Pathogenicity of Diplodia fraxini and Other Botryosphaeriaceae Identified on Fraxinus excelsior with Dieback Symptoms in Poland

In the current work, the analysis covered 70 isolates of fungi belonging to Botryosphaeriaceae obtained in the years 2007–2017 during research on the mycobiota of F. excelsior trees with dieback symptoms in various regions of Poland. Five botryosphaeriaceous species were identified: Diplodia fraxini, D. seriata, D. sapinea, Dothiorella omnivora, and Do. sarmentorum, supported by morphological characteristics and nucleotide sequence data from three genes. The effect of temperature on the in vitro growth of colonies of five identified botryosphaeriaceous species was assessed. Dothiorella omnivora achieved optimal growth at 19.0 °C, while the other four species have shown optimal growth between 22.8 °C (Do. sarmentorum) and 25.7 °C (D. seriata). The pathogenicity test was performed in field conditions on nine-year-old F. excelsior seedlings. In total, wound inoculation was performed on 176 shoots, using 22 isolates of five identified fungal species. Each isolate was inoculated onto eight F. excelsior shoots. The symptoms on shoots were examined at 12 weeks after the inoculation. Among the tested fungal species, necrotic lesion was caused by D. fraxini, D. seriata, and Do. sarmentorum. The extent of damage they caused showed statistically significant differences. The highest pathogenic properties were demonstrated by D. fraxini, which caused necrotic lesion with a length of 34.25–50.50 mm (mean 40.13 mm) on inoculated trees. D. seriata showed the lowest degree of virulence. Half of its strains caused necrotic lesions, which did not differ significantly from the control. Diplodia sapinea and Do. omnivora did not cause any visible lesions. None of the control shoots developed necrosis. The role of Botryosphaeriaceae species in intensifying disease symptoms in ash trees in the context of Hymenoscyphus fraxineus invasion and climate changes was discussed.