Phytophthora × pelgrandis causes root and collar rot of Lavandula stoechas in Italy

In 2007, Phytophthora isolates with atypical morphological and biological characteristics were found associated with root and collar rot of potted plants Stoechas lavender (Lavandula stoechas L.) in an ornamental nursery in Italy. A polyphasic approach, including morphological and cultural observations, sequencing the ITS-rDNA region, the Pheca and the mitochondrial coxI genes, multiplex PCRs with primers specific for P. nicotianae or P. cactorum, as well as RAPD-PCR, was used to characterize these isolates. On the basis of morphocultural and molecular analyses the isolates from Stoechas lavender were identified as Phytophthora × pelgrandis, a natural hybrid of P. nicotianae x P. cactorum previously reported in other European countries, Americas and Taiwan, as a pathogen of ornamentals and loquat plants. In pathogenicity tests using potted plants of Stoechas lavender the P. × pelgrandis isolates, similarly to the parental species P. nicotianae, induced the symptoms observed on plants with natural infections and were reisolated only from artificially inoculated plants. Dispersal of P. x pelgrandis on this host could exacerbate the damages caused by Phytophthora root and collar rot of which main causal agent presently is P. nicotianae on lavender in Europe. Application of hygienic measures are important to reduce the proliferation and spread of the Phytophthora hybrids.