Phytophthora cactorum on walnut seedlings in Italian nurseries

Phytophthora cactorum was consistently isolated from basal stem tissues of walnut seedlings affected by collar and root rot. The isolates were identified on the basis of morphological and cultural characteristics and of electrophoretic patterns in polyacrylamide slab gels of total native mycelial proteins and isozymes (arylesterase, malate dehydrogenase, acid and alkaline phosphatase). The electrophoretic phenotypes of walnut isolates were identical or very similar to those of P. cactorum 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 (Juglans regia) for and moderately pathogenic for seedlings of eastern black walnut (Juglans nigra); moreover, they did not rot apple seedlings, suggesting 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.