In the 25 years following the initial 1967 report of the disease, butternut canker was able to quickly spread throughout the entire range of butternut (Juglans cinerea) in North America, from Minnesota in the upper Midwest to Tennessee in the south and Quebec in the northeast. The speed of this dispersal is notable as butternut trees do not make up a significant proportion of any single forest type. Instead, they are usually found sparingly in most mixed hardwood forests. In this review, we synthesize the current knowledge of the invasion process of the butternut canker pathogen, Ophiognomonia clavigignenti-juglandacearum, an invasive fungal pathogen, that as its emergence has spread across North America and is now found wherever butternut naturally occurs. Taxonomic studies have determined that the fungus belongs in the genus Ophiognomonia, which includes a number of saprophytes, endophytes and pathogens of members of the Fagales, rather than the genus Sirococcus, which includes several important pine pathogens. The ability of fungus to be dispersed by rain splash, transported on and in beetle vectors, transmitted by infected seed and successfully colonized several species of Juglans and Carya have all likely contributed to the rapid increase in abundance and severity of disease and tree mortality in the invaded forest ecosystems. Recent genomic and population genetic analyses have determined that there were at least three emergence events. A less virulent strain of the fungus likely has been present in the north eastern United States for over a century, but it was the emergence of a more virulent strain of the fungus in Minnesota and Wisconsin in the 1960s that resulted in range-wide mortality and pushed butternut to be listed as an endangered species in Canada and a number of states in the United States.
The occurrence of multiple introduction events, or sudden emergence from a host jump, of forest pathogens may be an important factor in successful establishment in a novel environment or on a new host; however, few studies have focused on the introduction and emergence of fungal pathogens in forest ecosystems. While Ophiognomonia clavigignenti-juglandacearum (Oc-j), the butternut canker fungus, has caused range-wide mortality of butternut trees in North America since its first observation in 1967, the history of its emergence and spread across the United States and Canada remains unresolved. Using 17 single nucleotide polymorphic loci, we investigated the genetic population structure of 101 isolates of Oc-j from across North America. Clustering analysis revealed that the Oc-j population in North America is made up of three differentiated genetic clusters of isolates, and these genetic clusters were found to have a strong clonal structure. These results, in combination with the geographic distribution of the populations, suggest that Oc-j was introduced or has emerged in North America on more than one occasion, and these clonal lineages have since proliferated across much of the range of butternut. No evidence of genetic recombination was observed in the linkage analysis, and conservation of the distinct genetic clusters in regions where isolates from two or more genetic clusters are present, would indicate a very minimal or non-existent role of sexual recombination in populations of Oc-j in North America.
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