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The symptoms of early blight vary depending on the host and plant tissue.
Foliar symptoms are dark brown to black necrosis. The first symptoms usually appear on the older leaves as small, dark, necrotic lesions, a few millimetres in diameter, which increase in size. Sometimes the lesions are restricted by leaf veins and take on an angular shape. The size of the necrosis can vary in width, from a few millimetres to 2 cm. Within larger lesions, concentric rings (so call target spot or bullseye) are visible, often surrounded by a chlorotic, yellowing zone. The chlorosis can extend to the whole infected leaf. The infected lesions enlarge and the whole leaf becomes necrotic which results in premature defoliation. In tomato production, the premature defoliation can cause injury to the fruits due to sunscald.
On tomato, A. solani can cause symptoms on the stem. Dark and sunken lesions can appear on the stems of seedlings, so called collar rot. The infected seedling shows reduced plant vigour or can die when the stem is completely girdled by the lesion. The main stem of adult tomato plants can also be infected, showing small, slightly sunken lesions (stem lesion). As on the leaves, typical concentric rings are visible on the infected stem.
On green or ripe tomato fruits, dark lesions can occur at the end of the stem. Ripe fruits are less susceptible than semi-ripe ones (Mehta et al., 1975). Heavily infected fruits drop prematurely. On less resistant cultivars, the calyx and blossom also can be infected (Pandey et al., 2003) and show comparable symptoms.
The symptoms on potato tubers are dark, slightly sunken lesions (dry rot). The dry or hard rot of tubers causes storage losses, reduces the quality of table potatoes, and reduces germination capacity of seed potatoes.
The control of early blight requires the implementation of several approaches. Integrated pest management is based on good agricultural practices, cultivation of a less susceptible cultivar and the use of biological and synthetic components.
Reduction in the amount of initial inoculum
The fungus survives on plant debris and in the soil for several years. Crop rotation therefore reduces the initial soilborne inoculum. The control of host plants such as black nightshade, volunteer potatoes and tomatoes, and cull piles in the non-host crops is also important to reduce the inoculum. Clearing the infected debris from the field results in inoculum reduction.
Biofumigation is also a possibility to reduce soilborne inoculum. Isothiocyanates (ITC) produced by the hydrolization of glucosinolates by myrosinase in disrupted plant cells suppress the soilborne inoculum. Biofumigative active plants include white mustard and leaf radish (Volz, 2013).
Tillage practices such as autumn ploughing that bury plant refuse can be used (see: https://www.canr.msu.edu/resources/potato_diseases_early_blight_e2991)
Harvest and storage
The potato tuber gets infected during harvest, but the fungus cannot infect through the intact periderm of the tuber. Killing off the foliage before harvesting and allowing tubers to fully mature can therefore prevent tuber infection.
Avoiding wounding of the tubers at harvest, transport and storage and promoting wound healing in storage reduces tuber infection.
Plant nutrition should be based on balanced fertilization. Low nitrogen results in a significantly higher early blightdisease.
The kind of N-fertilizer also influences the disease progression of A. solani. In potatoes, calcium cyanamide results in a delay of early blight disease. The degradation products of calcium cyanamide have a side-effect on the fungus and therefore reduce the initial inoculum in the soil (Volz et al., 2013).
Biotic and abiotic stress
Biotic or abiotic stress results in plants becoming more susceptible to the fungus Alternaria solani. Promotion of plant health therefore results in lower disease progression. Abiotic stress is driven by drought, high temperature and overhead irrigation. Additionally, overhead irrigation, especially at night, can prolong the leaf wetness period and therefore increase the risk of a fungal infection. The use of drip irrigation instead of overhead irrigation reduces this risk.
There are different biotic stress factors for plants during the growing season, mainly associated with attack by insects (aphids, Colorado beetle, Aleyrodoidea species).
Certified seed and seed tuber
The use of virus- and disease-free seeds and seed tubers is important for healthy plant growth. Plant tissues of Potato virus Y-infected tubers are more susceptible to A. solani and PVY-infected plants showed an increased sensitivity for the pathogen (Metz, 2016).
In tomato and aubergine production, the application of plastic or an organic mulch provides a barrier between contaminated soil and leaves.
For greenhouse production, early blight has been reduced by covering houses with UV-absorbing vinyl film.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources: