Isolation and pathogenicity of Phytophthora species and Phytopythium vexans recovered from avocado orchards in the Canary Islands, including Phytophthora niederhauserii as a new pathogen of avocado

Abstract

Root rot, caused by Phytophthora cinnamomi, is the most important disease of avocado, but few studies have determined whether other Phytophthora or oomycete species are involved in crop decline. Avocado orchards in the Canary Islands were surveyed for the presence of Phytophthora and Phytophthora-like oomycetes. Isolates obtained were identified morphologically and by sequence analysis of their internal transcribed spacer (ITS) regions, and their pathogenicity was tested by root and stem inoculation of avocado seedlings. Phytophthora species were isolated in 41 of 99 orchards sampled, and 10% of orchards were infected with more than one species. The species most frequently isolated was P. cinnamomi, which was detected in 26 orchards. In addition, P. multivora (ten orchards), P. niederhauserii (four orchards), P. nicotianae (four orchards), P. palmivora (one orchard) and Phytopythium vexans (20 orchards) were isolated. Phytophthora nicotianae and P. palmivora have been previously reported as pathogens of avocado, but P. niederhauserii, P. multivora and Pp. vexans are reported for the first time to be associated with this host. Phytophthora niederhauserii was the most virulent of these species. It was isolated from declining trees, and root rot severity was comparable to that caused by P. cinnamomi in two independent pathogenicity tests. In addition, P. niederhauserii caused cankers after stem inoculation. The pathogenicity results for P. multivora and Pp. vexans varied depending on isolates and pathogenicity tests. This study increases the knowledge of oomycetes associated with avocado, highlighting the potential threat posed by P. niederhauserii to this important fruit crop.