Reflectance spectroscopy to characterize the response of Corymbia calophylla to Phytophthora root rot and waterlogging stress

Abstract

The health of Corymbia calophylla (marri), a keystone tree species in the native forests of southwest Western Australia, has been in decline for the past few decades. Phytophthora root disease and waterlogging have often been cited as contributing to this decline. Traditional methods (i.e., field surveys and sampling) of mapping Phytophthora root infection in the field are time-consuming and expensive; thus, the potential of reflectance spectroscopy to characterize marri response to Phytophthora and waterlogging stress was investigated. Twelve-month old marri plants were infected with either P. cinnamomi or P. multivora in two glasshouse trials and waterlogged for 24 h each fortnight. Spectral measurements with a portable high-resolution spectroradiometer were taken weekly. Plant biophysical measurements were taken at harvest time. Normalized difference spectral index (NDSI) was calculated for every combination of reflectance values between 400 and 2500 nm for all time points, correlated with the treatment effects and displayed as heat maps. Narrowband vegetation indices (VIs), utilizing different wavelengths of the electromagnetic spectrum, were also calculated from the spectral data. The Phytophthora treatments did not cause significant differences with the biophysical measurements in both trials. In the second trial, the waterlogging treatment significantly lowered plant top dry weight (P = 0.016) and diameter (P = 0.044). Reflectance values plotted against wavelength displayed differences between treatments as well as a seasonal trend. The NDSI heat maps indicated that the Phytophthora and waterlogging treatment effects were strongest correlated with bandwidths in the visible and near-infrared portions of the electromagnetic spectrum (538–558 nm and 701–709 nm). Six of the VIs (normalized difference nitrogen index 2, anthocyanin reflectance index 1, photochemical reflectance index, Carter index 1, Vogelman index 3 and water band index) were able to track the biochemical changes in the leaves over the 10 weeks, confirming the seasonal trend. The interaction effect between P. cinnamomi, waterlogging and elapsed time in the first trial was significant for water band index (P = 0.010). This study demonstrates that reflectance spectroscopy holds promise for characterizing marri response but more work needs to be done to identify the optimum wavelengths for identifying Phytophthora and waterlogging stress with marri.

Recruitment failure of keystone trees in Phytophthora infested forest

“…Seedling densities (recorded in 60 m 2 plots) were multiplied tenfold to represent tree densities (recorded in 600 m 2 plots) prior to analysis. Croeser, 2021;Tippett et al, 1985). Whether the pathogen was directly responsible for marri declines in this study or if harsh environmental conditions within P. cinnamomi infestations were a stronger influence is an open question worthy of future research.…”

“…Our observation of decreased marri tree densities within infestations and significant recruitment failure for marri (as evidenced by lack of marri trees <20 cm stem diameter) was therefore unexpected. This finding brings into question the supposed resistance of marri to P. cinnamomi under non‐waterlogged conditions (Cahill et al., 1992; Croeser, 2021; Tippett et al., 1985). Whether the pathogen was directly responsible for marri declines in this study or if harsh environmental conditions within P. cinnamomi infestations were a stronger influence is an open question worthy of future research.…”

“…Whether the pathogen was directly responsible for marri declines in this study or if harsh environmental conditions within P. cinnamomi infestations were a stronger influence is an open question worthy of future research. A combination of the two stressors is a likely scenario, as fine roots of marri can be heavily reduced by P. cinnamomi under temporarily waterlogged conditions (Croeser, 2021), which when followed by hot and dry summer conditions could lead to death of seedlings.…”

“…For example, high rainfall years (i.e., 2005–2009; Figure 3) may have contributed to waterlogged soils with increased P. cinnamomi activity. The pathogen can reduce root biomass of both jarrah and marri seedlings in waterlogged conditions (Croeser, 2021; Hardy et al., 1996), which could have led to many seedlings dying and thereby reducing recruitment of mature trees.…”

“…Marri trees are considered field resistant to P. cinnamomi infection, although roots can have low levels of colonization (Cahill et al., 1992; Tippett et al., 1985), and in pot trials, seedlings can experience considerable, but often non‐lethal reductions to root mass (Croeser, 2021). Marri saplings (up to 7 years of age) can be killed by P. cinnamomi when water ponds around the lignotuber.…”