Characterization and taxonomic reassessment of the box blight pathogen Calonectria pseudonaviculata, introducing Calonectria henricotiae sp. nov.

Abstract

Calonectria pseudonaviculata, the causal agent of the disease of Buxus spp. known as 'box blight', was first detected in the mid-1990s in the UK and New Zealand. Since then, the geographic range of box blight has rapidly expanded to at least 21 countries throughout temperate regions of the world, causing significant losses in nurseries, gardens and wild boxwood populations. This study determined the genetic diversity in a collection of 234 Calonectria isolates from diseased Buxus plants, originating from 15 countries and four continents. Two genetic clades, G1 and G2, were identified within this sample using multilocus phylogenetic analysis. The application of genealogical concordance phylogenetic species recognition criteria using four independent nuclear loci determined that the Calonectria isolates in these two clades are separate phylogenetic species. The isolates in the G1 clade were upheld as C. pseudonaviculata sensu stricto. Based on phylogenetic distinctiveness and the lack of mating, a new species is proposed, Calonectria henricotiae sp. nov., for the Calonectria isolates in the G2 clade. A PCR-RFLP assay and real-time PCR assays were developed to easily and reproducibly differentiate these species. To assess the practical implications of the identification of the two species, their physiology, fungicide susceptibility and pathogenicity were compared. No differences in pathogenicity were observed. However, C. henricotiae isolates exhibited greater thermotolerance and reduced sensitivity to specific triazole as well as strobilurin fungicides. The identification of a second phylogenetic species causing box blight may have a substantial impact on the epidemiology and control of this destructive disease.

Fungicide Efficacy Against Calonectria pseudonaviculata, Causal Agent of Boxwood Blight

“…We determined that EC50 values for conidial germination for all fungicides tested in both studies, including the (2) concluded that azoxystrobin did not effectively inhibit conidial germination. Gehesquiere et al (5) separated European isolates of C. pseudonaviculata into two clades, G1 and G2, using amplifled fragment length polymorphism markers. They determined that there was little variation within clades and that both clades had similar pathogenicity and morphology.…”

The forgotten Calonectria collection: Pouring old wine into new bags

“…This large number of species has arisen mainly due to the introduction of DNA sequence data and subsequent phylogenetic inference enabling delimitation of numerous previously unrecognised cryptic taxa. These species often share the same plant hosts, informing knowledge of the epidemiology and fungicide resistance (Graça et al., 2009, Vitale et al., 2013, Gehesquière et al., 2016). …”

“…have chiefly been reported only from South America and the Northern Hemisphere, where they are mostly associated with nursery diseases (Lombard et al., 2010c, Vitale et al., 2013), Cylindrocladium black rot of peanut (Bell and Sobers, 1966, Beute and Rowe, 1973, Hollowell et al., 1998) and box blight of Buxus spp. (Henricot et al., 2000, Crepel and Inghelbrecht, 2003, Brand, 2005, Saracchi et al., 2008, Saurat et al., 2012, Mirabolfathy et al., 2013, Gehesquière et al., 2016). …”

Two novel species of Calonectria isolated from soil in a natural forest in China

“…Species of Calonectria are also rapidly dispersed via aerial dissemination and water movement, which leads to the transmission of Calonectria disease . Based on previous studies, at least 145 Calonectria species have been identified using molecular data and have been described worldwide (Crous 2002, Crous et al 2004, Lombard et al 2010a, b, c, 2011, 2016, Chen et al 2011, Alfenas et al 2013a, Gehesquière et al 2015. Sixty species were isolated from soil samples collected in subtropical or tropical regions (Crous 2002, Crous et al 2004, Lombard et al 2010a, b, c, 2015, 2016, Chen et al 2011, Alfenas et al 2015.…”