Metabolic regulation of host-specific toxin production in Alternaria alternata pathogens. 3. Instability of pathogenicity in field isolates of A. alternata Japanese pear pathotype.

Host-specific toxins (HSTs) produced by several pathotypes of Alternaria alternata are now known to be an essential mediator for the induction of compatibility between the HST producers and their susceptible plants1,2). However, the field isolates of A. alternata pathogens frequently lose their ability to produce HST under laboratory3) or field4) conditions, particularly at considerably higher temperatures (e.g., in hot sum mer); no longer recognizable as a distinct pathotype. Experimental approaches to the mechanism of instability of pathogenicity are essential for understanding the disease ep idemiology. This article presents some of fundamental evidence on the concomitant loss of pathogenicity and toxin productivity in A. alternata Japanese pear pathotype, the caus al fungi of black spot disease of Japanese pear. This pathotype produces AK-toxin as a HST, and is characterized by affecting only certain cultivars of Japanese pear which are comparable to the AK-toxin-sensitive cultivars5). All isolates were obtained by single spore isolation from spore mass formed on les ions. Alternaria-infected pear leaves were sampled from two Nijisseiki pear orchards of Nagoya University and Anjo Agricultural High School in May and June, 1983. Ten sampled leaves were used in this experiment; three of them were from Nagoya Uni versity (Nagoya lesions) which were under short-term pear cultivation (5 years), and the other seven samples were from Anjo Agricultural High School (Anjo lesions) which were under long-term pear cultivation (ca. 50 years). Each lesion on diseased leaves