The slippery nature of ectomycorrhizal host specificity: fungi associated with novel pinoid () and abietoid () hosts.

is among the best-known examples of an ectomycorrhizal (ECM) fungal genus that demonstrates a high degree of host specificity. Currently recognized host genera of include , and , which all belong to the pinoid clade of the family Pinaceae. Intriguingly, sporocarps have been sporadically collected in forests in which known hosts from these genera are locally absent. To determine the capacity of to associate with alternative hosts in both the pinoid and abietoid clades of Pinaceae, we examined the host associations of two species ( and ) through field-based root tip sampling and seedling bioassays. Root tip collections underneath sporocarps were molecularly identified (fungi: nuc rDNA internal transcribed spacer region ITS1-5.8S-ITS2 [ITS barcode]; plant: L) to assess the association with multiple hosts. The bioassays contained both single- and two-species treatments, including a primary ( or ) and a secondary (, or ) host. For the bioassay, an additional treatment in which the primary host was removed after 8 mo was included to assess the effect of primary host presence on longer-term ECM colonization. The field-based results confirmed that fungi were able to associate with and hosts, representing novel host genera for this genus. In the bioassays, colonization on the primary hosts was detected in both single- and two-species treatments, but no colonization was present when and hosts were grown alone. Removal of a primary host had no effect on percent ECM colonization, suggesting that primary hosts are not necessary for sustaining colonization once they are successfully established on secondary hosts. Collectively, our results indicate that host specificity is more flexible in this genus than previously acknowledged and help to explain the presence of in forests where recognized hosts are not present.