Abstract
Corymbia calophylla (Marri) is an endemic tree in the forests of South-Western Australia. This iconic, keystone species is being threatened by soil-borne oomycete (Phytophthora spp.) root pathogens and an aerial fungal canker pathogen (Quambalaria coyrecup) that are causing widespread mortalities. Phytophthora is often isolated from the rhizosphere of dying marri and it is hypothesized that Phytophthora root infection compromise marri to attack by other pathogens, such as this canker pathogen. The aim of this study was to investigate if the spectral absorption features of the photosynthetic components in the leaves of marri could be used as an indicator of stress caused Phytophthora species. For this purpose, non-destructive spectral measurements were conducted using a handheld hyperspectral sensor (ASD FieldSpec portable spectroradiometer). Isolates of Phytophthora cinnamomi and P. multivora were used to infect the roots of marri seedlings in a glasshouse pathogen nicity trial. Weekly spectral measurements were taken during the course of this experiment. These measurements were correlated with measurements of above-and below-ground biomass at the completion of the experiment.
Seedlings inoculated with P. cinnamomi displayed decreased values in root volume, a belowground biomass indicator. The value s of the Red Edge Normalized Difference Vegetation Index (Red Edge NDVI), a narrow-band greenness vegetation index which makes use of the absorption features in the red and near-infrared regions of the electromagnetic spectrum, was also decreased. In contrast, the root volumes for seedlings inoculated with P. multivora displayed increased values. These values were mirrored by the increased values for the Red Edge NDVI for these seedlings. This study suggests that spectral measurements done with a hyperspectral sensor may provide early-warning signs of root pathogen stress in marri. Future work would include analyzing the time-series component of the weekly spectral measurements, investigating the change in the photochemical components in the leaves over time. Current studies are examining spectral indices and disease development with dual inoculations with Phytophthora species and the canker pathogen Quambalaria coyrecup.