Phoma stem canker

Leptosphaeria maculans is a significant ascomycete fungus and the primary causal agent of phoma stem canker, also known as blackleg, in various Brassicaceae crops, most notably oilseed rape. This pathogen is globally recognized as one of the most economically damaging diseases for oilseed production, capable of causing devastating yield losses if left unmanaged.

The disease primarily manifests as leaf spots, which later progress into severe necrotic lesions at the base of the stem near the soil line. These stem cankers weaken the plant structure, leading to lodging, impaired nutrient transport, and premature plant death. The presence of tiny black pycnidia within these lesions is a diagnostic symptom of the infection.

The lifecycle involves both sexual and asexual reproduction stages. During autumn, ascospores released from infected crop debris are wind-dispersed to infect new seedlings. Secondary infection occurs throughout the growing season via conidia produced in pycnidia, which are spread by rain splash. Moist, temperate conditions ranging from 15°C to 20°C create the optimal environment for disease progression.

The economic impact is severe, as the pathogen restricts the xylem and phloem functionality, preventing proper pod filling and seed development. Under severe infection, crops may suffer from significant reductions in biomass and oil quality. Because the inoculum can persist in the soil on crop residues for several years, managing the field environment is crucial.

Strategies to mitigate the impact of this pathogen include:

• Maintaining a wide rotation gap of at least 3-4 years between Brassica crops.
• Utilizing resistant cultivars that possess major resistance genes.
• Implementing stubble management practices to encourage the decomposition of infected debris.
• Applying foliar fungicides during critical growth stages, specifically targeting early autumn infections to protect the stem base.