Combined morphological and molecular approach for identification of Stemphylium vesicarium inoculum in pear orchards

• Molecular and morphological data should be combined for unambiguous identification of S. vesicarium isolates.
• Different species of Stemphylium coexist in pear orchards with S. vesicarium.
• S. vesicarium populations in pear orchards include nonpathogenic isolates.
• S. vesicarium inoculum monitoring through volumetric spore traps may overestimate the pathogenic population.

Stemphylium vesicarium is the causal agent of brown spot of pear, an important disease reported in pear-growing areas of Europe. The pathogen is able to colonize pear leaf debris and dead tissues of herbaceous plants and produce abundant ascospores and conidia that are capable of infecting pear trees. Inoculum monitoring in pear orchards is mainly achieved through spore traps and species identification is based on conidial morphology, but the similarities on conidial traits among species of Stemphylium make correct identification difficult. In this work a total of thirty-seven Stemphylium isolates from pear orchards were characterized at the morphological, pathogenic, and molecular level. Correspondence among ITS and gpd sequences and morphological traits were evaluated. Species identification based exclusively on morphological data was not feasible. Combined morphological and molecular data were necessary for unambiguous identification of isolates in the S. vesicarium species group. Only isolates identified as S. vesicarium were pathogenic on pear. The study revealed that several species of Stemphylium coexist in pear orchards with S. vesicarium, the causal agent of BSP, and that combined morphological and molecular data are needed to differentiate them. Consequently, direct measurements of the airborne inoculum using volumetric spore traps may overestimate the actual pathogen population.