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

Apple marssonina leaf blotch (Diplocarpon mali) is a severe disease of apple that mainly causes premature leaf defoliation in many apple growing areas in the word. Its epidemic development is closely related to temperature and rainfall. The effects of temperature and moisture on conidium germination, infection on leaves, and acervulus production were investigated under controlled environments. Temperature required for conidium germination and infection ranged from 5 oC to 30 oC with the optimum around 23 oC, and temperature required for acervulus formation was slightly wider with the optimum at 24.6 oC. Wetness was needed for conidia to germinate and infect; only few conidia germinated at RH=100%. However, lesions can produce acervuli in dry conditions. The minimum duration of leaf wetness required for conidia to complete the entire infection process were 14, 8, 4, and 6 hours 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 conidial infection has the optimum temperature at 22.6°C and the minimum wetness duration required of 4.8 hours. The model can be used to forecast the conidial infection of D. mali to assist in disease management in commercial apple production.