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Abstract
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.
Highlights
European ash (Fraxinus excelsior L.) is a valuable component of forest ecosystems where small pure or mixed stands are created with other broadleaved species in deep humid soil [1,2]
This paper aims to present the differences in: (i) the survival of F. excelsior seedlings over a period of two years (2016/2017); (ii) growth parameters including root development, (iii) re-isolation and PCR-based confirmation of the pathogens from dead and living seedlings; (iv) an exploratory analysis of triterpenes, phenols, and sterols in the cortical tissue samples; and (v) the chlorophyll-a fluorescence (ChlF) of leaves as an indicator of the plant metabolism and health status of the seedlings [43]
In April 2017, and six months after inoculation, 100% of the seedlings inoculated with H. fraxineus (Hf) and 55% of those inoculated with H. fraxineus + Phytophthora mix were dead in water treatment (Figure 2)
Summary
European ash (Fraxinus excelsior L.) is a valuable component of forest ecosystems where small pure or mixed stands are created with other broadleaved species in deep humid soil [1,2]. Due to fast growth and usable wood, ash for many years was the tree of choice for foresters [3]. About 25 years ago, reports of the decline of ash stands appeared in Poland [4]. There was not much attention directed to ash, because Europe was already facing a decline in oak [5], beech [6], spruce, and fir [7,8], which were both ecologically and economically more significant species. A decade later, the problem of ash dieback started becoming apparent, with reports from Poland, and from neighboring Baltic countries, indicating that the problem with ash health was present in the wider region of the northeastern part of continental
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