Diseases of tomato (
Alternaria, early blight
Early blight of tomato is caused by the fungus Alternaria alternata f sp. lycopersici. Symptoms become prevalent during the hotter months and with high humidity and rain. In Arizona, it occurs sporadically during the summer "monsoon" rainy season, especially in southeastern Arizona. This disease produces brown to black, target-like spots on older leaves that may coalesce into larger lesions (photo 1). Affected leaves may turn yellow then drop, leaving the fruit exposed to sunburn. If severe, the fungus also attacks stems and fruit. Fruit shows freckles, spots or lesions (photo 2 , photo 3 ). Often the spots are not evident until a few days after the fruit is harvested.
To manage disease, remove all diseased plant tissue on the ground, as the fungus overwinters on leaf debris. Sanitation is the best control. Do not plant tomatoes in the same place next year. Avoid overhead irrigation and space plants farther apart to improve air circulation. Plant resistant varieties. If the infestation is heavy, sulfur dust may help protect new leaves from infection (do not apply sulfur dust above 90oF). Other fungicides such as chlorothalonil (Bravo, Daconil) and azoxystrobin (Quadris) are effective in preventing diseasse.
Links:
http://www.ipm.ucdavis.edu/PMG/r783101311.html
http://www.nysaes.cornell.edu/ent/hortcrops/english/eblight.html
http://plantpathology.tamu.edu/Texlab/Vegetables/veg6.html
http://www.ext.colostate.edu/PUBS/Garden/02949.html
1
2
3
January 8, 2007