DR, also known as black leg, is caused by the fungus Leptosphaeria maculans. It can be extremely destructive in swede, rape and kale crops, and occurs throughout NZ, most commonly in the lower South Island.
DR appears as small brown lesions on leaves in summer and autumn. On stems and bulbs, DR develops into large brown-black dry and cracked lesions. On stems, black streaking can develop on the surface tissue. In wet weather, lesions ooze a rose-pink spore mass. Infected leaves die, bulbs collapse and stems break at the lesion.
DR can be very destructive, causing 100% crop loss in some instances.
DR over-winters in infected crop residue, and spores can spread up to 1-2 km following rain. DR enters bulbs and stems through growth cracks or insect feeding wounds. Although not common, infected seed can introduce DR into new areas.
Prevention and management
Cultivars with higher DR tolerance should be used in problem areas, and only treated, certified seed should be sown. Good crop rotations are essential. Where DR was noted in a first crop, do not follow up with the same crop in the second year. Good cultivation is necessary to ensure full breakdown of crop debris before resowing.