Virulence and the production of endo-1,4-β-glucanase by isolates of Alternaria alternata involved in the moldy-core disease of apples

Alternaria alternata, is the predominant fungal pathogen responsible for moldy-core in apple cultivars of the Red Delicious group. Here we report on the association between virulence of natural isolates of A. alternata, their production of endo-1,4-β-glucanase (EG) and moldy-core development in apple fruits. Based on decay development following wound inoculations of mature fruits, three of 150 isolates, collected in three orchards in northern Israel and representing low, moderate and high virulence, were selected and designated Rm44, Er30 and Sh42, respectively. All three isolates secreted EG when grown on enzyme-inducing medium (EIM) containing commercial cellulose or apple cell walls and this production was related to their degree of virulence. Polyacrylamide gel electrophoresis (PAGE) revealed quantitative differences between the three isolates, relative to their virulence. When fungal extracts were run in native gels, a single band with a molecular mass of 23 kDa showing EG activity was produced by the high- (Sh42) and the medium-virulence (Er30) isolate but not by the low-virulence (Rm44) isolate. A commercial cellulase preparation (containing endo- and exo-1,4-β-glucanase) placed on pricked fruit led to the formation of symptoms similar to those developing on A. alternata-inoculated fruits within 2-4 days. Inoculation of bloom clusters at full bloom with the highly virulent isolate (Sh42) of A. alternata resulted in a significantly higher infection in fruits (58%) than in those inoculated with the low-virulence isolate (Rm44) (30%). Our results suggest that the moldy-core symptoms caused by A. alternata in apple, could be related to the ability of the fungus to produce EG in developing lesions.