Spectral reflectance characteristics of Pinus radiata needles affected by dothistroma needle blight

Dothistroma needle blight, caused by Dothistroma septosporum (Dorog) Morelet, is an economically significant disease of several Pinus species in New Zealand, Australia, Chile, South Africa, and parts of Asia, North America, and Europe. The spectral reflectance properties of Pinus radiata D. Don needles infected by D. septosporum were examined over the visible and near-infrared wavelength region (400-1000 nm). The largest reflectance difference occurred on the shoulder of the near-infrared region at 763 nm. Wavelengths of greatest sensitivity to D. septosporum infection were located in the ranges of 675-691 nm, followed by wavelengths near 760 and 550 nm. Sensitivity minima occurred at 434, 493, 506, 709, and 1373 nm. The reflectance ratio best correlated to needle damage was 709/691 nm (r = -0.739, P < 0.001). Among the other reflectance indices tested, an index based on wavelengths of the upper red edge (710-740 nm) was also well correlated (r = -0.730, P < 0.001). There was not a strong linear relationship between the point of maximum slope in the red edge region (red edge position) and needle damage category. This may be because D. septosporum is a necrotrophic fungal pathogen producing a complex series of damage symptoms: initial chlorosis, production of red and brown metabolites, rapid loss of cellular integrity, cell necrosis, and eventual desiccation. Diagnostic features identified in the infected needle spectra may contribute to the formulation of remotely sensed spectral indices for detecting and monitoring dothistroma needle blight in plantations.