Abstract: 【Objective】 Leaf brown spot caused by Diplocarpon mali is the main disease causing defoliation of apple tree in early season. In order to understand the epidemics and design programs to control the disease, temporal incubation dynamics of the disease were examined. 【Method】 Fuji apple leaves at different ages were inoculated with conidia of D. mali for 4-6 times from June to September in 2011 and 2012. Disease development on each inoculated leaf was examined every 3 days after inoculation. Duration from the day of inoculation to the day that the first disease lesion appeared on each leaf, which was defined the incubation period of the leaves, was recorded. Average incubation period, the shortest incubation period and lesions development duration of the disease were calculated from the data. Temperature, rainfall and relative humidity near inoculated trees were recorded at 30 min intervals during the experiment period with automatic meteorological station. 【Result】 Average incubation period of the disease was 23.8 d, the shortest incubation period was 8-20 d with the mean of 13.6 d and the symptom development duration, from the first lesion was observed to last inoculated leaf showing symptoms, was 12-54 d with mean of 31.3 d. The shortest incubation period and the symptom development duration of the disease on the leaves inoculated in June and July were longer than those inoculated in August, as well as on the leaves incubated in indoor were much longer than those incubated in outdoor. Incubation period of the disease was shorter on aged leaves than on young leaves of head at vigorous physiological metabolic stage. The incubation period of the disease on leaves incubated at the early stage of growth (from June to July) was much longer than those incubated at the later stage of growth (from August to September). 【Conclusion】 Symptoms development of apple leaf brown spot was a dynamic process. The incubation period and the disease development duration were relatively longer than other diseases on apple leaves. Incubation period and symptom development duration depended mainly upon the combination of pathogen and the host and largely upon the leaf resistance, which affected by age, degree of senescence and vigorousness of metabolism of the leaf. During the main epidemic stages of the diseased, which occurred from June to September, temperature did not show significant impact on the incubation period of apple leaf brown spot. However, the rain and wetted environment could promote the pathogen sporulation and latent lesions to develop to visible lesions. According to the results, 15 days, even 20 days, after infected by pathogen, systemic fungicides could effectively prevent most infected pathogen developed to disease lesions.

Key words: apple , apple leaf brown spot , Diplocarpon mali , incubation period , symptom development duration