Occurrence, incidence and associations among fungal pathogens and Agrilus auroguttatus, and their roles in Quercus agrifolia decline in California

Synchronous decline of oak (Quercus spp.) trees in woodlands has been described in Europe and eastern North America as a complex interaction of stressors that predispose, incite or contribute to tree death. This study presents a 2-year (2010-2011) assessment of the role of pathogens in coast live oak (Quercus agrifolia) woodlands in southern California where oak mortality occurs in locations that are infested and uninfested by the goldspotted oak borer (GSOB, Agrilus auroguttatus). Cumulative coast live oak mortality was not significantly different between sites and was weakly correlated with Diplodia corticola and GSOB incidence and negatively correlated with annual relative humidity. Multiple logistic regression models explained the presence of individual fungi or GSOB at the tree level. Fisher's exact test analysis determined that the presence of D.corticola, Fusarium solani, Dothiorella iberica, Cryptosporiopsis querciphila and Diatrypella verrucaeformis were each related to origin of sample location on tree, and C.querciphila was additionally related to symptom type on the bole. Multiple linear regression models showed high correlation between environmental variables and plot-level incidence of both GSOB and D.corticola. Disease incidence (DI) for D.corticola was highest in GSOB-uninfested locations. Jaccard index of association (J) showed that D.corticola was negatively associated with the presence of GSOB, F.solani and C.querciphila. Results suggest that oak decline in California is an example of a complex syndrome involving strong regional differences in factors that are associated with the problem.