Diversity and pathogenicity of emerging and novel Paramicrosphaeropsis species associated with olive canker and dieback

Abstract

Olive (Olea europaea L.) is an economically and ecologically important crop that is increasingly threatened by trunk diseases such as canker and dieback. This study investigated the occurrence, taxonomy, and pathogenicity of Paramicrosphaeropsis species associated with these diseases in Fars Province, Iran. A total of 73 fungal isolates were recovered from symptomatic olive trees across five regions and six cultivars. Morphological characterization and RNA polymerase II second largest subunit (rpb2) sequence analysis of representative isolates identified Paramicrosphaeropsis eriobotryae as the dominant species, whereas multilocus phylogenetic analyses of representative isolates based on the internal transcribed spacer (ITS), rpb2, and β-tubulin (tub2) loci supported the recognition of two novel species, P. oleae sp. nov. and P. sarvestanensis sp. nov. This study provides the first evidence linking P. eriobotryae to olive canker and dieback. Pathogenicity assays on detached shoots showed that 95% of isolates caused necrotic lesions, and principal component analysis (PCA) of six traits clustered isolates along a continuum from highly aggressive to non-aggressive. The tested aggressive isolate of P. eriobotryae fulfilled Koch’s postulates on olive saplings and exhibited the greatest aggressiveness and the shortest incubation period among the three tested representative isolates. Pathogenicity assays on wounded detached shoots showed that the tested aggressive isolates of the three Paramicrosphaeropsis species were capable of causing necrotic lesions on all evaluated fruit tree hosts under controlled experimental conditions. The representative isolate of P. eriobotryae was the most aggressive, particularly on walnut, while olive inoculated with P. oleae sp. nov. showed the least susceptibility. Susceptibility of six international and Iranian olive cultivars to P. eriobotryae, identified ‘Conservalia’, ‘Koroneiki’, and ‘Tokhm-Kabki’ as highly susceptible cultivars, whereas the dominant native cultivar ‘Zard’ showed moderate susceptibility. Notably, ‘Abou-Satl’ was the least susceptible cultivar under the present inoculation conditions and showed no measurable internal lesion depth following inoculation with the tested aggressive isolate. This study provides the first comprehensive evidence of Paramicrosphaeropsis species associated with olive canker and dieback and introduces two novel species, contributing to the understanding of their taxonomy, aggressiveness, and potential impact on important fruit trees.