High Humidity and Age-Dependent Fruit Susceptibility Promote Development of Trichothecium Black Spot on Apple

Fruit bagging is a widely used orchard practice in China. Trichothecium black spot (TBS) is a disease highly associated with the fruit bagging. In this study, we characterized in vitro factors affecting the causal agent, Trichothecium roseum, and TBS development and infection histology on field-bagged apple fruit in situ. Under in vitro conditions, conidial germination required exogenous nutrients, and the germination rate was significantly promoted by high humidity, a condition mimicking the bag microenvironment. Germ tubes penetrated fruit via natural openings including stomata, lenticels, and surface cracks. To determine the chronology of infection by T. roseum, 'Fuji' fruit were inoculated in the field at different developmental stages. The earliest infection occurred 60 days after full bloom (dafb), and disease incidence increased as fruit maturity advanced. At harvest time (165 dafb), lesions on more recently inoculated fruit (105 dafb, 150 dafb) were larger than lesions from fruit inoculated on earlier dates. Histological observation showed that infection of younger fruit elicited stronger host lignification responses restricting lesion development. Taken together, our results support the hypothesis that high humidity in sealed bags and increased susceptibility associated with advancing fruit maturity are key factors promoting T. roseum infection and TBS symptom development on bagged apple fruit.