Dieback of Fraxinus excelsior L. associated with Chalara fraxinea is observed in Europe, and in some areas dying trees exhibit symptoms of root and butt rot. Our study was conducted (1) to estimate the impact of the rot on F. excelsior dieback severity; (2) to identify fungi colonizing roots of dieback-affected trees; (3) to check their pathogenicity to F. excelsior; and (4) to estimate sprouting incidence and sprout health condition in relation to presence/absence of rot. The extent of rot was measured in 33 trees with different dieback intensity, 150 fungal isolations were attempted from roots of 50 trees, 26 fungi were tested for pathogenicity to 286 saplings, and sprouting was evaluated for 328 stumps on three clear-felled sites. Root rot was mainly caused by Armillaria cepistipes, and the extent of rot correlated positively with dieback severity although it played a secondary role in tree decline. Four years after tree felling, root rot had a negative impact on sprouting frequency, yet rot did not enter sprouts from stumps, and when experimentally inoculated, fungi associated with root rot in mature ash had no visible impact on tree vigor, showing that after formation of sprouts, rot does not affect the subsequent phytosanitary condition. Sprouts on investigated sites exhibited Chalara dieback symptoms on leaves and bark irrespectively of presence/absence of rot, indicating that vigorous natural regeneration of F. excelsior in dieback-affected areas could not be expected.
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