The association of Boeremia lilacis with necrotic lesions on shoots and leaf petioles and its pathogenicity towards Fraxinus excelsior

This paper reports the results of the study on the new type of necrotic lesion observed on epicormic shoots and leaf petioles of Fraxinus excelsior showing ash decline symptoms in stands of the Miechow Forest District, southern Poland. The performed symptom analysis included: shape and size of necroses, discoloration of necrotic tissue and the occurrence of fungal fructification. The same species of fungus, which was initially identified as Boeremia exigua, was isolated from 97% of epicormic shoots and from 82% of leaf petioles. However, detailed morphological and physiological (NaOH test) analyses and combined ITS, ACT, TUB and EF1-α phylogeny proved these isolates to belong to Boeremia lilacis, the species known so far as Syringa vulgaris pathogen occurring occasionally also on Forsythia hybrids or Philadelphus. Fraxinus excelsior is reported for the first time as a host for this fungus. To clarify the possible role of B. lilacis in development of necrotic lesions on ash, 10 randomly selected isolates were tested for their pathogenicity toward young F. excelsior plants using the method of artificial wound inoculation under field conditions. The isolates used in the test caused tissue necroses on all 60 inoculated shoots and all 60 inoculated leaf petioles. Necrotic lesions developed as a result of artificial inoculation corresponded largely to the symptoms following natural infections.