Abstract
Since the 1990s, brown root rot caused by Phellinus noxius (Corner) Cunningham has become a major tree disease in Taiwan. This fungal pathogen can infect more than 200 hardwood and softwood tree species, causing gradual to fast decline of the trees. For effective control, we must determine how the pathogen is disseminated and how the new infection center of brown root rot is established. We performed Illumina sequencing and de novo assembly of a single basidiospore isolate Daxi42 and obtained a draft genome of ~40 Mb. By comparing the 12,217 simple sequence repeat (SSR) regions in Daxi42 with the low-coverage Illumina sequencing data for four additional P. noxius isolates, we identified 154 SSR regions with potential polymorphisms. A set of 13 polymorphic SSR markers were then developed and used to analyze 329 P. noxius isolates collected from 73 tree species from urban/agricultural areas in 14 cities/counties all around Taiwan from 1989 to 2012. The results revealed a high proportion (~98%) of distinct multilocus genotypes (MLGs) and that none of the 329 isolates were genome-wide homozygous, which supports a possible predominant outcrossing reproductive mode in P. noxius. The diverse MLGs exist as discrete patches, so brown root rot was most likely caused by multiple clones rather than a single predominant strain. The isolates collected from diseased trees near each other tend to have similar genotype(s), which indicates that P. noxius may spread to adjacent trees via root-to-root contact. Analyses based on Bayesian clustering, F ST statistics, analysis of molecular variance, and isolation by distance all suggest a low degree of population differentiation and little to no barrier to gene flow throughout the P. noxius population in Taiwan. We discuss the involvement of basidiospore dispersal in disease dissemination.
Highlights
Brown root rot caused by Phellinus noxius (Corner) Cunningham, a white rot fungus, occurs in tropical and subtropical areas worldwide
With the development of next-generation sequencing (NGS) technology and bioinformatics, obtaining a draft genome of an organism is becoming easier for genome-wide search of repeat regions and rapid simple sequence repeat (SSR) marker development
This approach has allowed for efficient development of large numbers of SSR markers for a wide range of organisms, including several plant fungal and oomycete pathogens such as Phytophthora ramorum [45] and Anisogramma anomala [46]
Summary
Brown root rot caused by Phellinus noxius (Corner) Cunningham, a white rot fungus, occurs in tropical and subtropical areas worldwide. When grown in potato dextrose agar (PDA), it forms colonies that are initially white to yellowish brown and with age are decorated with irregularly shaped lines or patches of dark brown tissues. Trichocysts and arthospores are formed in culture, no clamp connection is observed [1]. Formation of arthospores in culture is rarely seen in other species of Phellinus, so it may serve as a reference for the identification of P. noxius. When grown on sawdust medium, P. noxius can form thin-layered, flat basidiocarps that are initially yellowish brown and later turn brown and dark gray, similar to those found in nature [2]
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