Boxwood blight caused by Calonectria pseudonaviculata and C. henricotiae is destroying cultivated and native boxwood worldwide, with profound negative economic impacts on the horticulture industry. First documented in the United States in 2011, the disease has now occurred in 30 states. Previous research showed that global C. pseudonaviculata populations prior to 2014 had a clonal structure, and only the MAT1-2 idiomorph was observed. In this study, we examined C. pseudonaviculata genetic diversity and population structure in the United States after 2014, following the expansion of the disease across the country over the past 5 years. Two hundred eighteen isolates from 21 states were genotyped by sequencing 11 simple sequence repeat (SSR) loci and by MAT1 idiomorph typing. All isolates presented C. pseudonaviculata-specific alleles, indicating that C. henricotiae is still absent in the U.S. states sampled. The presence of only the MAT1-2 idiomorph and gametic linkage disequilibrium suggests the prevalence of asexual reproduction. The contemporary C. pseudonaviculata population is characterized by a clonal structure and composed of 13 multilocus genotypes (SSR-MLGs) unevenly distributed across the United States. These SSR-MLGs grouped into two clonal lineages (CLs). The predominant lineage CL2 (93% of isolates) is the primary contributor to U.S. disease expansion. The contemporary U.S. C. pseudonaviculata population is not geographically subdivided and not genetically differentiated from the U.S. population prior to 2014, but is significantly differentiated from the main European population, which is largely composed of CL1. Our findings provide insights into the boxwood blight epidemic that are critical for disease management and breeding of resistant boxwood cultivars.

由Calonectria pseudonaviculata和C. henricotiae引起的黄杨枯萎病正在全球范围内破坏耕种的和天然的黄杨木，对园艺业产生了深远的负面经济影响。该病于2011年首次在美国记录，目前已在30个州发生。先前的研究表明，2014年之前的全球假单胞菌种群具有克隆结构，并且仅观察到MAT1-2独特型。在这项研究中，我们检查了C. pseudonaviculata在过去5年中该疾病在美国各地蔓延之后，2014年之后美国的遗传多样性和人口结构出现了变化。来自21个国家的两个了118株被测序11简单重复序列（SSR）位点，并通过基因分型MAT1 idiomorph打字。所有分离株均呈现假虫假丝酵母特异性等位基因，这表明在美国采样的州中仍未存在亨氏假丝酵母。仅MAT1-2独特型和配子连锁不平衡的存在表明无性生殖的流行。当代C. pseudonaviculata人群的特征是克隆结构，由在美国分布不均匀的13个多基因座基因型（SSR-MLG）组成。这些SSR-MLG分为两个克隆谱系（CL）。优势谱系CL2（占分离株的93％）是导致美国疾病扩展的主要因素。2014年之前，当代美国假单胞菌种群并未在地理上进行细分，也未在遗传上与美国种群区别开来，但与主要由CL1组成的主要欧洲种群却有明显区别。我们的发现提供了对黄杨疫病流行的见解，这对于疾病管理和抗性黄杨品种的育种至关重要。