Relation of Temperature and Humidity to the Occurrence of Angular Leaf Spot of Cucumber Grown under Plastic House

Abstract

Angular leaf spot of cucumber caused by Pseudomonas syringae pv. lachrymans occurs severely under the conditions of low temperature and high humidity such as the open-field cultivation in the northern district of Japan and the winter cultivation under plastic house. The present studies were carried out to clarify the effects of humidity and temperature on the occurrence of angular leaf spot of cucumber, and to establish the control measure of the disease by means of humidity control in the plastic house. Angular leaf spot developed severely at 15-25C, and slightly at 10C and 30C in high humidity room. The optimum temperature for disease develpoment was 25C. On leaves of cucumber plants inoculated with causal bacteria and placed at various relative humilities for 7 days, angular leaf spot developed more severely at over 90-94% relative humidity than at 83-85%. Typical large lesions developed at over 90-94% relative humidity, but only minute lesions developed at 83-85% relative humidity. Plants kept in a moist chamber for 24 hours after inoculation showed large lesions, but plants removed earlier from moist chamber less than 6 hours after inoculation showed only minute lesions. Lesions were formed even when plants were transfered into a chamber of 40-60% relative humidity immediately after inoculation. Daily keeping of inoculated plants in saturated humidity conditions more than 6 hours formed typical large lesions. On the contrary, minute lesions were only formed by the daily keeping for less than 3 hours. Occurrence of angular leaf spot was suppressed to an appreciable extent by means of humidity control at night to the level of 80-90% relative humidity by using the dehumidifiers equipped in the plastic house. It was demonstrated also from the practical point of view that high humidity at night in the plastic house could be reduced to the level of 80-90% relative humidity by using an ordinary type of hot-air heater.