Screening native pepper, Tasmannia lanceolata (Poir.) A.C. Smith, for resistance against Phytophthora cinnamomi dieback

Abstract

Tasmannia lanceolata (native pepper) has been reported as susceptible to Phytophthora cinnamomi and the objective of this study was to identify variability in native pepper resistance to P. cinnamomi. Plant material was collected from native pepper populations across Tasmania (four regions) and selected commercially grown cultivars, and 47 clones were successfully propagated. Two disease screening experiments were conducted in “soil-free plant growth system” (SPS) units. Native pepper roots were inoculated with P. cinnamomi zoospores and maintained in controlled conditions. After one week, the pathogen was re-isolated to confirm infection success, and after two weeks root discolouration was visually assessed with images, which were then analysed with an automated machine learning system, using colour thresholds. The SPS was a successful approach to screen the early response of native pepper to P. cinnamomi. Based on pathogen re-isolation success and total root discolouration percentage, clones were categorised using a susceptibility rating method across multiple categories from highly resistant to highly susceptible. In the first experiment, all 47 propagated clones were challenged with one isolate of P. cinnamomi (Pc1), and pathogen re-isolation percentage and total root discolouration (brown and black) percentage differed significantly with clone (P < 0.001). In the second experiment, three representative clones of experiment 1 were challenged with two isolates of P. cinnamomi (Pc1 and Pc2) and clone response was similar. This study has highlighted that there is a range of responses to P. cinnamomi, from highly susceptible to highly resistant, in native pepper clones from different regions of Tasmania.