Fungal species assemblages associated with Phytophthora ramorum-infected coast live oaks following bark and ambrosia beetle colonization in northern California

Phytophthora ramorum, the causal agent of sudden oak death (SOD), has killed large numbers of coast live oaks (Quercus agrifolia), California black oaks (12 kelloggii), canyon live oaks (Q chrysolepis), and Shreve oaks (Q parvula var. Shrevei) in coastal California, and tanoaks (Notholithocarpus densiflorus) in California and Oregon. Native and introduced bark and ambrosia beetles selectively tunnel into stem cankers caused by the pathogen. Beetle attacks have been shown to considerably shorten survival of infected coast live oaks. We hypothesized that this behavior facilitates the introduction into, and/or stimulates the activity of, decay and pathogenic fungi, which, in turn, accelerate tree death. The associations of infected coast live oaks, beetles, and sapwood-inhabiting fungi in two California forests were investigated to: (1) determine whether beetles introduce fungi into P. ramorum-infected coast live oaks, (2) catalog the community of fungal species isolated in pure culture from the sapwood of infected and uninfected trees, with and without beetle colonization, and (3) identify the fungi found on beetles reared from infected trees. We inoculated coast live oaks, and monitored symptom development (i.e., bleeding cankers and colonization by beetles) for 2 years. Trees were categorized as (1) live inoculated, bleeding, and still alive, (2) inoculated, bleeding with beetle attacks, and still alive, (3) inoculated with beetle attacks, and dead, (4) felled, asymptomatic before beetle attacks, and (5) felled, asymptomatic after beetle attacks. Trees were felled at 6-month intervals, sapwood samples were plated on four types of growth media, and the ITS sequences of the rDNA regions of morphologically unique fungal cultures were determined. Thirteen species were identified in the sapwood of inoculated trees prior to beetle attacks and 26 species were identified in living inoculated oaks after beetle colonization. The known oak pathogens Botryosphaeria cord cola and B. sarmentorum were isolated from all categories of inoculated trees as well as from beetles. Fungi reported elsewhere as pathogens that were isolated from beetle-colonized trees include an Aureobasidium sp., Bartalinia robillardoides, Geosmithia fassatiae, a Monochaetia sp., Pleurostomophora richardsiae, Truncatella angustata, and Stereum hirsutum. Nineteen fungal taxa were isolated from beetles emerging from infected logs, including B. corticola, B. sarmentorum, a Pestalotiopsis sp., G. fassatiae, a Geosmithia sp., an Ophiostoma sp., and an uncultured Pleosporaceae sp. Preventing beetle attacks might limit introduction of fungi into infected coast live oaks, increasing the effectiveness of resistance mechanisms against P. ramorum. Where the disease is not fatal, beetles attacking trees infected by this introduced pathogen may facilitate new ecological associations between fungi, beetles, and a native host tree species. (C) 2012 Elsevier B.V. All rights reserved.