Leptosphaeria stem canker

Leptosphaeria stem canker, commonly known as Phoma leaf spot or blackleg, is a major fungal disease caused by members of the Leptosphaeriaceae family, primarily Leptosphaeria maculans and Leptosphaeria biglobosa. It is one of the most economically significant diseases affecting brassica crops, especially oilseed rape worldwide.

The symptoms begin as leaf spots that are pale grey with dark edges, containing numerous pycnidia (small black fruiting bodies). As the disease progresses, the fungus grows from the leaf petioles into the stem, causing necrotic lesions at the base of the plant. This characteristic canker weakens the stem, often leading to lodging and premature senescence of the crop.

The disease cycle is driven by the fungus surviving on crop debris in the soil. During autumn and spring, rainfall triggers the release of ascospores from pseudothecia, which are then wind-dispersed to infect healthy plants. Warm and wet weather conditions significantly increase the rate of spore dispersal and successful infection on young seedlings.

The impact of this pathogen is severe; it directly affects the vascular system of the plant, cutting off nutrient and water supply to the grain-filling parts. Yield losses can be substantial, often ranging from 10% to 50% depending on the timing of the infection. Early infection of the hypocotyl is particularly devastating, potentially killing the plant entirely.

Management strategies focus on an integrated approach:

• Planting resistant or tolerant cultivars.
• Ensuring proper crop rotation to avoid short intervals between brassica plantings.
• Deep plowing to bury infested stubble and reduce initial inoculum.
• Applying fungicides during critical growth stages, specifically in the autumn or at the onset of stem elongation.