Preventive Effect and Mode of Action of Repetitive Hot Water Spraying against Powdery Mildew in Strawberry

抄録

This study aimed to investigate the preventive effect of periodic hot water spraying against powdery mildew in strawberry as an alternative to chemical application as a protection method. Young upper leaves of ‘Sachinoka’ were dipped in hot water at 50°C for 20 s. Conidia of powdery mildew were inoculated 24 h after the treatment. The occurrence and growth rates of powdery mildew were apparently lower on leaves dipped in hot water than on those left untreated. Thus, resistance against powdery mildew was thought to be successfully induced by heat shock. Change in leaf temperature was tested using a prototype of a hot water sprayer at 57 ± 2°C. The resultant rise in temperature and temperature attained varied widely across the leaves. Nonetheless, the minimum requirement of a leaf temperature of 50°C in 10 s for inducing systemic resistance against crown rot could be attained in the treated plants. In this study, weekly hot water spraying treatments were conducted in ‘Sachinoka’ in 2008 and in ‘Tochiotome’ and ‘Hitachi-hime’ in 2009. In 2008, powdery mildew occurred explosively in the non-treated plants. However, its occurrence in plants treated with hot water spraying was less than that in the controls, but more than that after chemical application. In 2009, the occurrence rate was lower than that in the non-treated group of 2008 and extremely low after hot water spray treatment and chemical protection in both the varieties. Whether the difference in the occurrence of powdery mildew in both the years was because of the difference in environmental conditions or the difference between the varieties is not known. The plant height of ‘Hitachi-hime’ tended to become short after hot water spraying; however, no serious effects were noted on the growth and yield of strawberry plants. In conclusion, heat shock treatment by using hot water spraying can be considered an alternative method to chemical protection against mild powdery mildew incidence in strawberry.