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Discula destructiva, an exotic pathogen of Cornus spp. in North America: Evidence of independent introductions

Denita Hadziabdic: University of Tennessee

<div>Native to North America, <em>Cornus florida</em> (flowering dogwood) and <em>C. nuttallii</em> (Pacific dogwood) are valued for their ornamental characteristics and as an important food source for many animals due to the high fat content of berries. In the late 1970s, trees in the eastern and western United States were declining which resulted in massive die-offs of entire populations. The fungal pathogen, <em>Discula destructiva</em> was identified as the causal agent of dogwood anthracnose. For the next two decades, this fungus spread throughout most of the native ranges of <em>C. florida</em> and <em>C. nuttallii </em>resulting in high mortality rates. We evaluated genetic diversity and population structure of 93 <em>D. destructiva </em>isolates using 47 microsatellite loci. Our results indicated low genetic diversity and the presence of four genetic clusters that corresponded to two major geographic areas, the eastern U.S. and the Pacific Northwest, and to the two time periods when the isolates were collected (pre- and post-1993). Linkage disequilibrium was present in five out of six subpopulations, suggesting that the fungus only reproduced asexually. Evidence of population bottlenecks was found across four identified genetic clusters, and was probably the result of the limited number of founding individuals on both coasts. Our results support the hypothesis that <em>D. destructiva</em> is an exotic pathogen with independent introductions on the east and west coasts of the U.S.</div>