The role of stress factors in predisposing plum trees to invasion by bacterial pathogen Pseudomonas syringae pv. syringae

Our research objectives are to: i) determine if there is a link between ring nematode infestation and bacterial canker outbreaks, ii) determine if there is a link between Phytophthora and bacterial canker outbreaks, iii) study the effect of drought and ring nematode stress combination in predisposing plum trees to Pseudomonas syringae infection, and iv) the effect of Phytophthora as a biotic stress factor in predisposing plum trees to infection. Anecdotal evidence supports the hypothesis that abiotic and other biological factors influence the outbreaks of bacterial canker of stone fruit trees.

Orchard assessments were conducted in March and October 2017 and 2018 in major plum- producing farms in the Western Cape. Trees were assessed for symptoms of bacterial canker. Rhizospheric soil samples were collected from plum trees displaying these symptoms. Nematodes were extracted from the soil samples using a centrifugal flotation technique, and ring nematodes were counted per 250ml of soil using a stereo-microscope. The internal transcribed spacer (ITS) region was sequenced for representative ring nematodes, and a phylogenetic tree was constructed using a maximum likelihood analysis.

Oomycetes spp. were isolated from soil samples and infected plant material. The soil baiting technique was used to isolate Oomycetes from soil samples. Lesions that formed on baits were plated onto Phytophthora selective media. The lesions from infected plant material, collected from the trunk of trees displaying cankers, were plated onto the selective media.

Overall, there was no association between bacterial canker and ring nematode densities. A unit change of ring nematode densities increases the probability of bacterial canker infection by 1% only, and there was no significant difference in the least square means of ring nematode values between an asymptomatic and an infected plum tree. The majority of the ring nematode specimens were identified as Mesocriconema xenoplax based on the ITS phylogeny. These results support previous reports of M. xenoplax as the common ring nematode species present in plum orchards in the Western Cape. Fourteen Phytophthora, 15 Pythium and 15 Phytopythium isolates were obtained using the soil baiting technique.

The knowledge of the specific species of oomycetes, ring nematodes and relevant information on tolerant rootstocks, cultivars and time of the season of prevalence, can assist in the development of management strategies controlling the outbreak of bacterial canker in plum orchards.

Improvement in agricultural practices and proper management strategies, including the control of biological stresses, should improve the health of plum trees, and increased yield would be expected.