Apple Valsa canker: insights into pathogenesis and disease control

Apple Valsa canker (AVC) has caused significant losses worldwide, especially in East Asia. Various fungal species from the genus Cytospora/Valsa can infect tree bark and cause tissue rot, and Valsa mali (Vm) is responsible for the most severe tree branch deaths and yield losses. Since AVC was first reported in Japan in 1903, the pathogen species, biological characteristics, infection and pathogenesis, spore dissemination, and disease cycle have been intensively investigated. Based on the new cognition of the disease dynamics, the disease control strategy has shifted from scraping diseased tissue to protecting the bark from infection. In this review, we summarize new knowledge of the Vm infection process mediated by various kinds of virulence factors, including cell wall degrading enzymes, toxins, effectors, microRNA-like RNAs, and pathogenic signaling regulators. We also introduce progress in evaluating germplasm resources and identifying disease response-related genes in apples. In addition, we elaborate current understanding of spore dissemination and disease cycles in orchards and disease prevention techniques. Finally, we provide recommendations for developing more cost-effective strategies for controlling AVC by applying genetic resistance and biological fungicides.