Studies on a strain of Alternaria citri Pierce, the causal organism of brown spot of Emperor mandarin

Abstract

Brown spot of the Emperor of Canton mandarin in Queensland was found to be caused by a distinct strain of Alternaria citri Pierce. In morphological and cultural characters the fungus is identical with A. citri, but differs in pathogenicity and the production of toxic filtrates when cultured on liquid medium. No evidence of cuticular or stomatal penetration by the germinating spores was found, and hyphae were not found in young lesions. When the fungus was cultured on Czapek's sucrose nitrate solution a toxic filtrate was produced which induced the same disease symptoms and showed the same host specificity as the pathogen itself. Non-pathogenic isolates of A. citri did not produce this host-specific toxic filtrate. Chloroform and Celite treatments increased the susceptibility of mature fruit to damage, presumably by removing a barrier of waxes which was impermeable to the toxin. The host range in Queensland was found to include Emperor of Canton mandarin, Sovereign mandarin and Calamondin.