Recent insights into the pandemic disease butternut canker caused by the invasive pathogen Ophiognomonia clavigignenti‐juglandacearum

Summary 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 ( J uglans cinerea ) in N orth A merica, from M innesota in the upper M idwest to T ennessee in the south and Q uebec in the north‐east. 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, O phiognomonia clavigignenti‐juglandacearum , an invasive fungal pathogen, that as its emergence has spread across N orth A merica and is now found wherever butternut naturally occurs. Taxonomic studies have determined that the fungus belongs in the genus O phiognomonia , which includes a number of saprophytes, endophytes and pathogens of members of the F agales, rather than the genus S irococcus, 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 J uglans and C arya 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 U nited S tates for over a century, but it was the emergence of a more virulent strain of the fungus in M innesota and W isconsin in the 1960s that resulted in range‐wide mortality and pushed butternut to be listed as an endangered species in C anada and a number of states in the U nited S tates.