Ectomycorrhizal Inoculum Potential of Northeastern U.S. Forest Soils for American Chestnut: Results from Field and Laboratory Bioassays
American chestnut (Castanea dentata) was once a dominant overstory tree in the eastern United States but was decimated by chestnut blight (Cryphonectria parasitica). Blight resistant chestnut is being developed as part of a concerted restoration effort to bring this heritage tree back. Here, we evaluate the potential of field soils in the northern portion of the chestnut's former range to provide EM (ectomycorrhizal) fungus inoculum for American chestnut. In our first study, chestnut seedlings were grown in a growth chamber using soil collected from three sites dominated by red oak (Quercus rubra) as inoculum and harvested after five months. Of the 14 EM fungi recovered on these seedlings, four species dominated in soils from all three sites: Laccaria laccata, a Tuber sp., Cenococcum geophilum, and a thelephoroid type. In the second study, chestnut seedlings were grown from seed planted directly into soils at the same three sites. Seedlings with intermingling roots of established trees of various species were harvested after five months. Seventy-one EM fungi were found on the root tips of the hosts, with 38 occurring on chestnut seedlings. Multiple- versus single-host EM fungi were significantly more abundant and frequently encountered. The fungi observed dominating on seedlings in the laboratory bioassay were not frequently encountered in the field bioassay, suggesting they occur in soils as resistant propagules. These results show that soil bioassays can be used to inoculate chestnut seedlings with locally adapted fungi. We are now investigating whether chestnut developed for resistance to the blight fungus continue to associate with ectomycorrhizal fungi.