The identification of Oomycetes associated with plum orchards in the Western Cape Province, South Africa

Abstract

The aim of this thesis was to determine the role played by oomycetes in plum tree decline observed in the Western Cape Province of South Africa from 2016-2018. At this time, extreme drought conditions were experienced in the province. Thus, the focus of this study was to identify and characterize oomycetes isolated from both diseased plum tree tissue and rhizosphere soil, and to test their pathogenicity on two plum cultivars. Chapter 1 reviewed previous literature on plant diseases caused by oomycetes and P. syringae pv. syringae, and how the disease triangle and climate change influenced disease development. Temperature and moisture were reported as factors that influence disease development by weakening plant hosts when conditions are unfavorable, and also they influence pathogen occurrence and establishment. The literature also highlighted the importance of the interrelations of stress factors involved in decline, including the effect of co-infection in a single host plant. Chapter 2 of this thesis focused on conducting field surveys and sampling five plum orchards in the Western Cape Province. During the survey, a few trees displayed symptoms of bleeding cankers, which suggested that an oomycete might be a possible causal agent. Isolations from the diseased plant material and soil samples were conducted followed by molecular identifications. Six oomycetes species were identified, which are: Phytophthora multivora, Phytopythium vexans, Pythium coloratum, P. diclinum, P. irregulare and P. ultimum. Phytopythium vexans was the only oomycetes that was isolated from infected plant material. The pathogenicity of P. multivora and P. vexans (isolated in this study) was determined in Chapter 3 on Sun kiss plum cultivars in a greenhouse environment. Pseudomonas syringae pv. syringae was included because bacterial canker symptoms were concurrently observed in the field. The pathogenicity trials showed that neither P. vexans nor P. multivora were able to cause symptoms and were not re-isolated from the inoculated seedlings. Seedlings infected with P. syringae showed symptoms typical of bacterial canker and the pathogen was re-isolated from the infected seedlings. Co-infection trials revealed that seedlings inoculated with P. multivora and P. syringae had larger lesion size compared to other combinations.