DISEASES

All Regions

Sclerotinia stem rot is most severe in wetter areas and years. It is caused by fungus Sclerotinia sclerotiorum which has a wide host range. The source of fungus is sclerotia in soil where they can remain dormant for five years or more. During prolonged moist conditions and moderate temperatures (at least for ten days), sclerotia produce apothecia that are the primary source of spores. Spores are carried by air and land on flower petals which eventually fall onto leaves and the growing mycelium invades the plant tissue. Contact moisture is required for successfull infection. In the southeast, infection also can occur during the rosette stage where mycelium grow on dead leaves still attached to the plant crown during wet conditions in the winter. The recommended management strategy is crop rotation with at least a four-year break between suceptible crops (beans, sunflowers, peas, lentils, alfalfa, clover, carrot, and potato). However, crop rotation can reduce risk only within a particular field and does not protect against air-borne inoculum from nearby fields. No resistant varieties are available. No fungicides are labelled for use.

Blackleg is a disease also favoured by moist conditions. The disease is caused by fungus Leptospheria maculans. The source of inoculum could be infected seed or crop residue in the field. In areas not infected previously with blackleg, seed is the source of infection; whereas in areas with known incidence of the disease, infected crop residue is the main source of the disease. Long-range infection of up to several miles is due to ascospores produced from the last year's crop residue. Infected plants produce pycnidia spores that cause secondary or short-range infection. The disease spreads over the plant and could infect the pods and seeds. The recommended management strategy for preventing seed-borne infection is to use healthy seed or treat seed with a systemic fungicide. Other practices are crop rotation with an at least three-year break before planting canola and an isolation distance of at least five miles from previous year's infected field. Cultural control also includes crop residue sanitation and control of weedy and volunteer hosts. Some cultivars exhibit some degree of resistance, especially Argentine types. No fungicides are labelled for foliar application. Fungicide seed treatments of benomyl suppress seed transmission of blackleg.

Alternaria black spot is caused by fungi Alternaria brassicae and Alternaria raphani. The disease is favoured by warm humid conditions. The source of inoculating spores is infected crop residue, infected seed, or cruciferous weeds. The spores germinate, penetrate the plant tissue, and cause lesions that produce more spores. Management includes crop rotation with at least three years without a host plant, control of cruciferous hosts of the disease, sanitation, and using healthy seed. Seed treatment only suppress infection. Early swathing may reduce shattering. Captan suppressess seed transmission of alternaria.

This site was last updated on April 19, 2000 at 04:39 PM -0500