Abstract: Studies on the severe strain of Alternaria leaf spot in apple indicated that the main reason for serious occurrence of this disease is the extensive existence of severe strain of Alternaria mall Roberts. The occurrence and pathogenicity of the severe strain are different in each orchard. Toxicity tests in laboratoy showed that 50% rovral, 70% mancozeb, 10% polyoxin-Al and 1.5% Duokangmeisu gave the best effects on fungistasis. The tests also showed that their fungistasis was increased when 1 % boric acid was added into 1.5% Duokangmeisu and 10% bolyoxin-Al and in fact, boric acid itself could inhibit the pathogen. Herein proposed the principle of field control "stress the protection to spring shoots and leaves and care for autumn shoots and leaves".

Key words: Alternaria leaf spot in apple, Severe strain, Toxicity test