Relationship between phyllosphere fungal communities and tolerance of olive tree cultivars to olive leaf spot disease

Abstract: Spilocaea oleaginea is the causal agent of olive leaf spot (OLS) disease on olive trees, which causes significant yield losses in many countries, including Portugal. The disease incidence is variable according to the olive cultivars, being cv. Cobrançosa more tolerant to OLS than Verdeal-Transmontana. The present work aimed to elucidate the role of endo- and epiphytic fungal communities inhabiting the olive tree phyllosphere of these two cultivars in the mediation of plant host tolerance to OLS. For this purpose, leaves and twigs were collected from seven olive trees of each cultivar (Cobrançosa and Verdeal-Transmontana), present in a single olive grove located in Mirandela (Northeast of Portugal). The endo- and epiphytic fungi inhabiting the collected plant material were isolated in both PDA and PCA media. OLS incidence (% infected leaves) was assessed by evaluating the percentage of infected leaves per tree. The obtained isolates were further identified by sequencing the ITS region of rDNA. Both endo- and epiphytic fungal communities differ significantly between cultivars, especially within endophytes. Cv. Cobrançosa was distinguished by the presence of Chromelosporium carneum, Pseudocercospora cladosporioides, Hyalodendriella betulae and Plectania rhytidia, within endophytic community, and by Placopsis rhodophthalma, Plenodomus enteroleucus, Cladosporium sp. and Penicillium sp., within the epiphytic community. The Verdeal-Transmontana was distinguished by exhibiting Leptosphaeria avenaria and Fusarium sp. within the endophytic and epiphytic community, respectively. Cultivar Cobrançosa showed lower disease incidence (3%) than cv. Verdeal-Transmontana (15%). A correlation between data regarding fungal community and disease incidence displayed by both cultivars was performed by using univariate analyses. The results indicate that some fungal species, either epiphytes or endophytes, may have a direct effect on olive tree tolerance to OLS.