European oak species have long been considered relatively resistant to different disturbances, including drought. However, several recent studies have reported their decline initiated by complex changes. Therefore, we compared mature sessile oak trees (Quercus petraea (Matt.), Liebl.) infested versus non-infested by hemiparasitic yellow mistletoe (Loranthus europaeus Jacq.) during the relatively dry vegetation season of 2019. We used broad arrays of ecophysiological (maximal assimilation rate A_sat, chlorophyll a fluorescence, stomatal conductance g_S, leaf morphological traits, mineral nutrition), growth (tree diameter, height, stem increment), and water status indicators (leaf water potential Ψ, leaf transpiration T, water-use efficiency WUE) to identify processes underlying vast oak decline. The presence of mistletoe significantly reduced the Ψ by 1 MPa, and the WUE by 14%. The T and g_S of infested oaks were lower by 34% and 38%, respectively, compared to the non-infested oaks, whereas the A_sat dropped to 55%. Less pronounced but significant changes were also observed at the level of photosystem II (PSII) photochemistry. Moreover, we identified the differences in C content, which probably reduced stem increment and leaf size of the infested trees. Generally, we can conclude that mistletoe could be a serious threat that jeopardizes the water status and growth of oak stands.

In temperate regions of Europe, the large pine weevil, Hylobius abietis, is a major pest of coniferous forests mostly at sites where clear-felling is followed by planting of saplings. Control measures against this pest are based on silvicultural techniques, an application of physical barriers on stems of saplings and insecticide treatments. To avoid the use of insecticides, alternative measures such as biological control have been investigated. The goal of the present study was to obtain local strains of entomopathogenic fungi (Ascomycota, Hypocreales) from natural populations of H. abietis, and to investigate their efficacy against the weevil. A survey on entomopathogenic fungi was undertaken at clear-felled areas of spruce forests in northern Slovakia. Two Beauveria species, B. bassiana and B. pseudobassiana, were identified, and 22 in vitro strains were obtained. Mean prevalence of infected adults was low (2.10% ± 0.67%) and the mycosis was mostly recorded during May and June. Virulence of Beauveria strains against the weevil was tested in laboratory. B. bassiana strain AMEP20 was significantly most virulent (LC₅₀ of 0.65 ± 0.10 × 10⁸ conidia mL⁻¹). Treatment with conidia of AMEP20 strain affected feeding damage by the weevil on bark of Scots pine twigs. Daily bark consumption by B. bassiana-treated weevils was lower than by untreated individuals and decreased with increasing conidia concentration used for the treatment. In the outdoor experiment, AMEP20 strain killed weevils that fed on spruce saplings treated with conidia suspensions. Mortality due to mycosis on weevils exposed to the conidia-treated saplings reached 30.0%–76.5% and 55.0%–88.2% after 32 and 46 days, respectively.

Acute Oak Decline (AOD) is an oak disease that was first described in the United Kingdom. From the time the first symptoms appear, the disease can kill trees even within 6 years. One of the factors causing this disease is the action of the pathogenic bacteria Brenneria goodwinii, Rahnella victoriana and Gibbsiella quercinecans. In recent years, a deterioration in the condition of oak stands has been observed throughout Slovakia. These trees show exudates characteristic of AOD, which may indicate the presence of pathogenic bacteria. Samples of such symptomatic exudates from tree trunks were collected and analysed on 20 different plots in central and western Slovakia. Using real-time PCR, the presence of Brenneria goodwinii and Gibbsiella quercinecans was confirmed in ten and three stands, respectively. This report is the first information about the observation of these bacteria in weakened stands of Quercus robur in Slovakia.