Early Blight of Tomato
Symptoms of early blight occur on older foliage, stem, and fruits. On the leaves, gray to brown spots appear that 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. The same pattern of dark concentric lesions with a clear center appears on stems and fruits. Rotting of tissues follows.
Alternaria solani overwinters primarily on infected crop debris in soil or on alternative hosts. Lower, older leaves might catch the infection when in contact with contaminated soil. Warm temperatures (24-29°C) and high humidity (90%) favor development of the fungus. It penetrates the leaf epidermis directly or enters through leaf pores. A long wet period (or alternating wet/dry weather) enhances the growth of spore cushions on the leaf spots and the release of spores. The spores are spread via wind, splashing rain or overhead irrigation.
You can spray algal limestone or rock flour on infected plants.
Rotate different chemical compounds to avoid resistance development. Apply treatments in a timely manner, taking into account weather conditions. Fungicides containing Copper, mancozeb, chlorothalonil, azoxystrobin, pyraclostrobin, trifloxystrobin, fenamidone and famoxidone have been successfully used to control Early Blight in tomato.