Symptoms of early blight occur on older foliage, stem, and fruits. Gray to brown spots appear on leaves and gradually grow in a concentric manner around a clear center - the characteristic “bullseye” formation. These lesions are surrounded by a bright yellow halo. As the disease progresses, entire leaves may turn chlorotic and shed, leading to significant defoliation. When leaves die and fall, fruits become more vulnerable to sun scald. The same type of spots with a clear center appears on stems and fruits. The fruits begin to rot and may eventually fall off.
Symptoms are caused by Alternaria solani, a fungus that overwinters on infected crop debris in soil or on alternative hosts. Purchased seeds or seedlings may also be already contaminated. Lower leaves often get infected when in contact with contaminated soil. Warm temperatures (24-29°C) and high humidity (90%) favor development of the disease. A long wet period (or alternating wet/dry weather) enhances the production of spores, which may be spread via wind, splashing rain or overhead irrigation. Tubers harvested green or in wet conditions are particularly susceptible to an infection. It often strikes after a period of heavy rainfall and is particularly destructive in tropical and subtropical areas.
Application of products based on Bacillus subtilis or copper-based fungicides registered as organic can treat this disease.
Always consider an integrated approach with preventive measures and biological treatments if available. There are numerous fungicides on the market for controlling early blight. Fungicides based on or combinations of azoxystrobin, pyraclostrobin, difenoconazole, boscalid, chlorothalonil, fenamidone, maneb, mancozeb, trifloxystrobin, and ziram can be used. Rotation of different chemical compounds is recommended. Apply treatments in a timely manner, taking into account weather conditions. Check carefully the preharvest interval at which you can harvest safely after the application of these products.