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

Abstract

In Mediterranean Europe and the United States, oak species (Quercus spp.) have been in various states of decline for the past several decades. Several insect pests and pathogens contribute to this decline to varying degrees including Phytophthora cinnamomi, Armillaria spp., various insect defoliators, and additionally in the U.S., the oak wilt pathogen, Bretziella fagacearum. More recently, two emerging canker pathogens, Diplodia corticola (Dc) and Diplodia quercivora (Dq) have been implicated in causing dieback and mortality of oak species in Europe and in several regions in the United States. In 2019, a fungal survey was conducted in the Mid-Atlantic region of the Eastern U.S., including Maryland, Pennsylvania, Virginia, and West Virginia to determine the range and impact of Dc and Dq on forest health within the U.S. A total of 563 oak trees 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 from 5,335 total plugs collected from the 13 of 18 sampled counties. Recovered Diplodia species included Dc, Dq, and D. sapinea (Ds), as well as Botryosphaeria dothidea (Bd), a closely related canker pathogen 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 conducive for disease. In an attempt to better understand the relationships among Dc and potentially their geographic origin(s), a multi-gene phylogenetic study and corresponding morphological study were conducted. A total of 49 Diplodia isolates from Spain, France, Italy, and the U.S. were assessed. Across all isolates and geographic regions, Dc formed a strongly supported monophyletic clade sister to Dq and included two strongly supported subclades, one that included isolates from Spain and California and a second that included isolates from Italy, Maryland, and West Virginia. Both subclades also exhibited overlapping spore measurements. These results support Dc as a cosmopolitan pathogen, native to both Europe and the U.S. with the possibility of secondary introductions.

Cork Oak Endophytic Fungi as Potential Biocontrol Agents against Biscogniauxia mediterranea and Diplodia corticola

“…The presence of D. corticola, D. quercivora and Discula quercina in declining cork oak trees agrees with their role as emerging pathogens to Quercus spp. in different regions of the world [11,41], which is emphasized by their risk to cork oak health as previously reported [39,42]. Concerning the fungi displaying a beneficial role, AL forest presented the highest number of OTUs with a potential beneficial role (all described as pathogenic-beneficial), but GV forest was the richest with exclusive beneficial fungi (Figure 2A).…”

First Report of Diplodia quercivora and Neofusicoccum vitifusiforme Associated with Cankers and Necrosis of Holm Oak (Quercus ilex) in Declining Stands in Southern Italy

“…D. quercivora is recognized as a pathogen of Quercus spp. in other countries [29][30][31]37]. However, our study has shown that this species is also associated as a pathogen with holm oak in Italy.…”

“…In Italy, as well as in the rest of Europe, increasingly frequent reports of tree decline in oak forests have been recorded and a range of oak pathogens, for example, Armillaria spp., Cephalosporium spp., Cladosporium spp., Cylindrocarpon spp., Diplodia spp., Biscogniauxia mediterranea (Hypoxylon mediterraneum), Phoma cavae, Phomopsis quercina, Sporotrix spp., and Phytophthora spp. have been found to be associated with this decline [27][28][29][30][31][32][33][34]. It is not yet clear whether the disease development in many of these cases is primarily caused by emerging pathogens, as, for example, found in Italy for Diplodia corticola and Phytophthora cinnamomi on declining holm oak in Caprera Island (Sardinia) [28] and in the Salento Peninsula (Apulia) [35], or whether tree decline is due to physiological stresses caused by climate change, as in the case of the Lucanian Apennine (Basilicata Region) in Italy [36].…”

Cytospora paraplurivora sp. nov. isolated from orchards with fruit tree decline syndrome in Ontario, Canada

“…In this study, using the sequence data of act and tef1-α genes suggested by Urbez-Torres et al [39] improved the phylogenetic resolution of this species complex. This approach has also been extensively used to resolve phylogenies of other plant-associated fungal pathogens such as Diaporthe [71], Valsaria [72] and Diplodia [73].…”