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 characterized. Colony morphologies on carrot-agar medium led us to separate them into two groups: A (36 isolates) and B (16 isolates). The optimum growth temperature for Group A was about 21°C, with slow growth at 30°C. In contrast, Group B isolates had an optimum temperature for growth of 26°C, and grew rapidly at 30°C. Growth rates, sporangial and oogonial characteristics of the Group A isolates conformed to P. megasperma "BHR-type" sensu stricto. This designation was supported by a sequence analysis of their ITS rDNA regions. Colony patterns, sporangial characteristics and temperature-growth relationships of the Group B isolates conformed closely 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 were either entirely self-sterile, self-sterile A1s or weakly self-fertile. Pathogenicity tests showed that both taxa were highly aggressive on roots of olive trees. The association of flooding with Phytophthora infection indicates that the previously reported high sensitivity of olive to root asphyxiation may be more properly regarded as root-rot caused by Phytophthora spp.