Occurrence and etiology of Alternaria leaf blotch and fruit spot of apple caused by Alternaria alternata f. sp. mali on cv. Pink lady in Israel.
Severe outbreaks of Alternaria leaf blotch and fruit spot were recently observed in cv. Pink Lady apples in northern Israel, especially on fruit. Such severe outbreaks have not been reported from other countries. Symptoms involved cracks and rot around the calyx and external rot of the fruit body. Up to 80% of the fruit in some orchards were affected by the disease. Microscopic examinations, fulfillment of Koch's postulates and molecular (genetic) analyses confirmed the causal agent as Alternaria alternata f. sp. mali. The incidence of Alternaria increased as the degree of calyx cracking increased, or if fruit were both cracked and rotted. Injecting spore suspensions into the fruit produced typical rot symptoms. Injection assays of detached fruit of eight apple cultivars showed that cvs. Pink Lady and Golden Delicious were susceptible whereas cv. Jonathan was resistant. Pink Lady and Golden Delicious produced more fruit rot as the inoculum concentration increased. Rot in all three cultivars was moderate close to the skin but more severe close to the seed locule. Aqueous extracts taken from Jonathan fruit peel inhibited germ tube elongation of A. alternata f. sp. mali in vitro. This is the first report on heavy infection of Pink Lady fruit in Israel caused by A. alternata f. sp. mali.