Use of low-dose UV-C irradiation to control powdery mildew caused by <i>Podosphaera aphanis</i> on strawberry plants

Abstract

Strawberry powdery mildew, caused by Podosphaera aphanis, significantly reduces fruit yield and quality and predisposes fruit to other diseases. Fungicides have been routinely used to control this disease; however, their limitations necessitate the development of alternative approaches, especially for protective culture and organic production. A disease-control strategy was developed that combines treating strawberry plants with a low dose of UV-C (low dose/short duration) followed by a specific period of darkness, which greatly increases the lethality of UV-C. A leaf disc assay was developed to evaluate the effectiveness of the UV-C treatment in controlling powdery mildew on strawberry. Irradiation for only 15 s by UV-C lamps with an output of 20.6 µW cm−2 followed by a 4-h dark period resulted in a significant decrease (P = 0.05) in P. aphanis as determined by the percentage of the leaf disc quadrants colonized by the fungus. An increase in irradiation exposure to 60 s followed by 4-h dark period resulted in complete control of the powdery mildew in most cases. Such treatment of strawberry plants once a week for 3 weeks resulted in more than a four-fold reduction (P = 0.01) of conidial production on adaxial leaf surfaces exposed to the UV-C irradiation, and did not affect leaf photosynthesis (P = 0.05). The UV-C treatment of plants over 15 weeks reduced the amount of diseased fruit and increased fruit yield and quality (P = 0.05).