Fungal trunk pathogens and drought stress are serious threats to London plane (Platanus x hispanica) trees in northern Italy

Sudden diebacks and the mortality of London plane (Platanus x hispanica) trees have been investigated at two sites in northern Italy over several years. The main objective of this study was to test whether the observed syndrome was caused by fungal trunk pathogens whose virulence was triggered by drought. Seventy-six trees were inspected for several disease symptoms. The fungi associated with the symptoms were isolated and identified through macro-micromorphological and/or molecular methods, including multilocus phylogenetic analyses of relevant DNA loci. Overall, 1352 fungal isolates were obtained and assigned to 33 morphotypes and 24 species. The most prevalent fungal family associated with symptomatic tissues was Botryosphaeriaceae (51 % of isolates), followed by Diaporthaceae (11 %). Pathogenicity tests on healthy five-year-old London planes for the most common Neofusicoccum parvum species revealed that 60 % of isolates caused necrotic lesions of different size (p < 0.05), and this was interpreted as virulence diversity. Pathogenicity tests were conducted on five Botryophaeriaceae species and Seiridium aquaticum (Pestalotiopsidaceae) under two irrigation regimes. All the tested isolates were re-isolated thereby fulfilling one of Koch's postulates, but Diplodia seriata, Dothiorella iberica and S. aquaticum isolates were not pathogenic under the test conditions, regardless of the irrigation regime. Instead, Diplodia mutila and Lasiodiplodia theobromae isolates, and virulent strains of N. parvum were pathogenic, irrespective of the irrigation regime, and their virulence was significantly increased on drought-stressed trees. The results indicate a clear role of fungal trunk pathogens and drought stress, the latter being further supported by climate analyses, in the onset of the syndrome.