Morphological and Phylogenetic Resolution of Diplodia spp., Emerging Canker Pathogens of Oak (Quercus spp.), in the United States

In Mediterranean Europe and the United States, oak species (Quercus spp.) have been in a state of declining health and vigor for the past several decades. Several biotic agents have been found to contribute to this decline to varying degrees including Phytophthora cinnamomi and Armillaria spp., both causal agents of root rots, and various insect defoliators. In the U.S., Bretziella fagacearum, the causal agent of oak wilt, has also been implicated. More recently, Diplodia corticola (Dc) and other Diplodia spp. have been implicated in causing dieback and mortality of oak species in Europe and in several regions in the United States. In an attempt to determine the origin(s) of Dc and whether it can be considered an introduced pathogen in the U.S., a phylogenetic study and associated morphological studies were conducted to resolve relationships among Dc isolates from geographically discrete populations in both Europe and the U.S. A total of 23 Diplodia isolates from Spain, France, Italy and the U.S. were studied, including 20 previously ITS barcoded Dc isolates, two Dq isolates, and one D. mutila (Dm) isolate. Many of these isolates had been previously included in pathogenicity tests. Across all isolates and geographic regions, Dc formed a strongly supported clade sister to Dq and included two subclades, one that included isolates from Spain and California and a second that included isolates from Italy and West Virginia. Both subclades had moderate bootstrap support. The Spain and California isolates also exhibited overlapping culture morphology and spore measurements, but larger trends based on these morphological features across Dc were not consistent or phylogenetically informative. The results of this study offer support for Dc as a cosmopolitan pathogen, native to both Europe and the U.S. Concurrent to investigations between European and U.S. Diplodia populations, a study was initiated to determine the range and impact of Dc and Dq on forest health within the U.S. To this end, a fungal survey was conducted in the Mid-Atlantic region, including Maryland, Pennsylvania, Virginia and West Virginia in 2019. A total of 563 oak species between red and white oak family members were evaluated across 33 forests spanning 18 counties. A total of 32 Diplodia isolates encompassing three Diplodia spp. were recovered out of some 5,335 total plugs sampled from the 18 sampled counties. Diplodia species recovered included Dc, Dq, and D. sapinea (Ds), as well as Botryosphaeria dothidea (Bd), another close relative in the Botryosphaeriaceae. Both Dc and Ds were recovered from red and white oak family members, whereas Dq was exclusive to white oak family members and Bd to red oak family members. Of these species Dc was most frequently isolated followed by Dq, Ds, and Bd. Overall, mortality was relatively low across all sampled counties, indicating that these fungi, at the levels that were detected, are not widely inciting oak decline across the region but more likely are acting opportunistically when the environment is favorable