Leptosphaeria maculans (anamorph: Phoma lingam)
Symptoms manifest as oval, sunken, light-brown cankers with purple-to-black margins near the base of stems. Cankers enlarge and girdle stems, causing plant collapse. Lesions may also develop on cotyledons and hypocotyls of young seedlings and appear on leaves as pale, irregular spots. Leaf spots gradually enlarge, becoming circular with gray centers. Under favorable conditions, small black fruiting structures (pycnidia) develop in stem cankers and leaf spots. Severely infected plants are stunted and often wilt. The leaves remain attached and the plant turns a dull blue-red color. The root system may be destroyed, although new roots may form above the stem cankers, allowing the plant to remain alive. When infected cabbage heads are stored, the infection can spread to the base of leaves where brown to black spots develop. On root crops, a dark, dry rot can occur in storage.
Conditions for Disease Development
The fungus can survive in crop debris and cruciferous weeds. Infected seed, however, may also be a source of primary inoculum. In seed beds, infected seedlings generally develop symptoms in two to three weeks. Irrigation water can spread the spores of the fungus to surrounding healthy seedlings. Secondary infection may also occur when the young plants are dipped in water prior to transplant. The disease may also spread by splashing rain, workers and equipment.
Use Leptosphaeria maculans-free seed. Eradicate cruciferous weeds, remove or deep plow plant debris and practice a three-to-four year rotation to non-host. Fumigate, solarize or flood infested fields to help reduce field inoculum levels.