Vegetation Indices & Disease Monitoring
Plant Reflection of Light
NDVI and PRI are calculated from measurements of electromagnetic radiation reflected from plant canopy surfaces.
NDVI (Normalized Difference Vegetation Index) is a standardized index use to measure the state of plant health. Leaf chlorophyll absorbs red light (approximately 680mm), and the cellular structure of the leaves strongly reflect near-infrared light, approximately 730mm. When the plant is water stressed or diseased the spongy layer deteriorates and the plant absorbs more of the nearinfrared light, rather than reflecting it.
By observing how NIR changes compared to red light provides an accurate indication of the presence of chlorophyll, which correlates with plant health.
PRI (Photochemical Reflectance Index) originally defined as an index of the diurnal xanthophyll cycle activity, provides a measure of photosynthetic light-use efficiency
(LUE) which can be used as an indicator of stress. PRI bands are centred at 532nm and 570nm.
Leaf wetness refers to the presence of free water on the canopy, and is caused by ntercepted rainfall, dew, or guttation. The duration of the time period during which the leaves are wet is generally referred to as leaf wetness duration (LWD).
Leaf wetness is a concern for the development of disease and the dispersal of pathogens; LWD is an important input (along with temperature) in many crop disease models which are used for determining the appropriate time for the use of preventative measures, such as fungicide application.