The Role of Phytophthora and Water Shortage in the Canker Disease of Corymbia calophylla as Determined by Reflectance Spectroscopy and Biophysical Variables

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ABSTRACT Corymbia calophylla (marri), an iconic keystone species in the northern jarrah forest of southwestern Australia, is suffering from a stem canker disease caused by an endemic fungus, Quambalaria coyrecup. It is unusual for an endemic pathogen to have such a detrimental effect on a co‐evolved host, unless host defence mechanisms have been compromised. This study investigated the role of Phytophthora cinnamomi root infection and water shortage in predisposing C. calophylla to this canker disease, and whether these two stresses work synergistically to intensify the effect of the canker pathogen on C. calophylla. The roots of two‐year‐old C. calophylla plants were inoculated with P. cinnamomi in pot infestation trials, and 8 weeks later in the stems with the canker pathogen Q. coyrecup. Half of the plants were exposed to a water shortage treatment for the duration of the trial. Biophysical variables related to plant responses to the treatments were measured at harvesting. Reflectance spectroscopy measurements with a portable high‐resolution spectroradiometer were also taken weekly. The normalised difference spectral index (NDSI) was calculated for every combination of reflectance values between 350 nm and 2500 nm for all time points, correlated with treatment effects, and displayed as heat maps. Fifty‐seven vegetation indices (VIs), using wavelengths from different regions in the electromagnetic spectrum, were also calculated from the spectral data. Neither P. cinnamomi nor the water shortage treatments exacerbated the effect of the canker pathogen on the plants. The canker treatment increased plant stem diameter and canker volume significantly (p < 0.001). The NDSI heat maps indicated that wavelengths in the electromagnetic spectrum's visible and shortwave infrared portions displayed the strongest correlations with the P. cinnamomi and water shortage treatments. For the canker treatment, it was the shortwave infrared portion. Six of the VIs responded significantly to the water shortage treatment: Carter index 1 (p < 0.001), renormalised difference vegetation index (p < 0.001), normalised difference water index (p = 0.012), normalised phaeophytinization index (p < 0.001), photochemical reflectance index (p < 0.001) and red‐green ratio index (p = 0.018). The renormalised difference vegetation index was also sensitive to the canker treatment (p < 0.001), and the Carter index 1 to the P. cinnamomi treatment (p < 0.001). Reflectance spectroscopy was able to track biochemical changes in C. calophylla leaves due to inoculation with P. cinnamomi, Q. coyrecup, and the water shortage treatment. However, more work must be done to identify optimum wavelengths specific to C. calophylla and its responses to pathogens.

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