Identity and pathogenicity of two Phytophthora taxa associated with a new root disease of olive trees

Fifty-two Phytophthora isolates from necrotic roots of olives were characte rized. Colony morphologies on carrot-agar medium led us to separate them in to two groups: A (36 isolates) and B (16 isolates). The optimum growth temp erature for Group A was about 21 degreesC, with slow growth at 30 degreesC. In contrast, Group B isolates had an optimum temperature for growth of 26 degreesC, and grew rapidly at 30 degreesC. Growth rates, sporangial and oog onial characteristics of the Group A isolates conformed to P. megasperma "B HR-type" sensu stricto. This designation was supported by a sequence analys is of their ITS rDNA regions. Colony patterns, sporangial characteristics a nd temperature-growth relationships of the Group B isolates conformed close ly to those of the 'O-group' taxon of Phytophthora. They also conformed to this unusual taxon in their ITS sequence. In addition, Group B isolates wer e either entirely self-sterile, self-sterile A1s or weakly self-fertile. Pa thogenicity tests showed that both taxa were highly aggressive on roots of olive trees. The association of flooding with Phytophthora infection indica tes that the previously reported high sensitivity of olive to root asphyxia tion may be more properly regarded as root-rot caused by Phytophthora spp.