Effects of Temperature and Moisture on Conidia Germination, Infection, and Acervulus Formation of the Apple Marssonina Leaf Blotch Pathogen (Diplocarpon mali) in China.

Apple Marssonina leaf blotch (AMLB; Diplocarpon mali) is a severe disease of apple that mainly causes premature leaf defoliation in many apple growing areas worldwide. AMLB epidemic development is closely related to temperature and rainfall. In this study, the effects of temperature and moisture on conidium germination, infection on leaves, and acervulus production were investigated under controlled environments. The temperature required for conidium germination and infection ranged from 5 to 30°C, with the optimum at approximately 23°C. The temperature required for acervulus formation was slightly higher, with the optimum at 24.6°C. Wetness was needed in order for conidia to germinate and infect; only a few conidia germinated at 100% RH. However, lesions can produce acervuli in dry conditions. The minimum duration of leaf wetness required for conidia to complete the entire infection process was 14, 8, 4, and 6 h at 10, 15, 20, and 25°C, respectively. A model describing the effect of temperature and leaf wetness duration was built. The model estimated that the optimum temperature for conidial infection was 22.6°C and the minimum wetness duration required was 4.8 h. This model can be used to forecast D. mali conidial infection to assist in disease management in commercial apple production.