Table 1.

Comparison of thermal and optical features indicative of healthy and diseased crops

Waveband	Diagnostic ability
Visible (400–700 nm)	Increased reflectance in diseased canopies, especially in the chlorophyll absorption bands due to loss of chlorophyll and presence of surface spores or mycelium
Near Infra-Red (NIR 700-1200nm)	The red edge (rapid transition from low reflectance to high reflectance) is shifted from 730 nm in healthy canopies to shorter wavelengths (e.g. 670 nm) in diseased canopies. Reflectance is also reduced in diseased canopies due to senescence and defoliation.
Short-wave Infra-Red (SWIR 1200–2400 nm)	Relatively small effects occur due to water content
Thermal infrared band (TIR ≈ (8000–14000 nm)	Leaf temperature is increased by reduced transpiration rate, caused by root diseases and wilts (xylem infection) and some foliar diseases (that cause closure of stomata at early stages). Water-soaked leaf lesions (caused by cell lysis) can appear cooler at the start of the day or warmer at the end of the day as the rest of the leaf changes temperature more quickly according to ambient conditions.