Page 97 - University of Pretoria RESEARCH REVIEW 2016

GOOD AND BAD
OPPORTUNISTIC BACTERIAL SPECIES
Understanding how opportunistic plant pathogenic bacteria function in different environments is central to understanding tree health.
Professor Teresa Coutinho, a member of the Centre for Microbial Ecology and Genomics (CMEG) and the Forestry and Agricultural Biotechnology Institute (FABI) at UP, has been leading research projects on the pathology of opportunistic bacterial species that infect Eucalyptus and stone fruit
trees for several years. Her research group mainly focuses on four bacterial species: Pantoea ananatis, Ralstonia solanacearum, R. pseudosolanacearum and Pseudomonas syringae pv. syringae. In their association with woody hosts, these pathogens are all regarded as opportunistic.
Pantoea ananatis is a ubiqitous bacterium. As a pathogen it infects a number of plants across a wide geographic range, but can also associate with plants without causing a detrimental effect to their health. Some strains are even used in the agricultural sector as biocontrol agents against insect pests while others enhance plant growth. There are also strains that infect humans, causing bacteriaema. P. ananatis is thus regarded as an opportunist pathogen, based on its ability to survive and proliferate in such different environments.
Using a comparative and functional genomic approach, Professor Coutinho’s group has aimed
to determine how this bacterium can function so successfully in such diverse ecological niches, and why it is sometimes a pathogen, and sometimes not.
The two Ralstonia species and Ps. syringae pv. syringae are believed only to infect Eucalyptus
and stone fruit trees, respectively, and only when they are stressed by abiotic and/or biotic factors. Poor planting practices enhance the appearance
of bacterial wilt, caused by the Ralstonia spp., in Eucalyptus plantations. In a project funded by HORTGRO and in collaboration with ARC-Infruitec, the role of abiotic (drought) and biotic (Phytophtora and the ring nematode) effects are being investigated
as factors that enhance bacterial canker in the
stone fruit tree industry in the Western Cape. From extensive surveys undertaken in this province,
it is evident that these factors, together with the bacterial pathogen, contribute substantially to the loss of orchard trees, particularly plums, cherries and apricots.
Symptoms of bacterial canker on the branches of an apricot tree.
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