Alternaria solani

Alternaria solani infects stems, leaves and fruits of tomato (Solanum lycopersicum L.), potato (S. tuberosum), eggplant (S. melongena L.), bell pepper and hot pepper (Capsicum spp.), and other members of the family Solanaceae. Distinguishing symptoms of A. solani include leaf spot and defoliation, which are most pronounced in the lower canopy. In some cases, A. solani may also cause damping off. On tomatoes On tomato, foliar symptoms of A. solani generally occur on the oldest leaves and start as small lesions that are brown to black in color. These leaf spots resemble concentric rings – a distinguishing characteristic of the pathogen – and measure up to in diameter. Disease severity due to A. solani is highest when potato plants are injured, under stress or lack proper nutrition. High levels of nitrogen, moderate potassium and low phosphorus in the soil can reduce susceptibility of infection by the pathogen. ==Disease cycle==

Alternaria solani is a deuteromycete with a polycyclic life cycle. Alternaria solani reproduces asexually by means of conidia. A.solani is generally considered to be a necrotrophic pathogen, i.e. it kills the host tissue using cell wall degrading enzymes and toxins and feeds on the dead plant cell material The life cycle starts with the fungus overwintering in crop residues or wild members of the family Solanaceae, such as black nightshade. In general, development of the pathogen can be aggravated by an increase in inoculum from alternative hosts such as weeds or other solanaceous species. Disease severity and prevalence are highest when plants are mature. ==Environment==

Alternaria solani spores are universally present in fields where host plants have been grown. ==Management==

Cultural control • Clear infected debris from field to reduce inoculum for the next year. • Water plants in the morning so plants are wet for the shortest amount of time. • Use a drip irrigation system to minimize leaf wetness. Wet leaves provide optimal conditions for fungal growth. • Use mulch so spores that were already in the soil are blocked from splashing onto the leaves. • Rotate to a non-solanaceous crop for at least three years. The more potato-free years, the less infection. • If possible control wild population of Solanaceae. This will decrease the amount of inoculum to infect your plants. • Closely monitor the field, especially in warm damp weather when it grows fastest, to reduce loss of crop and spray fungicide in time. • Plant resistant cultivars. • Increase air circulation in rows. Damp conditions allow for optimal growth of A. solani and the disease spreads more rapidly. Better air circulation can be achieved by planting farther apart or by trimming leaves. Chemical control There are numerous fungicides on the market for controlling early blight. Some of the fungicides on the market are (azoxystrobin), pyraclostrobin, Bacillus subtilis, chlorothalonil, copper products, hydrogen dioxide (Hydroperoxyl), mancozeb, potassium bicarbonate, and ziram. ==Economic significance==

Early blight caused by A. solani is the most destructive disease of tomatoes in the tropical and subtropical regions. Each 1% increase in intensity can reduce yield by 1.36%, and complete crop failure can occur when the disease is most severe. Yield losses of up to 79% have been reported in the U.S., of which 20–40% is due to seedling losses (i.e., collar rot) in the field. A. solani is also one of the most important foliar pathogens of potato. In the U.S., yield loss estimates attributed to foliar damage, which results in decreased tuber quality and yield reduction, can reach 20-30%. In storage, A. solani can cause dry rot of tubers and may also reduce storage length, both of which diminish the quantity and quality of marketable tubers. Because A. solani is one of numerous tomato/potato pathogens that are typically controlled with the same products, accurately estimating both the total economic loss and the total expenditure on fungicides for control of early blight is difficult. Best estimates suggest that total annual global expenditures on fungicide control of A. solani is approximately $77 million: $32 million for tomatoes and $45 million for potatoes. ==Historical impact==

Though the causal pathogen is distributed worldwide and can cause crop yield reductions, early blight has never caused widespread famine or other sudden and major detrimental effects on humanity. The disease is not to be confused with late blight, which is caused by the oomycete Phytophthora infestans. Late blight disease together with the socio-economic situation at the time was responsible for the Great Famine of Ireland in the 1840s. ==References==