheat. While this practice can reduce the incidence of smut, it rarely eliminates the disease altogether. Although this practice can reduce the incidence of smut, it rarely eliminates the disease altogether. Generally, stinking smut is more of a problem in winter wheat than in spring wheat due to the longer period of more favorable temperatures for teliospore germination in the autumn when winter wheat is planted.

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