Canola Council

Canola Watch

Sclerotinia Stem Rot - Causes and Appearance

Stem rot is caused by the fungus Sclerotinia sclerotiorum which occurs in all canola growing areas of Canada. The disease is usually most severe in wetter areas. The severity of stem rot varies from year to year, and even from field to field within a region. With the right combination of crop density and weather conditions or irrigation, heavy infections can develop almost anywhere.

Plants are infected when the canola crop is in bloom. Variation in the percentage of infected plants is due to differences in the quantity of infectious spores, plant population, crop height and vigor, rainfall, soil moisture, and temperature. Even after plants are infected, the severity of stem rot symptoms and the resulting effect on yield will vary according to temperature, rainfall, crop density and especially the stage of crop growth at the time of infection. In rare cases half the potential yield of a crop may be lost to sclerotinia.

Yield losses reflect yield reduction per infected plant and the percentage of infected plants in a crop. In general, when conditions for the disease are favorable and infections occur throughout the flowering period, yield reduction per infected plant can equal 50% (0.5). In this situation, if 25% (0.25) of the plants in a crop are infected and the yield potential is 40 bushels per acre, the bushel loss would be 0.5 x 0.25 x 40 = 5 bushels per acre. However, if infections are delayed until late bloom or if dry weather set in after early bloom, yield reduction per infected plant may be as little as 10% (0.1). Again using the example of a crop with 25% (0.25) infected plants, the bushel loss would be 0.1 x 0.25 x 40 = only 1 bushel per acre. These type of calculations may be very important when deciding whether to spray to control the disease.

Appearance

Two to three weeks after infection, soft watery lesions or areas of very light brown discoloration become obvious on the leaves, main stems and branches. Lesions expand, become grayish white in color, and may have faint concentric markings. Plants with girdled stems wilt, ripen prematurely and become conspicuously straw-colored in a crop which is otherwise still green.

Figure 45

The stems of infected plants eventually bleach and tend to shred and break. Infected plants may produce fewer pods per plant, fewer seeds per pod or small shriveled seeds that blow out the back of the combine. The extent of damage depends on whether the main stem or a branch is infected and at what stage during flowering infection occurs. Severely infected crops frequently lodge, shatter at swathing, and are difficult to swath.

When the bleached stems of diseased plants are split open, a white moldy growth and hard, black resting bodies (sclerotia) are visible. Sclerotia vary in size and shape. They may be small and round like a canola seed, or up to 2 cm (3/4 in.) long and cylindrical, ovoid or irregular in shape. Under moist conditions, sclerotia and the white moldy growth may also occur on the surface of infected areas of the plant. At harvest the sclerotia are either threshed out with the seed or left in the field.

Sclerotia inside Stems

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