Sclerotina and blackleg [Leptosphaeria maculans]
Sclerotina and blackleg - image 1
Better known under the name Phoma or Phoma lingam (asexual form of the pathogen)
Winter and spring rape. Other cruciferous species serve as host plants.
Pattern of damage
The leaves exhibit sharply delineated, yellowish spots with a center that appears light-gray. On the light-gray area, small black dots (pycnidia = spore structures) are visible. If the spots on the leaves expand, the tissue starts to rupture. In case of a severe infection, the leaves die off before winter. The sclerotina and blackleg originates from the pycnidia. Small, dark-brown to black spots develop at the root crown. These expand to stem-encompassing constrictions and decompositions. In spring, the infection at the stem expands and it appears dark-brown, fissured, and decomposed. The plants may die or break off (Picture 1). In the upper stem section, a dark edge delimits the spots from the healthy tissue. Consequence of the disease are increased storage risk, premature ripening, and a reduced thousand-seed weight. There is a likelihood of confusion with Cylindrosporium disease and white mold, downy mildew, cylindrosporiosis, annular spot disease, black spot, stem rot, Verticillium wilt and gray mold disease.
Good tilling of oilseed rape residue. Cultivation of more tolerant varieties. Prevent plant damage (application of insecticide against the cabbage stem flea beetle and ceuthorrynchus species).
Spread/ transmission: Ascospores and pycnidiospores infect the new seeds in the fall (primary infection). Stoma and wounds serve as entry sites. A necrotic tissue forms around the affected sites on which pycnidia develop. These fruiting bodies release numerous pycnidiospores which are responsible for the secondary infection in the spring. They are propagated via wind and rain. The fungi survives on affected harvest residues and forms spore structures from which a renewed infection can originate. A seed infection is also possible if spores reach the grain during harvest.