Relationship between leaf injury and colonization of cabbage by Sclerotinia sclerotiorum

Mechanical and insect feeding injuries of cabbage provided sites for ingress by Sclerotinia sclerotiorum. In field studies, injuries sustained by hitting cabbage heads with a blunt object penetrated several leaf layers and were readily infected by S. sclerotiorum. Incidence of Sclerotinia rot ranged from 15 to 36% infected plants in the hitting treatment. Disease incidence was low (0–5%) when wounds were created by allowing lepidopterous larvae (the imported cabbageworm, Pieris rapae L., the diamondback moth, Plutella xylostella L., and the cabbage looper, Trichoplusia ni Hubner) to feed on cabbage leaves. The incidence of colonization of lepidopterous larvae feeding sites by S. sclerotiorum was greater in greenhouse studies (5–60% infected feeding sites) than in field studies. In greenhouse studies, disease incidence was significantly greater in plants receiving hitting injuries than tearing injuries. There was little evidence of inoculation day or injury day effects. Colonization frequency of tissue injured mechanically was similar whether wounds were inoculated immediately or 4 days later. Plants that were incubated in a dry environment developed fewer infections than plants held in a moist environment. Inoculated plants that were not injured did not develop disease.