Relation of climatic factors on the outbreak of black rot of sweet potato in fields

Abstract

Evidences have been confirmed concerning climatic conditions in relation to the severeness of black rot of sweet potatoes in fields. The data were obtained from the variety experiment at Shirai-mura near Chiba from 1941 to 1948.
The basal portion of inoculated slips healed not infrequently after planting, though they used to be attacked there. The healing was distinguished in the year with the highest mean temperature being at about 30°C for 10-day-period. The temperature observations were taken at 10 o'clock. The higher in the temperature the more remarkable is this phenomenon (Fig. 1). This is referred to as the effect of a curing as such mentioned by YOSHII (1945).
The varietal difference of the infection of the roots has a tendency to be correlated with the injury by wire worm, Melanotus caudex LEWIS. Similar tendency is acertained in regard to the yearly deviation on the same varieties (Fig. 2).
The number of the pits was in inverse proportion to the precipitation at the first 10-day period of March (Fig. 3). In this case the correlation coefficient reaches to -0.89 and is highly significant. At about this period, the wire worm in that field is surmised to move upward from deep to just beneath the surface of soil, and subject to the effect of precipitation. Deviation of the dots Nos. 16 and 17 in Fig. 3 is explained by the earlier upward movement resulted by warmer winter (Fig. 4) and larger precipitation of the preceding 10-day period in the year Showa 16 (1941), and by the delayed upward movement caused by colder spring (Fig. 4) and meagre precipitation of succeeding two 10-day periods in the year Showa 17 (1942). The number of the pits may be presumably in proportion to the number of the worm itself. The precipitation of the first 10-day period of March has a curvilio, near relation to the percentage of the diseased roots (Fig. 5), similar to that between the latter and the index of the wire-worm pits.
The length of the lesion at the basal portion of the slips correlates with (1) the mean temperature for the last two 10-day periods of August (Fig. 6). Correlation coefficient reaches to 0.92 and is highly significant. (2) the mean temperature for the growing season of sweetpotato, (3) the mean temperature for September and October, and (4) the mean of maximum temperature for the first two 10-day periods of September also suggest tendencies of positive correlation to the enlargement of the lesion, though it is not significant. As a whole, the higher the temperature in August or thereafter, thel arger is the development of the diseased part at the base of the stem.
This is apparently in contradiction to the result on the healing mentioned first. However, the curve B in the figure 6 suggests a rather suppressive effect of high temperature in 1948 (22 in the figure).
The distribution of the dots in fiure 7 is suggestive of a certain influence of concentration of the pathogen on the outbreak of this disease in the field, since the meagre outbreak of the disease in 1941 (16 in the figure) cannot be explained otherwise, i. e. the lower temperature in summer and autumn might prevent the increase of the pathogen, in spite of the worm pits were rather numerous in this year.