One clonal lineage of Calonectria pseudonaviculata is primarily responsible for the boxwood blight epidemic in the United States

Authors: CASTROAGUDÍN, VANINA - Oak Ridge Institute For Science And Education (ORISE); Weiland, Gerald - Jerry; BAYSAL-GUREL, FULYA - Tennessee State University; CUBETA, MARC - North Carolina State University; DAUGHTREY, MARGERY - Cornell University; GAUTHIER, NICOLE - University Of Kentucky; LAMONDIA, JAMES - Connecticut Agricultural Experiment Station; Luster, Douglas - Doug; HAND, FRANCESCA - The Ohio State University; Shishkoff, Nina; WIILIAMS-WOODWARD, JEAN - University Of Georgia; LEBLANC, NICHOLAS - Oak Ridge Institute For Science And Education (ORISE); YANG, XIAO - Oak Ridge Institute For Science And Education (ORISE); Crouch, Jo Anne

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/2020
Publication Date: 6/25/2020
Citation: Castroagudín, V., Weiland, J.E., Baysal-Gurel, F., Cubeta, M., Daughtrey, M., Gauthier, N., Lamondia, J., Luster, D.G., Hand, F., Shishkoff, N., Williams-Woodward, J., LeBlanc, N., Yang, X., Crouch, J.A. 2020. One clonal lineage of Calonectria pseudonaviculata is primarily responsible for the boxwood blight epidemic in the United States. Phytopathology. 110(11):1845-1853. https://doi.org/10.1094/PHYTO-04-20-0130-R.
DOI: https://doi.org/10.1094/PHYTO-04-20-0130-R

Interpretive Summary: Boxwood plants worldwide are threatened by a deadly fungal disease known as boxwood blight. Currently the disease is spreading rapidly across the U.S. and parts of Asia, but has been present in Europe since the late 1990s. Early U.S. outbreaks of the disease have been characterized, but the genetic composition of the fungus as it has spread since 2014 is unknown. Therefore, a nationwide collection of 218 new samples of the fungus were typed using custom DNA markers called SSRs. Scientists learned that just one major genetic group is responsible for the majority of U.S. boxwood blight outbreaks, although individuals from a second major genetic group are also present at low frequency. The boxwood blight fungi are continuing to multiply by making clones of themselves, but new genetic types have evolved from the dominant genetic type since 2014. This research is important because it shows how spread of the blight fungus has occurred and suggests how it will continue to move in the U.S. This research will be used by plant pathologists, plant breeders extension personnel and quarantine officials to minimize the transmission and impact of boxwood blight.

Technical Abstract: Boxwood blight caused by Calonectria pseudonaviculata (Cps) and C. henricotiae (Che) is decimating cultivated and native boxwood worldwide, with profound negative economic impact in the horticulture industry. First documented in the U.S. in 2011, the disease is now present in 28 states. Previous research showed that global Cps populations prior to 2014 had a clonal structure, and only the MAT1-2 idiomorph was observed. Here we set out to determine Cps genetic diversity and population structure in the U.S. after 2014, following expansion of the disease across the country during the past five years. Two hundred eighteen new isolates from 21 states were genotyped by sequencing 11 simple sequence repeat (SSR) loci and by MAT1 idiomorph typing. All isolates presented Cps-specific alleles, indicating that Che is still absent in the U.S. The presence of only the MAT1-2 idiomorph and linkage disequilibrium suggests the prevalence of asexual reproduction. The contemporary Cps population is characterized by a clonal structure and composed of 13 multilocus genotypes (SSR-MLG) unevenly distributed across the U.S. The 13 SSR-MLGs grouped into two clonal lineages (CL). The predominant lineage CL2 (93% of isolates) is the primary contributor to U.S. disease expansion. The contemporary U.S. Cps population is not geographically subdivided and is not differentiated from the U.S. population prior to 2014, yet it is significantly differentiated from the main European population, which is largely composed of CL1. Our findings provide insights into the contemporary boxwood blight epidemic that are critical for disease management and breeding of resistant boxwood cultivars.