Hot Water Spray Over Strawberry Plants Effectively Controls the Occurrence of Strawberry Powdery Mildew in Everbearing Strawberry Production

Extensive use of sterol demethylation inhibitor (DMI) fungicides against strawberry powdery mildew has resulted in DMI-resistant genotype emergence. In this study, we examined the preventive effect of hot water spraying (HWS) on the occurrence of powdery mildew in everbearing strawberry, predominantly infected by DMI-resistant powdery mildew fungi from 2016 to 2018. Hot water of 54 ± 1°C was sprayed on Natsuakari for 20 s to achieve the target leaf temperature of 50°C. Disease occurrence under the chemical fungicide (F) + HWS treatment was less severe than that under the F treatment until the end of the cultivation period in February 2018. However, the preventive effect of HWS was not observed in already infected leaves. The incidence of DMI-resistant genotype was 0% at the start, and it increased to 62% and 100% in the infected fruits and leaves (including F+HWS) in September and November 2016 until the end of cultivation, respectively. On the contrary, disease severity increased between August 2017 and February 2018 under the F treatment, whereas only a slight infection was detected under the F+HWS treatment. Thus, HWS could induce resistance in strawberry against powdery mildew fungi, including DMI-resistant fungi, and effectively control the occurrence of powdery mildew in long-term everbearing strawberry cultivation.