New understanding on infection processes of Valsa canker of apple in China

Abstract

Apple canker caused by Valsa mali is a destructive disease that causes serious economic losses in China. In this study, the dynamics of release of conidial masses (cirri) in apple orchards were studied in different seasons, and in laboratory tests the effects of temperature on conidial germination were determined. Using artificial inoculations in the field, the influence of the pruning process on year-round disease development, and the possibility of the pruning saws carrying conidia of V. mali were tested to determine disease transmission in the winter months. Conidial masses released from pycnidia demonstrated strong adaptability to the changing environment. Conidial exudation from pycnidia could last up to a year and take place even in the winter, in the period when the bark was wet. The germination rate of conidia reached 90 % after 6 days incubation at 5 °C and 50 % after 19 days at 0 °C. Infection rates of conidia in different seasons were determined in a 3-year experiment. Fresh pruning wounds were more susceptible to infection than old wounds, and conidia were able to infect apple trees and induce canker lesions at different times of the year, and especially in winter. Pruning in early winter may promote heavy infections of apple by V. mali and could play an important role for the spread of the disease.