Diana M. Kennedy

Relatedness of Group I species of Phytophthora as assessed by randomly amplified polymorphic DNA (RAPDs) and sequences of ribosomal DNA

Relatedness of Group I species P. iranica, P. clandestina, P. pseudotsugae, P. cactorum and a new species, P. idaei with specificity in pathogenicity tests to raspberry (Rubus idaeus), were examined at the molecular level using twenty random ten-mer primers to amplify total DNA (RAPDs) and by sequencing of the internal transcribed spacer regions, ITS1 and ITS2, of the ribosomal RNA gene. Cluster analysis of five of the RAPD banding patterns, separately or as a combined analysis, ranked P. idaei, P. pseudotsugae and P. cactorum more or less equally in each case. The separation between them and P. iranica and P. clandestina was much greater. Within P. cactorum, collar rot isolates from apple clustered separately from strawberry crown rot isolates, while isolates from raspberry appeared to have affinities with both clusters. Sequence analyses of ITS1 and ITS2 revealed only a few differences among isolates of P. idaei, P. pseudotsugae and P. cactorum. Their sequences were much more similar to one another than to P. infestans and in ITS1 they shared a characteristic motif, resulting from a base pair inversion, that was not present in any other Phytophthora species. The inversion occurred within a sequence that binds to a DNA probe that has been used as a genus-specific probe for all Phytophthora spp.

Identity of a Phytophthora species attacking raspberry in Europe and North America

Severe root rot of raspberry is caused in Europe and North America by a homothallic, non-papillate Phytophthora sp., which has been identified by different investigators as P. erythroseptica, P. fragariae or as a highly pathogenic variant of P. megasperma . Two collections of such highly pathogenic raspberry isolates from Europe and North America were compared with recognized specimens of the three above named species, which were originally isolated from potato, loganberry and strawberry, and a variety of perennial host plants, respectively. All such raspberry isolates, regardless of previous identification, formed an essentially homogeneous group with respect to colony and growth characteristics, the production and morphology of gametangia, the morphology and dimensions of sporangia, electrophoretic banding patterns of mycelial proteins and pathogenicity. They were readily distinguishable from recognized isolates of P. erythroseptica and P. megasperma with respect to cultural, morphological, and electrophoretic criteria. In contrast, they were very similar to isolates of P. fragariae from strawberry (and a single isolate from loganberry) although the two groups could be separated by differences in growth rate on some agar media, production of oospores in culture, and small differences in electrophoretic banding patterns, as well as in pathogenicity. It was concluded that the raspberry isolates should be assigned to P. fragariae , but that they should be given a subspecific epithet at the varietal level to distinguish them from strawberry isolates of the species i.e. P. fragariae var. rubi .

A papillate Phytophthora species with specificity to Rubus

The morphology, physiology, protein patterns and pathogenicity of a papillate Phytophthora sp. that is widely distributed in raspberry stocks, are described and compared with those of other Group I Phytophthora species, especially P. cactorum , and with species from other groups that attack raspberry ( P. citricola and P. syringae ). The papillate species, although similar to ‘typical’ isolates of P. cactorum from raspberry and other hosts, could be distinguished from them on virtually all the criteria examined. Distinctive characteristics that separated them from P. cactorum isolates were absence of growth patterns on certain media; persistent sporangia; low rates of growth; low cardinal temperatures; inability to utilize nitrate nitrogen; host range limited to raspberry but with more severe symptoms; and differences in protein patterns. It was concluded that these papillate Phytophthora isolates from raspberry represent a new species which has been named as Phytophthora idaei sp. nov.