The influence of lower temperature induction of Valsa mali on the infection of apple trees.

Apple valsa canker (AVC), caused by Valsa mali (Vm), is one of the most important diseases of apple trees in China. AVC occurred severely along with cold winter or cold spring. However, the effect of lower temperature on Vm is poorly understood. This study evaluated the influence of lower temperature pre-treatment of Vm on the infection of apple twigs and leaves. The results showed that exposing of Vm at lower temperature (between -10°C and 10°C) for more than 18 h significantly increased the disease severity of apple leaves and twigs, with a higher LAR, lesion length and DI than that at 25°C. In addition, cold treatment ranging from -5°C to 10°C promoted the colony growth. Meanwhile, the relative expression of four cell wall degrading enzymes (CWDEs) related genes pre-treated at -5°C and 5°C were significantly higher than that at 25°C. The results indicated that the virulence of Vm mycelium is sensitivity for lower temperature change. After sensing lower temperature changes, Vm can adjust its infection of apple trees by regulating the expression of pathogenicity gene and growth rate. Spring is the season with very frequent temperature changes, and Vm is highly invasive in this season. Therefore, more attention should be paid in spring to protecting apple trees from infection by Vm, such as reduction of pruning wounds formation in spring, and application of protective agents to the pruning wounds in time.