PHYTOPHTHORA-CACTORUM ON WALNUT SEEDLINGS IN ITALIAN NURSERIES

Phytophthora cactorum was consistently isolated from basal stem tissue s of walnut seedlings affected by collar and root rot. The isolates we re identified on the basis of morphological and cultural characteristi cs and of electrophoretic patterns in polyacrylamide slab gels of tota l native mycelial proteins and isozymes (arylesterase, malate dehydrog enase, acid and alkaline phosphatase). The electrophoretic phenotypes of walnut isolates were identical or very similar to those of P. cacto rum isolates from different hosts (apple, loquat, pineapple-guava and strawberry). In pathogenicity tests, P, cactorum isolates from walnut proved to be highly pathogenic for seedlings of English walnut (Juglan s regia) for and moderately pathogenic for seedlings of eastern black walnut (Juglans nigra); moreover, they did not rot apple seedlings, su ggesting host-specificity. Poor soil drainage and excessive irrigation can be not rot considered as predisposing factors to collar and root rot incited by P. cactorum.