[Leaf spot]
Introduction
There are two Alternaria species that cause spots on leaves, seed capsules and tops. They can occur independently or as a combined infection in all major brassica species. Losses are often the result of effects on appearance and quality. A related species, Alternaria Raphani, attacks radish.
Geographic distribution
Alternaria leaf diseases occur in every part of the world, in both vegetable crops and seed crops.
Symptoms and diagnosis
Early symptoms are small dark spots (1-2 mm in diameter) that resemble spot patterns associated with other pathogens or physiological problems. The leaf spots enlarge and are brown in colour, with concentric rings of sporulating fungal growth. These characteristic ring spots sometimes have a yellow halo. The development of small, secondary lesions around these larger ring spots is a useful aid to diagnosis. It is difficult to distinguish A. brassicae from A. brassicicola by the symptoms on the leaf, but the distinction is easily made based on the shape of the spores when using a microscope. In the event of an epidemic, very large numbers of leaf spots can develop, causing the more mature leaves to die off. During the growing season, the disease spreads across the crop and infection can occur on the stems and sprouts of Brussels sprouts, cabbage heads and cauliflower heads. Alternaria leaf blight also causes problems in stored white cabbage, although the infection can often be cut away from the outside leaves. In seed crops, the seed capsules are damaged; seriously infected seed capsules ripen prematurely and lose their seed.
Conditions for disease development
These pathogens are seed-borne and occur on both the outside and the inside of the seeds. Infected seed can have a lower germination capacity and produce seedlings with infected cotyledons. While levels of Alternaria spp. on seeds have been low in recent years, there are many sources of infection via airborne spores in crop residues, adjacent crops and weeds. In addition, forage brassicas and rape are also affected. Moreover, in some places, airborne spores dispersed during the harvesting of these crops can also be important. Sporulation requires a high level of humidity for at least 12 hours, and infection requires a leaf wetness period of approx. 10 hours for the disease to develop significantly. Both pathogens show little activity below 5 °C, but spore production is optimal at 18-24 °C for A. brassicae and at slightly higher temperatures for A. brassicicola (20-30 °C). The disease cycle lasts just five to seven days and epidemics can develop quickly during hot, wet weather. In radish, the temperatures required for A. brassicae and A. brassicicola and sporulation to occur lie between 13 and 31 °C.
Impact and importance
The most important effects of Alternaria leaf blight concern the appearance of the saleable product. Yields may be lower if a severe infection has caused leaf drop or if additional trimming is required, especially in vulnerable crops such as Chinese cabbage, pak choi and turnip. In seed crops, the disease may affect the yield and seed size. Crops may also be rejected due to a serious infection of the seeds. The severity of the disease may be greater if lodging occurred during or shortly after the crop's flowering. Little is known of resistance to this disease in any brassica species, and growers have no option but to use fungicides for foliar application as soon as crops start to show symptoms. Fungicides are also used in seed crops to protect the seed capsules.
Controlling seed-borne Alternaria spp. with seed treatment contributes to the general management of these diseases.
