Assessment of Acute Oak Decline in Latvia

Acute Oak Decline (AOD) is a complex disease affecting oaks (Quercus spp.). Typical symptoms include a dark, sticky  exudate from bark cracks. Larval galleries of the beetle Agrilus biguttatus under the bark of affected trees are often reported. AOD is characterized by oak decline, after which a large proportion of the oaks die, but some survive. In Latvia, oaks with AOD symptoms were first observed in 2017, and the presence of Gibbsiella quercinecans and Brenneria goodwinii in the bark exudate was confirmed. A country-wide survey was initiated to assess the prevalence of oaks with AOD symptoms. In addition, experimental plots were established to monitor the progression of AOD in infected sites. A total of 1,329 forest stands were surveyed; 111 exudate samples were collected and 36 samples were positive for one of both B. goodwinii and G. quercinecans. Larval galleries or other signs of the beetle A. biguttatus were not found in association with AOD symptoms. Four trial plots were established and routinely monitored over a four-year period for AOD symptoms, presence of G. quercinecans and B. goodwinii, and an assessment of fungal species was done. In the trial plots, AOD symptoms peaked in 2019. Presence of the bacteria was not detected in all exudate samples collected regularly over summer from the trial plots. The presence of the bacteria was confirmed only in a few soil samples collected along a transect in one trial plot. One or both bacteria were identified in a few soil and wood samples collected from asymptomatic oak stands. Assessment of fungal species in both symptomatic and healthy trees did not identify significant differences in species composition, soil pH, fine root vitality, and mycorrhization. Environmental factor analysis indicated that precipitation in the previous autumn is related to a higher detection possibility of G. quercinecans and B. goodwinii in exudate samples collected in the following year. Continued monitoring for AOD symptoms, focussing on mature oak trees is required to identify future potential AOD outbreaks in Latvia. Keywords: Quercus robur; AOD; monitoring; qPCR; phytopathology; environmental factors