1986 Volume 52 Issue 5 Pages 779-784
The susceptible cv. Nijisseiki of Japanese pear requires fruit covers during the growing season in addition to foliar cover spray against catastrophic pests such as the black spot disease caused by Alternaria alternata Japanese pear pathotype. During 1984 summer season, the disease prevailed throughout Tottori orchards, and often caused serious fruit damages in bags without showing significant black spots on leaves and young shoots. High rate of fruit apex rotting was the main damage encountered, which was an unusual phenomenon. Pistils during blooming were found to be latently infected by pathogenic A. alternata. Removal of the colonized pistils was closely associated with a marked reduction in the rate of apex rotting of immature and fruits that were covered with protecting paper bags during the growing season. An infection model in the field using an auxotroph of the pathogen as a tracer was also investigated. When spores of choline-less mutant were inoculated carefully to stigmas during blooming with or without choline, they were able to survive dominantly on immature and growing fruits in paper-wraps more than 2 months after inoculation, even under several foliar sprays of fungicides. Spores produced on pistils occasionally caused many black spot lesions on the fruit apices due to secondary and short-ranged infection in closed paper bags, and led to severe blossom-end rotting of fruits. In black-spot-epidemic years such floral infection by the black spot pathogen may be involved in large scale damages of paper-wrapped fruits growing on trees, regardless of heavy sprays with fungicides.
