Suppression by Red Light Irradiation of Corynespora Leaf Spot of Cucumber Caused by Corynespora cassiicola

The effect of red light irradiation on development of Corynespora leaf spot of cucumber plants (Cucumis sativus L. cv. Hokushin) caused by Corynespora cassiicola (Berk. & Court.) was investigated in greenhouses. In a greenhouse without red light (?Red), lesions enlarged, coalesced, and finally covered the entire leaves of cucumber. In a greenhouse with red light (+Red), however, lesion appearance was delayed relative to that under ?Red and its development was also significantly suppressed. Such difference in disease development was also observed in cucumbers grown under +Red and ?Red in the same greenhouse. Disease suppression under red light was also observed in glasshouse‐grown C. cassiicola‐inoculated cucumbers. Red light did not inhibit the infection behaviour of the pathogen. Our results indicated that the delay and suppression of Corynespora leaf spot of cucumber under +Red were due to induction of resistance in cucumber, and not to differences in environmental conditions or fungal population between the two greenhouses. Red light‐induced resistance might contribute to the development of new methods for controlling Corynespora leaf spot of greenhouse‐grown cucumber.