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Strawberry powdery mildew caused by Podosphaera aphanis, one of the most important diseases affecting strawberry production in Japan, can be effectively controlled by supplemental ultraviolet (UV) irradiation. Here, we investigated disease severity on plants irradiated with UV light at 270-380 nm with/without inoculation and demonstrated significant decreases in disease severity on the UV-treated plants throughout almost all of the experiment periods. Daily UV irradiation for 3 hr (0.17-0.96 kJ ⋅m −2 ⋅day −1 ) significantly reduced the incidence of powdery mildew on UV-treated strawberry at day 58 after initiation of UV irradiation in a natural infection and at day 168 in artificial inoculation tests compared with non-UV-treated controls. Furthermore, we found that two pathogenesis-related protein (PR) genes were up-regulated 7 days after initiation of UV irradiation on plants before or after transplantation into raised beds. Transcript levels of five defence-response genes were estimated using quantitative reverse transcription PCR to determine whether UV irradiation induced a defence response; PR-3 and PR-5 transcripts were significantly higher in the UV-treated strawberry leaves after UV irradiation than in the non-irradiated controls. In addition, transcript levels of both genes were significantly up-regulated in plants irradiated with a higher radiant dose (0.90-0.96 kJ ⋅m −2 ⋅day −1 ) within 7 days after initiation of UV treatment, compared with plants exposed to a lower dose (0.17 kJ ⋅m −2 ⋅day −1 ) and with controls. Thus, PR-3 and PR-5 are induced in strawberry leaves as part of a defence response that is induced by UV irradiation.

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Even if parsley is irradiated by UV-B to 60.0 μw/cm 2 , its growth is not influenced. We controlled powdery mildew by irradiation at more than 9.0 μw/cm 2 . However, when there are many sources of infection, UV-B cannot control it. We effectively restricted powdery mildew growth, by a combination of fatty acid glyceride emulsion and sodium bicarbonate·copper wettable powder applied for 3 weeks.

Onion downy mildew caused by Peronospora destructor has been widespread in Japan since 2016. Soil disinfection and use of fungicides have been implemented as control measures against oospores in the soil, which are the primary source of infection. Measurements of oospore density are needed to clarify the risk of disease development and inform disease management. In this study, an experimental system capable of detecting P. destructor DNA from field samples was developed. A TaqMan probe‐type primer was used to target the coxII (cytochrome c oxidase subunit II) region of the mitochondrial genome of P. destructor and no false positives were observed. Using this method, the detection limit was equivalent to 5 oospores of P. destructor/g soil in grey lowland soil and andosol. The correlation coefficient between oospore density and the primary infection rate was investigated by testing soil samples from 115 fields. Although no correlations between oospore density and the primary infection were observed in pesticide‐treated fields, significant correlations were observed in untreated fields. Continuous cropping of onions increased the oospore density. The correlation between primary and secondary infection was relatively weak, and the negative predictive value of primary infection was relatively high (89.0). These results suggest that the disease‐risk potential of onion downy mildew is very high, and thus it is necessary to set a low pathogen detection threshold. The system presented here provides a highly sensitive method for supporting decision‐making aimed at disease control.