Acquisition of Ophiostoma quercus and Ceratocystis fagacearum by Nitidulids from O. quercus–Colonized Oak Wilt Mats

Oak wilt infection centers are initiated by overland spread of the causal fungus, Ceratocystis fagacearum (T. W. Bretz) J. Hunt, by insects (10). Sap-feeding beetles (Coleoptera: Nitidulidae) are considered a primary vector of the pathogen in the north central states (7,9). Successful transmission by sap-feeding beetles depends on several coexisting conditions, including (i) the availability of viable inoculum on C. fagacearum mycelial mats; (ii) the presence of active nitidulids to transmit the pathogen; (iii) a fresh, xylem-exposing wound receptive to infection by the pathogenic propagules present on the beetles; (iv) the coexistence of these events during the spring when Quercus spp. are most susceptible to infection; and (v) the absence of microbial deterrents to infection at or introduced to the wound surface (21). Ophiostoma quercus (Georgev.) Nannf. (previously the hardwood type of O. piceae (Münch.) Syd. & P. Syd.) (19), a common sapwood colonizer of hardwood species, has been implicated as a biological control organism in the oak wilt disease cycle (8, 20). In an inoculation experiment on healthy northern pin oak, Q. ellipsoidalis E. J. Hill, Gibbs (8) found that when O. quercus was introduced into a fresh wound 24 h before C. fagacearum was introduced, infection by the pathogen was prevented. He speculated that O. quercus colonization of fresh wounds on healthy oaks may be partly responsible for the wounds being receptive to C. fagacearum for only a few days (3,14,17). Propagules of O. quercus have been isolated from both free-flying (2,12) and mat-inhabiting nitidulids (12). In addition, nitidulids are common in fresh wounds on healthy oaks (4,18). Thus, it is very likely that nitidulids initiate O. quercus colonization when they visit fresh wounds on oaks. Oak wilt mats overgrown by the Graphium anamorph of O. quercus are the most likely places for nitidulids to acquire O. quercus (8). On the basis of microscopic and isolation studies, Reutze and Parameswaran (20) suggested that O. quercus colonization of mats would reduce the acquisition of C. fagacearum by insect vectors. They speculated that nitidulids leaving such mats might well be carrying spores of O. quercus more frequently than C. fagacearum, thus increasing the probability that nitidulids would successfully vector O. quercus to fresh wounds more often than they would vector the oak wilt pathogen. The O. quercus-colonized wound would then no longer be receptive to C. fagacearum infection (8); and in this sense, nitidulids and O. quercus would appear to play a role in biological control of overland transmission of C. fagacearum in nature (11,12). Our research was undertaken to determine whether the frequency of O. quercus and C. fagacearum propagule acquisition by nitidulids visiting oak wilt mats is impacted by the presence of O. quercus on the mats. Because O. quercus is a common natural colonizer of the mats (13), we wished to artificially establish a different level of colonization for comparison, i.e., either no colonization (via exclusion) or complete colonization (via augmentation). The latter strategy was chosen for this study. Our objectives were to determine (i) whether augmentation sprays with O. quercus would result in significantly greater colonization of oak wilt mats compared with the untreated mats and (ii) the frequencies with which nitidulids associated with differentially colonized mats acquire C. fagacearum and/or O. quercus.