Delimiting cryptic pathogen species causing apple Valsa canker with multilocus data

Abstract

Fungal diseases are posing tremendous threats to global economy and food safety. Among them, Valsa canker, caused by fungi of Valsa and their Cytospora anamorphs, has been a serious threat to fruit and forest trees and is one of the most destructive diseases of apple in East Asia, particularly. Accurate and robust delimitation of pathogen species is not only essential for the development of effective disease control programs, but also will advance our understanding of the emergence of plant diseases. However, species delimitation is especially difficult in Valsa because of the high variability of morphological traits and in many cases the lack of the teleomorph. In this study, we delimitated species boundary for pathogens causing apple Valsa canker with a multifaceted approach. Based on three independent loci, the internal transcribed spacer (ITS), β-tubulin (Btu), and translation elongation factor-1 alpha (EF1α), we inferred gene trees with both maximum likelihood and Bayesian methods, estimated species tree with Bayesian multispecies coalescent approaches, and validated species tree with Bayesian species delimitation. Through divergence time estimation and ancestral host reconstruction, we tested the possible underlying mechanisms for fungal speciation and host-range change. Our results proved that two varieties of the former morphological species V. mali represented two distinct species, V. mali and V. pyri, which diverged about 5 million years ago, much later than the divergence of their preferred hosts, excluding a scenario of fungi–host co-speciation. The marked different thermal preferences and contrasting pathogenicity in cross-inoculation suggest ecological divergences between the two species. Apple was the most likely ancestral host for both V. mali and V. pyri. Host-range expansion led to the occurrence of V. pyri on both pear and apple. Our results also represent an example in which ITS data might underestimate species diversity.

Two members of the velvet family, VmVeA and VmVelB, affect conidiation, virulence and pectinase expression in Valsa mali

“…It is well established that conidial reproduction is important for fungi to survive in nature, especially as V. mali mainly infects the host bark via conidia (Wang X. et al ., ). Our results suggest that VmVeA and VmVelB negatively regulate conidiation (Fig.…”

“…Valsa mali is an ascomycete which causes Apple Valsa Canker (AVC). The disease is very important for apple production in eastern Asia, especially in China, where yield losses can reach 100% (Li et al ., ; Wang X. et al ., ). This necrotrophic pathogen mainly infects host bark by means of conidia entering through wounds (Ke et al ., ; Wang X. et al ., ).…”

“…The disease is very important for apple production in eastern Asia, especially in China, where yield losses can reach 100% (Li et al ., ; Wang X. et al ., ). This necrotrophic pathogen mainly infects host bark by means of conidia entering through wounds (Ke et al ., ; Wang X. et al ., ). After successful invasion in wounded tissue, infecting hyphae develop and colonize the bark tissue, leading to severe tissue maceration and necrosis (Yin et al ., ).…”

Genome sequence of Valsa canker pathogens uncovers a potential adaptation of colonization of woody bark

“…display a strong host preference in a sense that one species occurs only on hosts of one plant family or even only one genus (Farr et al ., ; Kepley & Jacobi, ). For example, V. mali is more aggressive on apple than on other species in Rosaceae, regardless of the host plant origin, while V. pyri , the causal agent of pear Valsa canker, mainly infects pear (Wang et al ., , ). Given the host preference of Valsa spp., the variations of secondary metabolism gene clusters between the two Valsa genomes suggest that small polypeptides secreted by these canker pathogens may act as host selective toxins (also known as effectors), which are probably responsible for their host specificity.…”

Agrilus maliMatsumara (Coleoptera: Buprestidae), a new invasive pest of wild apple in western China: DNA barcoding and life cycle

“…mali (Vmm), has been one of the important threatening factors for the wild apple forests (Wang, Li, et al, 2014a;Wang, Zang, Yin, Kang, & Huang, 2014b). With the attack of wild apple branches by A. mali, the vulnerability to V. mali invasion increases, which can accelerate tree mortality (Wang, Li, et al, 2014a;Wang, Wei, Huang, & Kang, 2011;Wang, Zang, et al, 2014b). Currently, A. mali is the primary threat to wild apple in the Tianshan region of Xinjiang-Uyghur Autonomous Province and is considered as a quarantine pest.…”