Pathogenicity and Relative Virulence of SevenPhytophthoraspp. on Mahaleb and Mazzard Cherry

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Abstract
Isolates of P. cryptogea, P. cambivora and P. megasperma from cherry, isolates of P. cinnamomi and P. citricola from walnut, and an isolate of P. cryptogea from safflower individually caused 88-100% root rot and 27-100% crown rot on ''Mahaleb'' cherry seedlings [Prunus mahaleb] that were grown for 15 wk in artificially infested UC mix and periodically flooded. In contrast, cherry isolates of P. drechsleri and an unidentified Phytophthora sp. caused 62 and 41% root rot, respectively, but caused no crown rot under the same conditions. ''Mazzard'' cherry seedlings [Prunus avium, cv. Silverbrook Mazzard] frequently appeared less susceptible than ''Mahaleb'' seedlings to root and crown rot, although this varied with the Phytophthora species involved. ''Mazzard'' appeared to be significantly more resistant than ''Mahaleb'' to both root rot and crown rot caused by P. cambivora, P. megasperma, and the safflower isolate of P. cryptogea, and to crown rot caused by P. cinnamomi and P. citricola. ''Mazzard'' roots appeared nearly as susceptible as ''Mahaleb'' roots to the latter 2 Phytophthora spp., and roots of both cherry species appeared moderately susceptible to P. drechsleri and the unidentified Phytophthora sp. Roots and crowns of Mazrard also appeared as highly susceptible as ''Mahaleb'' to the cherry isolate of P. cryptogea. This is believed to be the 1st report experimentally implicating P. cryptogea as a pathogen of a commercial stone fruit tree species in the USA.

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