Pathogenicity of Phytophthora and Pythium species associated with citrus root rot in South Africa

Root rot is a serious disease problem facing the citrus industry in South Africa. Pythiaceous fungi, especially Phytophthora spp., are well known causal agents of root diseases in citrus orchards and nurseries. In contrast, little is known regarding the role played by Pythium spp. in the development of citrus root rot. This is despite their abundance in the rhizosphere of diseased citrus trees. In this study, we isolated and identified a large collection of Phytophthora and Pythium spp. obtained from diseased citrus trees as well as from rhizosphere soil. Pathogenicity was tested using a rapid screening technique. A subset of the most pathogenic Phytophthora and weakly pathogenic Pythium isolates were further inoculated into commercial citrus rootstocks, Rough Lemon and Troyer Citrange. All Phytophthora isolates were found to be pathogenic on citrus fruit and rootstocks. Significant differences in pathogenicity were, however, found amongst the different Phytophthora isolates tested. Pythium spp. were found to be either avirulent or only weakly pathogenic. Results obtained indicate that Pythium spp. are not serious pathogens of citrus and probably do not play a significant role in the development of citrus root rot.