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Causal Agent: 

Fungus (Alternaria solani)

Characteristic Symptoms:
 
The disease can affect the leaves, stem, flowers, and fruits.
   
It is first observed as small, brown spots, often with yellow margins usually on the older leaves.
   
The spots eventually enlarge and concentric zonations/rings are observed at the center of the spot.
   
Stem lesions are dark brown to black, slightly sunken and elongated spots with pronounced concentric rings.
   
Fruit spots occur in the stem end and usually dark brown, leathery, sunken lesions with concentric rings.
   
Conditions for Disease Development:
 
The pathogen overwinters primarily on infected crop debris and can survive in the soil for many years.
   
Warm and humid environments favour infection. 
   
Lesions generally appear quickly under warm, moist conditions on older foliage and are usually visible within 5-7 days after infection.
   
Spores are dispersed mainly by wind and occasionally  by splashing rain or overhead irrigation.
   
The disease is considered polycyclic with repeating cycles of new infection, thus, it has the potential to spread and build up to damaging levels.
   
Management and Control:
 
Use pathogen-free seeds and seedlings.
   
Good field sanitation (i.e., removal of heavily affected leaves, infected crop debris) can help reduce inoculum source.
   
Remove weeds and volunteer crops that may serve as alternate hosts of the fungus.
   
Provide good air circulation to minimize periods of leaf wetness.
   
Keep the plants healthy as they become susceptible when under periods of stress.
   
Crop rotation with nonhosts can help reduce buildup of inoculum in the soil.
   
Apply protectant fungicides (mancozeb, chlorothalonil) at weekly intervals when necessary.
   
Use resistant varieties, if available.
   
Apply protectant fungicides such as mancozeb (e.g. Attain M-80Ⓡ, Achem Mancozeb 80 WPⓇ, Micron 80 WOⓇ, VanzebⓇ) or chlorothalonil (e.g. Daconil®, Agronil®, Yoda®, Rover®, Yoda 500Ⓡ) at weekly intervals.
   
References:
Kemmitt, G. 2002. Early blight of potato and tomato. The Plant Health Instructor. DOI: 10.1094/PHI-I-2002-0809-01 (http://www.apsnet.org/edcenter/intropp/lessons/fungi/ascomycetes/Pages/P...) ; Tomato Early Blight (http://www.ext.colostate.edu/mg/gardennotes/718.html)
   

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