Abstract:

【Objective】Valsa canker, mainly causing dead branches and dead trees, is an important branch disease of apple trees. The objective of this study is to clarify the impact of environmental factors such as low temperature and host factors such as the age of branches on the infection and occurrence of valsa canker, to analyze the causes of the disease epidemics, and to provide a basis for the prediction and control of the disease. 【Method】Inoculation experiments were conducted under artificial controlled environmental conditions. The conidia colonized ratios of Valsa mali on different parts of apple branches were detected, the colonization sites of the pathogen in wounds were examined, and effects of the factors such as freezing, ice forming on branches, loss of water of branches, and age of branches on the infection of the pathogen and occurrence of the disease were tested in the experiments. 【Result】The colonized percentages of V. mali, i.e. the site carried the pathogen, on the 7 different branch sites of the apple trees inoculated with conidial suspension in the August were close to or more than 90% when examined in March of the following year. The pathogen inoculated on the wounds mainly colonized, grew and expanded in the necrotic tissue of the wounds, but did not penetrate the cork layer around the wound and invade the living cortical tissue to cause canker disease. Among the 6 examined branch sites, fresh wounds were the most susceptible to infection of V. mali, with the highest disease incidence of inoculated sites, followed by fruit stalk scars. The forks, bud eyes and fruit moss were less susceptible and disease incidences of inoculated sides were slightly lower. The lenticels were resistant to pathogen infection and no inoculated sites developed to canker. Both freezing and freezing after soaking (form ice on branch surface) can increase the susceptibility of branches to infection of V. mail. In the tested branches, the disease incidence of inoculated sites on the branches treated at -25 and -18℃ were significantly higher than that treated at -10, -7 and 0℃, and the incidence on branches frozen after soaking (simulating branch freezing after precipitation in winter) was significantly higher than that of uniced branches treated at the same temperature. Among the branches that were frozen and frozen after soaking, the disease incidence of inoculated sites on 1-year-old branches was significantly higher than that of 2-year-old branches. The disease incidence of inoculated sites at the tips of 1-year-old branches was significantly higher than that at the base of the same branches when frozen after soaking. The branches subjected to water loss after frozen (simulating the branches that lose water due to wind and high temperature after overwintering) were more susceptible to the infected V. mali, and the disease incidence of inoculated sites was significantly higher than that on frozen branches after soaking. The greater the amount of water losing, the higher the disease incidence, and the highest disease incidence of inoculated bud eye can reach 85%. 【Conclusion】V. mali is easy to colonize on apple branches, and colonized pathogens mainly survive and grow in dead tissues in wounds or on the surface of branches. Whether colonized pathogens can infect and cause disease mainly depends on environmental factors destroying the cork layer of the branches. Freezing, especially freezing damage at the temperature below -15℃ can destroy the cortex and cork layer of the branches, and increase the susceptibility of apple branches to infection of V. mali. Compared with freezing, freezing that form ice crystals on the surface of branches lead to a greater damage to the cork layer of branches, and the branches are more susceptible to the infection of the pathogen. The damage of water loss after freezing to the branches is particularly serious. The loss of water after the branches overwinter can significantly increase susceptibility of the branches to infection of the pathogen. The age of the branches is different, the developmental maturity, strength and toughness of the cork layer are different, and the degree of damage to the cork layer is also different when the branches were affected by adverse environmental factors. The structure of cortex in different sites of the tree is different, and there are obvious differences in the susceptibility to the pathogen infection.

Key words: valsa canker of apple tree, cortex structure, Valsa mali, pathogen colonization, pathogen infection, freezing, ice forming, water lose