Blackleg of Brassicas and Canola Leptosphaeria maculans
Black Leg is derived from the typical black lesion which ring barks the stem close to the ground.
Grey circular spots on leaves that enlarge and develop tiny black specks in the centre with time. They are somewhat constricted by the veins on leaves. Light brown to purple streaks often with a darker purple halo and eventually with tiny black specks develop on stems and petioles. These increase in size, turn black and girdle the stem causing the plant to wilt and die. Stems may be cracked and corky. Seedlings can be infected and die before the 3 or 4 leaf stage. The first symptoms that are often noticed are wilting of the plants and slightly red edges on the leaves. A dry brownish rot may develop on the tubers of Swedes and Turnips. This may develop cracks and black fruiting bodies may be produced.
Brassicas, Canola, Rapeseed, Swede, Turnip
Favoured by wet, windy conditions. Survives on crop residues and in the soil for up to 4 years. Spread by spores discharged during rain or irrigation and spread mainly by water. Insects and machinery can spread spores. Seed may also carry spores. Has a sexual stage which allows it to become more virulent on resistant varieties with time. 95% of airborne spores come from last year's canola stubble.
Origin and History:
Very damaging to sensitive varieties of Canola. Breakdown of Blackleg resistance in Canola in the 1970's crippled the industry and breakdown of the sylvestris based resistance in SA in 2003 resulted in 90% yield reductions for many growers. Resistant varieties often form lesions but they have little economic effect.
Management and Control:
Treat seed with hot water. Remove crop residues by deep burial, burning or grazing soon after harvest. Allow 3-4 years between Brassica or Canola crops.
Sow resistant varieties See Disease Susceptibility of Canola Varieties Avoid varieties with sylvestris based resistance. Rotate canola varieties to reduce the risk of encouraging virulent strains of the disease. New varieties have a number of minor genes that confer tolerance. Don't plant Canola crops close to paddocks with infected stubble - leave at least a 500 metre buffer. Use fungicidal seed dressings or fungicides applied in furrow on the fertiliser. See http://www.agric.wa.gov.au/cropdiseases for a forecast of the disease in WA.
Brassica vegetable crops
Use healthy seedlings if transplanting. Apply fungicides at the first sign of disease.