Testing variability in pathogenicity of Phytophthora cactorum, P-citrophthora and P-syringae to apple, pear, peach, cherry and plum rootstocks

The relative virulence of Phytophthora cactorum and P. syringae originating from almond trees, and of P, citrophthora originating from citrus, to apple, pear, peach, cherry and plum rootstocks, was studied in vivo and in vitro. Results of the different experiments were in good agreement. All tested Phytophthora isolates showed little virulence to pear rootstocks - causing only minor crown rot symptoms - and no virulence at all to apple rootstocks. In contrast, they were highly virulent to stone fruit rootstocks, causing crown rot disease. The non-pathogenicity of these isolates to pome rootstocks could be interpreted as strict host specificity.