Leptosphaeria maculans previously named Phoma lingam blackleg stem and leaf canker Fig
Hot water treatment can be used to free seed of L. maculans infestation, by soaking for 25 min at 50°C for cabbage and Brussels sprout (B. oleracea var. capitata and gemmifera) and 20°C minimum for cauliflower and broccoli (B. oleracea var. botrytis and italica). The technique is unreliable, however, often impairing subsequent germination. Treatments with fungicidal slurries are effective in disinfesting artificially infected cabbage (B. oleracea var. capitata) seed, and the use of zinc-based compounds has been successful in eradicating soil borne infection for cabbage (B. oleracea var. capitata) crops.
Breeding for resistance has been made difficult because of the wide variability in the pathogen populations. Crop types can be qualitatively categorized as very susceptible - red and green cabbage, savoy cabbage, Chinese cabbage, Brussels sprout, some radish cultivars, some swede
- Fig. 7.16. Blackleg (Leptosphaeria maculans, previously Phoma lingam) symptoms on Brassica.
cultivars, white mustard and kohlrabi; medium susceptible - cauliflower, broccoli, rape, kale, collards, some turnip cultivars, wild radish and black mustard; mildly susceptible - some turnip and swede cultivars, Chinese mustard, garden cress and many strains of mustard; and resistant -horseradish, penny-cress, ball mustard, yellow rocket, shepherd's purse and pepper cress.
Swede cultivars often appear to be a mixture of susceptible and resistant populations, but the Wilhelmsburger types contain a high proportion of resistant material. Carmelina sativa, S. alba, S. arvensis and B. nigra are good sources of resistance, and attempts are being made to transfer this resistance to B. napus and B. oleracea through both embryo rescue and fusion and resistance tests applied to protoplasts using toxins. Some savoy cabbage, kale, turnip rape and white mustard express quantitative resistance and are being used to transfer resistance to white cabbage.
Husbandry controls include the use of rotations that ensure breaks of 4-5 years between Brassica crops. Legumes such as lucerne (M. sativa) or clover (Trifolium spp.) are particularly good break crops. Provision of adequate soil drainage and manipulation of plant stand density ensuring rapid air movement are essential, thereby discouraging the build up of a moist microclimate within the crop. Ploughing should be done deeply in the autumn to hasten destruction of infested debris.
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