Spectral characterization of necrosis from reflectance of Eucalyptus globulus leaves with Mycosphaerella leaf disease or subjected to artificial lesions

Abstract

Necrosis is one of a range of symptoms resulting from a number of different biotic and abiotic damage agents that may be detected and quantified with remote sensing as part of an operational forest health monitoring system. Mycosphaerella leaf disease (MLD) caused by Teratosphaeria spp. (formerly known as Mycosphaerella spp.) is the most common foliar disease in young Australian Eucalyptus globulus plantations. Necrosis often occurs in conjunction with other visible symptoms such as chlorosis and reddening, and we have tested whether these symptoms alter the ability of spectral approaches to detect necrosis. We completed two studies of necrosis with pot-grown E. globulus plants; one in which necrosis was induced by artificial infection of Teratosphaeria spp. and one in which necrosis was induced by injury, superimposed on plants with established reddening or chlorosis. Using spectral sensitivity analysis we found that across the two studies, wavelengths between 679 and 695 nm were most sensitive to the presence of necrosis and those between 706 and 726 nm were least sensitive. A new vegetation index (VI) was able to statistically group necrotic treatments together while grouping non-necrotic treatments together, regardless of reddening and chlorosis, which other relevant indices could not. Multivariate methods utilized many wavelengths throughout the spectrum and enabled much greater distinction of all treatment groups related to necrosis, compared with the VIs. Wavelengths in the 679–695 nm range were only occasionally selected as key wavelengths; therefore, results were not similar to the spectral sensitivity data.