桑裏うどんこ病菌の分生胞子および子のう殻形成数
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概要
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During the 1959 growing season, studies were conducted to observe epidemiologically the mode of occurrence and subsequent development of powdery mildew of mulberry tree caused by <I>Phyllactinia Corylea</I>. The average number of lesions per one hundred leaves was used as the criterion of mildew severity. On July 21 in Ichihei variety (summer harvest) the infection was minor, the 2511 leaves tested showing only 11 lesions. By the end of July the disease development showed a marked increase and the average number of lesions per 100 leaves became 7. 1. The epidemic potential of the pathogen thus rose correspondingly. From then on through August, the disease developed rapidly until, in the early autumn, nearly all the leaves became heavily mildewed. With reference to the Ichinose variety (summer harvest), however, the circumstances were strikingly different from those of the Ichihei variety. On August 7, 3557 leaves were investigated, of which 3 showed single lesion. After that, during the later part of August and the beginning of September, the mildew development was minor. By October 5 the average number of lesions per 100 leaves reached 51.7, on the 15th it was 115.3, and on the 25th it became 155.6. It would appear from this that as the season advanced the Ichinose variety gradually changed from resistance to susceptibility. On July 29, moreover, powdery mildew was observed on four varieties (spring harvest), viz., Ichihei, Kairyoroso, Kairyonezumigaeshi, and Jumonji-all of which were grown side by side in a field. The Ichihei variety proved the most susceptible among the four varieties used in the test. Between July 21 and August 30, at intervals of about ten days, the number of lesions newly-formed throughout each interval was plotted against leaf orders. With regard to leaf orders the uppermost fully opened leaf was designated as No.1 leaf. Considering the incubation period and growth rate of shoots at each interval, the leaves most susceptible to the infection in the natural conditions were between No.12 and No.16 leaves. Furthermore, to study the relation between mildew severity and leaf order on shoots, mulberry shoots of the Ichihei variety 1.5-2 meters tall were used. The size of lesions was greater on older (lower) leaves than on younger (upper) leaves
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