Stem canker (blackleg disease) on oilseed rape

Stem canker / blackleg disease is caused by the ascomycetous fungus Phoma lingam. The disease is one of the most economic important diseases on oilseed rape, cabbage and other crucifers. Typical symptoms on oilseed rape are leaf symptoms, which may become visible on cotyledons or true leafs with the onset of ascospore discharge starting at mid of September (fig.1). Furthermore, significant stem lesions may be obvious after elongation of the stem, which in most instances possess a dark margin (fig.2) and small dark dots representing the fruiting bodies of the fungus. Symptoms at the stem basis (rotting, girdling) are of uttermost economic importance due to the restrictions to water and nutrient supply of the upper sprout parts. Transport of water and nutrients may be blocked by rotting of plant tissues including the vascular system.  Furthermore, lodging of plants will be a consequence of stem basis tissue. Yield losses encompassing 30% were related to this disease.
From autumn to spring fruiting bodies of the sexual stage of the fungus (Leptosphaeria maculans, L. biglobosa) - pseudothecia -are ripening on stem debris of oilseed rape (fig.3). These fruiting bodies are also the resting stages of the fungus. Under favorable conditions (humidity, temperature) sexual recombination procesess result in six celled, fusiform ascospores (fig.5). These ascospores are actively discharged and distributed by wind over huge distances (>8km).

Distributed ascospores may find new host plants and will germinate on their leaf surface. Hyphae penetrate the plant tissue through wounds or stoma and spread intercellularly. After a latence period pale leaf lesions will be produced (fig.1). Vegetative fruiting bodies - pycnidia - will be formed within these lesions (fig. 4). Pycnidia produce masses of single celled rod shaped conidiospores (bottom-right). Grey or pinkish spore loops will be released. These spores can be transmitted to new plants via direct contact, rain splashes or insects like the stemweevil (Ceutorhynchus napi). After early infection the fungus may grow systemically to the stem basis and cause rot of the stem basis. After the harvest stem debris will be left on the soil. If isolates of different mating types are present, again pseudothecia and ascospores will be formed under favourable conditions closing the life cycle of the fungus.
Beside the above described ways of infection also french studies report on seed contaminations which were related to pod infections.