Genetic variability of Phytophthora × serendipita and pathogenicity on Rhododendron

We have developed an rDNA ITS real-time PCR method to discriminate between P. ×serendipita hybrids and the parental species P. cactorum and P. hedraiandra. This technique, which is more sensitive than PCR-RFLP or sequencing analysis to identify hybrids, was applied to 184 isolates from 10 European countries that were previously designated P. cactorum or P. hedraiandra based on morphological characteristics. The results revealed that 49% of these isolates were hybrids. In most cases the hybrids carried the coxI of P. hedraiandra, in a few cases, coxI of P. cactorum was present, suggesting different hybridization events. At least four groups of hybrid isolates could be distinguished based on different P. cactorum/P. hedraiandra ITS copy number ratios. AFLP was used to study the genetic variability between isolates of P. cactorum, P. hedraiandra and representatives of the four hybrid groups. Most of the fragments were shared with one or both of the parental species. Some fragments were unique for P. cactorum, P. hedraiandra or the hybrids. The presence of differential fragments between certain hybrid isolates also suggests distinct hybridization events. Pathogenicity of P. cactorum, P. hedraiandra and their hybrids was tested using Rhododendron leaf inoculation assays. Some of the hybrids were more pathogenic than either of the parental species. These data indicate that
Phytophthora ×serendipita hybrids are more common than previously recognized and that several hybridization events between these species may have taken place. The higher pathogenicity may explain the recent increase in prevalence of P. ×serendipita on Rhododendron.