Virulence of Stemphylium vesicarium isolates from pear and other host species

Brown spot, caused by Stemphylium vesicarium, is one the most important pear disease in Europe. The disease is caused by fungal strains producing host-specific toxins which are responsible for the disease symptoms on some pear varieties. It is known that there is a high degree of differentiation in host specificity among the different isolates of S. vesicarium. Pathogenicity and virulence of 78 S. vesicarium strains obtained from pear and other host species were studied by a leaf necrosis assay on 3 pear varieties showing different susceptibility to natural brown spot epidemics. The bioassay was performed using conidial suspensions and autoclaved fungal culture filtrates. Strains of S. vesicarium showed high variability for both progress of necrotic spot appearance and final disease incidence. Four virulence groups were defined using a multivariate data analysis. Group I included 49 strains from pear, which caused severe necrosis on all the varieties. Group II included only 5 strains isolated from pear which caused severe necrosis on \u2018Abate F\ue9tel\u2019 and \u2018Conference\u2019, as the strains of group I did, but symptoms on \u2018William\u2019 were very light. In group III there were 19 strains from pear which showed less severe symptoms on all the varieties. Finally, group IV was formed by the S. vesicarium strains isolated from asparagus, pea, and onion, as well as the un-inoculated test. These fungal strains showed only small sporadic necrosis at the end of incubation