Climate changes influencing forest ecosystems include increased air temperatures and CO 2 concentrations as well as droughts and decreased water availability. These changes in turn effect changes in species composition of both host plants and pathogens. In the case of Armillaria, climate changes cause an increase in the activity of individual species and modify the growth of rhizomorphs, increasing the susceptibility of trees. The relationship between climate changes and the biotic elements of Armillaria disease are discussed in overview.

As recently reported, European mistletoe has become an important agent of damage to forest trees in Central Europe. To document the extent of its impact, host and environmental preferences, as well as spatial and temporal dynamics, we have analyzed the results of 10 years of annual assessments of tree damage in level 1 plots of country-wide national forest monitoring. Mistletoe was found on 12 forest tree species, most frequently on silver fir and Scots pine, much less often on birches and deciduous admixture species. Common beech, pedunculate, and sessile oaks, as well as coniferous admixture species, were found to be free from the parasite. Mistletoe was more common on larger trees, mainly those in the 1st and 2nd Kraft classes, characterized by a higher DBH value than trees without V. album. Infestation was also age-dependent with low numbers of infested trees in young age classes and a much higher proportion of infested trees being over 80–100 years in age. Defoliation of trees colonized by mistletoe was found to be significantly higher than in trees free from the parasite, confirming the adverse effect of infestation on tree health. The increase of relative incidence of tree infestation by mistletoe in forests in Poland was high throughout the entire period 2008–2018 and significant for the most threatened host species: silver fir, Scots pine, and birches. It was also found that spread direction was from west to east and that the occurrence of V. album shows clear regionalization. The most endangered forests were located within a 200 km wide zone ranging through lowland Central Poland from the border with Germany down to the border with Ukraine and Belarus. The highest concentration occurred in the Western part of the country. Outside this zone (to the north and south) the mistletoe was much less abundant as expressed by the amount of injuries caused by the parasite or infested trees per plot.

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The European Fraxinus species are threatened by the alien invasive pathogen Hymenoscyphus fraxineus, which was introduced into Poland in the 1990s and has spread throughout the European continent, causing a large-scale decline of ash. There are no effective treatments to protect ash trees against ash dieback, which is caused by this pathogen, showing high variations in susceptibility at the individual level. Earlier studies have shown that the application of phosphites could improve the health of treated seedlings after artificial inoculation with H. fraxineus. Three-year-old F. excelsior seedlings were inoculated with the following pathogens: a H. fraxineus, Phytophthora species mixture (P. plurivora, P. megasperma, and P. taxon hungarica), in combination with two pathogens and mock-inoculated as the control, and then either watered or treated with ammonium phosphite (Actifos). Results showed significant differences in the survival of seedlings and symptoms of disease development among the treatments. Chlorophyll-a fluorescence parameters indicated a decrease in photosynthetic efficiency in infected plants, suggesting that they were under strong biotic stress, but none of the parameters could be used as a reliable bioindicator for ash decline disease. The application of Actifos enhanced the production of triterpenes (ursolic and oleanolic acid), and decreased the production of phenols (tyrosol) and sterols (β-sitosterol) in seedlings infected with H. fraxineus. Treatment with Actifos caused seedlings to enhance their response to pathogen(s) attack and increase their survival probability.

An investigation was carried out of the composition of metabolites in pine seedlings tissues at the initial stages of the infectious process caused by pathogenic fungi Armillaria ostoyae, which causes a root rot of trees and degradation of forest resources. With the help of successive extraction with organic solvents of different polarity, more than 190 metabolites were extracted from the needles and roots of the seedlings and then identified by GC-MS method. The composition of the extracts from control plants and those inoculated with Armillaria ostoyae were compared. It was established that part of secondary metabolites (glucosamines and free amino acids, carbohydrates raffinose and trehalose) were present only in the tissues of inoculated plants. Possible roles of some of these compounds appearing in the roots of seedlings infected with the fungus are also discussed in the paper.

The goal of the current study was to evaluate the efficiency of Norway spruce wood decomposition by the saprotrophic fungus Phlebiopsis gigantea (Fr.) Jülich (used in European forests to control root rot pathogens). We identified the following indices as reliable measures of wood decay: decay acceleration index (DAI), final decay index (FD), and decay intensity index (DI). DAI was used to assess acceleration and deceleration of spruce wood loss, FD to estimate the final wood decay, and DI to evaluate the ability of the fungus to decompose wood, taking into consideration changes in mycelial activity over time. We tested the hypothesis that the wood decay activity of any fungus can be evaluated more objectively when: i) several isolates of the tested fungus are assessed simultaneously (in this case, six P. gigantea strains registered in Europe), ii) tests are performed for at least two time points, and iii) samples vary in wood density. Allowing a three-month period for wood decomposition was inadequate for reliable assessment of the wood decay ability of P. gigantea. Conversely, six months after inoculation, the tested isolates showed significant differences in their ability to generate dry wood loss, which depended on wood density. In view of these results, the DAI, FD, and DI indices are practical tools for assessing fungal activity to predict wood loss.

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Identification of the <em>Armillaria</em> species in selected coniferous stands (Scots pine stands, Norway spruce stands and fir stands) was the aim of the work carried out on the basis of mating tests and consideration of macroscopic traits of fruit-bodies. One species of <em>Armillaria</em> [<em>A. ostoyae</em> (Romagnesi) Herink] was found in Scots pine stands, three species [<em>A. ostoyae, A. cepistipes Velenovský</em> and <em>A. borealis</em> Marxmüller et Korhonen] were found in Norway spruce stands and two species [<em>A. ostoyae</em> and <em>A. cepistipes</em>] were found in fir stands.