Mycosphaerella brassicicola (Asteromella brassicae): ring spot of brassicas.
Especially prevalent on winter cauliflower, Brussels sprout and cabbage in Great Britain, usually associated with intensive cropping and overlapping cycles of production.
Disease results from the sexual stage of this fungus, which is seen most frequently on older leaves as small, black specks arranged in concentric circles, hence the name "ring spot". These produce ascospores, which are the infective stage. The asexual stage, Asteromella brassicae, may cause leaf spots and form conidia but these are not thought to be infective. Some strains of this fungus may have a restricted host range, attacking only cabbage, but there is little evidence for true physiological specialisation.
This pathogen is capable of invading brassicas at all growth stages, from seedlings to mature plants. It is normally most prevalent on older crops. Foliage in ware crops and seed pods is most vulnerable to M. brassicicola.
Symptoms develop initially as small dark-green/brown or grey/black spots, 3-5mm in diameter. These increase up to 2-3cm in diameter and are grey when dry and blackened when wet.
Veins restrict the expansion of the lesions so they can be angular in shape — a feature that distinguishes M. brassicicola from Alternaria spp. A yellow halo may surround the ring spot. In cases of severe infection the entire leaf yellows and leads to early senescence and defoliation. Concentric rings of fruiting bodies are visible after about four weeks.
Ring spot symptoms can be differentiated from Phoma by the small size and high density of the fruiting bodies. Usually ring spot affects the older and more mature foliage but in severe cases younger leaves are invaded.
Serious economic damage results from the invasion of buds of Brussels sprout, where lesions 3-5mm in diameter form initially on the lower parts of the crop and these expand to 1-2cm in diameter in favourable weather conditions.
Disease symptoms may develop while cabbage heads are stored. The ring spots can penetrate though several layers of leaves resulting in the need for extensive trimming in preparation for sale. Where infection is severe the cabbage heads shrivel and become leathery. Infected seed pods carry symptoms similar to those found on leaves. These can be confused with damage caused by Alternaria or white leaf spot.
Temperatures favouring infection are 16-20°C with brief periods of leaf wetness. The pathogen penetrates by invading through the stomata. Symptoms develop in about 10-14 days after infection. Sexual reproductive bodies appear following four consecutive days of 100 per cent relative humidity, usually taking about three weeks after the initial infection. Severe disease develops after prolonged wet weather extending over a week to a fortnight.
Once ring spot is established in a crop, ascospores from the leaf lesions rapidly spread through the canopy. These spores are also transmitted in air currents between crops.
Although ring spot is aerially spread and affects crop foliage, soil husbandry may influence the severity of disease. Deep, well-structured soils drain more quickly, limiting moisture build-up in crop canopies. Crop nutrition should be adequate but not excessive, avoiding unnecessary levels of soil nitrogen resulting in lush growth, which is susceptible to infection and severe disease.
Applications of potassium at 400kg per hectare are considered as a means of avoiding this disease, possibly by stimulating generalised resistance within the plants. Some similar claims have been made for the use of seaweed extracts applied to developing and maturing crops, especially cauliflower. Irrigation where used should avoid leaving the foliage wet overnight.
Integrated disease management
Some levels of resistance are claimed for cultivars of Brussels sprout and cauliflower.
This pathogen may be seed-borne so only use certified seed that has been hot water treated or dressed with a permissible fungicide. Careful crop management is essential, ensuring that young crops are not placed adjacent to older maturing and infected ones. The chief source of infection is attributed to diseased crop debris.
Agricultural brassicas such as oil seed rape and kale used as game cover are reservoirs of infection that will aggressively spread onto horticultural crops. To break infection cycles, crops should be rotated with non-brassicas. Crop residues should be deeply ploughed in to bury infected tissues. Crop monitoring and disease-forecasting systems should be used to apply agrochemicals effectively with minimal cost and maximum effect.
Currently allowable chemical treatments include:
Leaf and flower head brassicas
• Boscalid + pyraclostrobin
• Chlorothalonil + metalaxyl-M (reduction)
• Tebuconazole (off-label)
• Tebuconazole + trifloxystrobin
• Tebuconazole (off-label)
For use on all fresh produce, vegetable crop growers must in advance of use ensure that a particular commercial agrochemical formulation is legally acceptable for their particular crop/husbandry regime and also accepted by the intended purchaser’s crop-quality standards specification as agreed with the relevant crop technologist.