Screening Tasmannia lanceolata for resistance against Phytophthora cinnamomi dieback

Tasmannia lanceolata (native pepper) is wild harvested, and the species has been reported as susceptible to Phytophthora cinnamomi. However, surveys in plantations suggest that clones differ in susceptibility and the key objective of this study was to identify native pepper clones resistant to P. cinnamomi, and the defence mechanisms responsible. Plant material from over 70 genotypes was collected from native stands throughout Tasmania from June-October 2020, and 47 clones were successfully propagated as cuttings. Two disease screening experiments were conducted in “soil-free plant growth system” units, with four replicates of each clone placed in separate control and inoculated units. P. cinnamomi zoospores (1×10⁵ zoospores/mL) were applied to all root tips of all plants in the inoculation units. Successful inoculation and infection were confirmed via observation of symptoms on simultaneously inoculated lupin roots and re-isolation of the pathogen from native pepper roots. Image analysis and machine learning was used to quantify root infection and discolouration. In the first experiment with 47 clones and one isolate of P. cinnamomi, the pathogen could not be re-isolated from 17 clones, which were tentatively considered less susceptible (LS) than the other 30 clones, which were tentatively considered more susceptible (MS). In the second experiment, 2 of the MS clones and 5 of the LS clones were included and challenged with two different isolates of P. cinnamomi. Re-isolation results for these clones were identical to experiment 1 and the same for both P. cinnamomi isolates. The putative defence compounds callose, lignin, hydrogen peroxide and peroxidase were assessed visually and quantitatively in the clones used for experiment 2 and their abundance was found to be significantly (P<0.05) affected by both clone and inoculation treatment. The constitutive and induced levels of these compounds could have a role in defence of native pepper and their detection a new way of selecting clones tolerant/resistant to P. cinnamomi.