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

Abstract

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.

Impact of the goldspotted oak borer, Agrilus auroguttatus, on the health of coast live oak before and after treatment with two systemic insecticides

“…However, Haavik et al 13 also reported generally low rates of appearance of symptoms of A. auroguttatus infestation (emergence hole density, woodpecker activity and bark staining), as well as tree mortality (5 out of 722 trees), at six survey sites on the periphery of the zone of infestation in San Diego County. Lynch et al 41 also reported low levels of oak mortality in the same general area. In some instances, crown dieback rating decreased during the survey period (Tables 1 and 2).…”

Occurrence of Diplodia corticola, including new oak host records, in Wisconsin, USA

“…Earlier reports of D. corticola in North America (including identification of strains to which other names had been applied) document presence within the extreme eastern, southern and western United States (Figure a). These include Maine (Aćimović et al., ), Massachusetts (Munck et al., ), Pennsylvania (Martin & Munck, ), Washington, D.C. (Alves et al., ; Jacobs & Rehner, ), West Virginia (Martin et al., ), North Carolina (Alves et al., ), Florida (Dreaden et al., ), Texas (Úrbez‐Torres et al., ) and California (Alves et al., ; Jacobs & Rehner, ; Lynch et al., ; McPherson et al., ; Úrbez‐Torres et al., ) (Figure a). Some of these reports describe occurrence of the pathogen at only a single location and/or point in time.…”

“…Lynch et al. () examined 857 coast live oak trees in two California counties in which large‐scale oak die‐off occurred. Diplodia corticola was one of the three most frequently cultured fungi considered “aggressive” pathogens in this study.…”

“…As reported for the rare North American example of such a detailed study, however, the role of this pathogen may be less clear. Lynch et al (2014) examined 857 coast live oak trees in two California counties in which large-scale oak die-off occurred.…”

“…However, D. corticola is much less well‐known in North America. Description of this fungus, followed by reports from multiple Quercus species in the extreme eastern, southern and western United States (Figure a) (Aćimović et al., ; Alves et al., ; Dreaden, Shin, & Smith, ; Lynch, Zambino, Scott, & Eskalen, ; Martin et al., ; McPherson, Erbilgin, Bonello, & Wood, ; Munck, Wyka, Bohne, Green, & Siegert, ; Úrbez‐Torres, Peduto, Rooney‐Latham, & Gubler, ), prompted reconsideration of strains in our collection that had been obtained in Wisconsin from native oaks. Collected in the late 1990s and early 2000s, these strains had been only tentatively identified as Diplodia species based on anamorph characteristics.…”