Abstract
Apple rust is incited by Gymnosporangium yamadae Miyabe ex Yamada, a demicyclic rust with some kinds of chinese juniper such as Juniperus chinensis L. and apple as alternate hosts. Seventeen fungal specimens were collected in 14 areas of Hokkaido and Honsyu islands. Two pathogenic races of the fungus were distinguished by inoculation of McIntosh, Empire and other five cultivars under the controlled conditions. All cultivars inoculated were susceptible to race 1. McIntosh and Empire (Delicious×McIntosh) were resistant to race 2. Only pycnium was incited on the lesions. These pathogenic races inoculated on J. chinensis showed the same reaction on the indicator cultivars after two years.
Akane (Jonathan×Worcester pearmain) and Worcester pearmain showed a different resistance from that of cvs. McIntosh and Empire to race 2. No lesions were detected on either cv. upon field observation. When potted plants, cvs. Akane and Worcester pearmain grown under a screenhouse with glass roof, were inoculated under the controlled conditions (plants were kept 24hrs in a moist chamber at 17.5±1.5C), many small but clear lesions with aecia were incited by all fungal collections. On the other hand, lesions with aecia were rarely incited, when Akane was inoculated under the field conditions.
