Severe Dieback of European Ash Shelterbelts in Northeastern Bulgaria Associated with Diplodia fraxini

The Common European ash (Fraxinus excelsior L.) is one of the main species constituting the field protection forest belts in Northeastern Bulgaria. Studies conducted in shelterbelts in Dobrich and Balchik in July 2020 and in Tutrakan and Dulovo in June–July 2022 revealed severe dieback of ash. The observed symptoms included density thinning of the crowns, dieback of branches, presence of sunken necrotic cankers, and light green to yellow foliage and premature defoliation. Parts of the shelterbelts were completely destroyed with 100% tree mortality. To determine whether the invasive Hymenoscyphus fraxineus or other pathogens are present in the ash field protective forest belts in Northeastern Bulgaria, fungal isolation was undertaken. Samples were collected from four locations: Dobrich and Balchik in June 2020, and Tutrakan and Dulovo in June–July 2022. The morphology, temperature–growth rate relationships, and pathogenicity of the two pathogenic fungal species isolated in this study—Diplodia fraxini and Diplodia seriata—were examined. Morphological and physiological studies confirm the molecular identification of the obtained plant pathogens. The Diplodia fraxini isolates (Dobrich 3, Tutrakan 2, and Dulovo 4) showed mycelial growth between 5 °C and 35 °C, with minimal growth at 5 °C (0.20–0.27 mm/day) and an optimum growth rate of 3.9–4.5 mm/day at 20–25 °C. Growth declined sharply above 30 °C, ceasing entirely at 35 °C. In contrast, D. seriata (Dulovo 5) exhibited higher growth rates, showing limited growth above 5 °C (~1 mm/day), and maximum growth of approximately 8 mm/day at 25 °C. Growth in D. seriata remained moderate up to 35 °C and ceased near 40 °C, indicating a broader temperature tolerance and higher upper thermal limit than D. fraxini. The results from the pathogenicity tests show that D. fraxini can cause necrosis on ash—both on leaves and twigs—and is likely involved in the investigated ash decline cases. Further studies of the spread and epidemiology of D. fraxini are needed in order to establish its occurrence on the territory of Bulgaria.