Genome analysis of the ubiquitous boxwood pathogenPseudonectria foliicola

Abstract

Boxwood (Buxus spp.) are broad-leaved, evergreen landscape plants valued for their longevity and ornamental qualities. Volutella leaf and stem blight, caused by the ascomycete fungi Pseudonectria foliicola and P. buxi, is one of the major diseases affecting the health and ornamental qualities of boxwood. Although this disease is less severe than boxwood blight caused by Calonectria pseudonaviculata and C. henricotiae, its widespread occurrence and disfiguring symptoms have caused substantial economic losses to the ornamental industry. In this study, we sequenced the genome of P. foliicola isolate ATCC13545 using Illumina technology and compared it to other publicly available fungal pathogen genomes to better understand the biology of this organism. A de novo assembly estimated the genome size of P. foliicola at 28.7 Mb (425 contigs; N50 = 184,987 bp; avg. coverage 188×), with just 9,272 protein-coding genes. To our knowledge, P. foliicola has the smallest known genome within the Nectriaceae. Consistent with the small size of the genome, the secretome, CAzyme and secondary metabolite profiles of this fungus are reduced relative to two other surveyed Nectriaceae fungal genomes: Dactylonectria macrodidyma JAC15-245 and Fusarium graminearum Ph-1. Interestingly, a large cohort of genes associated with reduced virulence and loss of pathogenicity was identified from the P. foliicola dataset. These data are consistent with the latest observations by plant pathologists that P. buxi and most likely P. foliicola, are opportunistic, latent pathogens that prey upon weak and stressed boxwood plants.

Boxwood Blight: Threat to Ornamentals

“…The white appearance helps to distinguish the boxwood blight pathogen from Pseudonectria foliicola and Pseudonectria buxi (syn. Volutella buxi), which are frequently seen on dead leaves and twigs of boxwood (83). Pseudonectria spp.…”

Comparative genomic analysis reveals contraction of gene families with putative roles in pathogenesis in the fungal boxwood pathogens Calonectria henricotiae and C. pseudonaviculata

“…Lastly, we compared rapidly evolving gene families in Che and Cps to non- Calonectria Buxaceae pathogens Pseudonectria buxi ( Pbu ), P. foliicola ( Pfo ), and Coccinonectria pachysandricola ( Cpa ). Pbu , Pfo , and Cpa are the causal agents of Volutella leaf blight on different hosts in the family Buxaceae and are considered non-aggressive pathogens that typically occur on plants experiencing abiotic stress compared to Che and Cps [ 56 , 73 ]. Pbu causes disease on species of Buxus , while Pfo causes disease on both Buxus and Sarcococca spp., and Cpa causes disease on Sarcococca and Pachysandra spp.…”

Boxwood Blight Disease: A Diagnostic Guide

“…8A to C). Because sporulation of Pseudonectria buxi and P. foliicola, commonly seen on moribund tissues of boxwood (Rivera et al 2018; Shi and Hsiang 2014), appears white before it acquires its characteristic pink pigmentation, Pseudonectria can easily be mistaken for the boxwood blight fungi without examination at higher magnification. Fusarium buxicola with white sporulation is also commonly encountered on boxwood leaves and stems (Farr and Rossman 2020).…”

“…Spores germinate within a few days, and single-spore isolates may also be obtained by transferring germinated spores to new plates. When collecting spores from leaves, care must be taken to avoid areas containing salmon-colored sporodochia of Pseudonectria buxi and P. foliicola (Rivera et al 2018), which are other common boxwood pathogens, because Pseudonectria rapidly outcompete Che and Cps in culture. Calonectria and Pseudonectria often co-occur on the same tissues; however, the rod-like spores of Che and Cps are easily distinguishable from the shorter, elliptical spores of Pseudonectria at 400×.…”