Abstract: 【Objective】 Marssonina leaf blotch is a main disease of apple leaf in China, which causes severe defoliation of apple tree in early growth season. The objective of this study is to understand the epidemics of Marssonina leaf blotch in whole apple growth season and provide data or information for prediction and management of the disease. 【Method】 From March to July during 2009 and 2010, apple leaves with Marssonina leaf blotch lesions were randomly picked every 15 days from ground of two apple orchards in Laiyang and Qingdao, Shandong Province. Fruiting bodies of the fungus were cut out from lesions on the upper side of sampled leaves and examined for asco-spores and pseudo-conidia under a microscope. Development dynamic of primary infection inoculum on overwintered diseased leaves was analyzed base on the percentage data of apothecium and pseudo-acervulus. From June to October during 2008, 2009 and 2010, incidences of diseased and defoliated leaf on the same shoots were surveyed every 15 days in orchards located in Laiyang and Qingdao. The incidence data were used to fit logistic models and model parameters were calculated to describe the epidemic dynamics of the disease. From September to November during 2010 and 2012, apple leaves with typical Marssonina leaf blotch lesions were sampled every 10 days. Acervula were cut out from the lesions and examined under a microscope for the small type spore and the conidia. Percentages of the small type spore relative to conidia were used for analyzing formation dynamics of the small type spores. 【Result】 Diplocarpon mali produced asco-spores and pseudo-conidia on overwintered disease leaves. Pseudo-conidia were produced from early March to the end of June with peak in middle of May. Pseudo-conidia were mainly dispersed with rain splashing water and mainly infected leaves on the bottom of the trees form the beginning of apple leaf growth. The infected leaves by pseudo-conidia often defoliated before the end of June and had little influence over other leaves on late onset. Asco-spores matured from middle of May to the end of June, discharged in rain and dispersed with airflow. Asco-spores chiefly infected leaves of main parts in apple tree and the infected leaves provided inoculum for the epidemic of the disease in late growing season. The primary infected leaves by D. mali on main part of apple trees began to show symptoms from early or middle of July. Diseased leaves accounted for approximately 2% of all the leaves at the beginning of disease development. In July, the primary infected leaves began to show symptoms and sporulation, and the infected new health leaves. The infected leaves showed symptom continuously in July and incidence of diseased leaves rose up to 5% at the end of the month. In August, most of the infected leaves in early season showed symptom, produced a large amount conidia and infected healthy leaves, resulted in rapid increase of disease leaves. The disease development reached its’ peak on late August and 12 days later the diseased leaves were defoliated. Dynamics of the disease epidemic and defoliation from June to September can be well described by two logistic models. From the beginning of September, the pathogen began to produce a small kind of spore, which was consider as sexual spore, and relative proportion of the small type spore on acervulus were increased in linear style with time elapse. In October, the pathogen gradually stopped to produce conidia, the only infection spores in growing season, on lesions and prepared for overwintering. 【Conclusion】 Epidemics of apple Marssonina leaf blotch in the central area of Shandong peninsula can be divided into four periods: From apple leaves growth to the end of June is the primary infection stage, and infection by asco-spores is mainly from late May to the end of June; July is the exponential growth stage of the disease; August and September are the logistics growth stage of the disease; October and November are period for the pathogen to prepare for overwintering. The asco-spores infection stage and exponential growth stage of the disease are key period to control the disease epidemics with fungicides.

Key words: apple Marssonina leaf blotch, asco-spores infection stage, exponential growth stage, logistics growth stage, development of sexual spores