Abstract
The leaves of Ichinose and Ichihei varieties of mulberry tree were used in the present inoculation experiments. Mildew inocula, namely, conidia of the causal fungus, Phyllactinia Corylea (PERS.) KARST., were obtained in ample quantity from Ichihei variety grown in a field. The leaves situated next to the brightest leaf were used for inoculations in all experiments reported herein. In Ichinose variety, the inoculation was first attempted on June 25, 1959 and continued until September 18 at intervals of about a week. In Ichihei variety, however, the inoculation experiments were performed from July 10 to September 7. In Ichinose variety much variation in the length of incubation periods was observed in the course of growing season. When leaves of the same variety were inoculated on June 25, powdery mildew took, on an average, 13.5 days to produce recognizable symptoms. After that, the length of the incubation periods tended to decrease gradually and by August 6 it reached a minimum value, namely, 8.7 days. From then until September 7, the necessary incubation time was in the neighbourhood of 10 days. This value was considerably shorter than that obtained during the first part of experiments. Thus, the susceptibility of Ichinose variety to powdery mildew appeared to increase correspondingly with the growing season. In Ichihei variety, similarly, the leaves inoculated on August 6 took the shortest time for incubations, showing that the natural environmental conditions at that time were most favorable for powdery mildew. The length of the period of incubation was slightly longer on Ichinose variety than on Ichihei variety throughout all the experiments.
In general, the reproduction period which included the duration from the inoculation to the first appearence of conidia of the causal fungus played an important role in epidemics. In both varieties used in the test, the variation in the length of the reproduction period was almost the same as the variation in the period of incubation.
In an attempt to investigate the infection period, the leaves of Ichinose variety were inoculated during the evening of September 5. After a various period of time, diluted “mel” solutions were sprayed on the inoculated leaves to prevent the further development of the pathogen. After 12 hours the leaves were washed carefully with water. There was no infection when the diluted solution of disinfectant was sprayed within the period of 36 hours from inoculation;100% infection occured when disinfection treatment was performed 48 hours after inoculation. Therefore, under favorable natural conditions, within 36-48 hours after inoculation the pathogen concerned could establish a relationship with the host cells so stable that it could no longer be destroyed by disinfection treatment.
