Valsalnicola oxystoma (Rehm) D.M. Walker & Rossman - primary biotic agent responsible for the dieback and high mortality of green alder in Călimani and Rodnei Mountains (North-Eastern Romania)

Abstract

The primary pathogen responsible for the green alder (Alnus alnobetula (Ehrh.) K. Koch subsp. alnobetula) decline in two mountain ranges pertaining to Eastern Carpathians and harbouring protected areas, Călimani Mountains National Park and Rodnei Mountains National Park was identified as stem and branch canker and dieback agent, Valsalnicola oxystoma (Rehm) D.M. Walker & Rossman (Melanconidaceae, Diaporthales). The identification was based on in situ observations and cultivation on PDA medium of diseased inner bark fragments. Field observations performed on transects in both areas showed that the incidence of the disease was high, up to 100% of the inventoried shrubs. The severity of the disease assessed on a five-level scale was variable, reaching high mortality (over 50% of the inventoried shrubs) in the Rodnei Mountains suggesting that the decline was in earlier stages in the Călimani Mountains. Morphometric analysis of ascomata structures showed significant differences compared to measurements reported in the literature (Wilcoxon test) suggesting that there are geographic and ecological differences among the populations of the species, at least at the morphometric level. The confirmation of the canker-inducing agent was supported by Koch’s postulate: infected branches of Alnus glutinosa (L.) Gaernt., also a host for V. oxystoma, were used to assess the symptoms and for the reisolation of the pathogen on PDA medium. Several other species, members of the diseased bark microbiome were identified, among which secondary canker and dieback-inducing pathogens, Phomopsis alnea (Sacc.) Höhn. and Melanconis alni Tul. & C. Tul.