Changes in spectral reflectance of dry bean (Phaseolus vulgaris) leaves in response to atrazine

--- Daniel M. Adamson

Drift and carryover of herbicides present challenges to land managers because of damage to sensitive crops. For many chemical groups, herbicide injury to plants may not be visible for several days or weeks after application. Remote sensing offers the potential to detect herbicide injury prior to visually apparent symptoms. This study tested the ability of a hand-held reflectance spectrometer to detect changes in leaf reflectance of dry bean (Phaseoulus vulgaris) plants in response to atrazine.

Dry bean (Phaseolus vulgaris) plants 21 hours (left) and 213 hours (right) after atrazine application.
After 213 hours, symptoms were visible as interveinal chlorosis (right-middle) and necrosis (right-right)

Atrazine was applied at 100% and 25% rates on V1 dry beans in a greenhouse. Results show that significant changes to the spectral reflectance curve occurred on treated plants within 21 hours of application. Indices such as NDVI and GNDVI were poor indicators of plant health because of the reduction in reflectance at 560nm and 645 nm.

Orange Normalized Difference Vegetation Index (ONDVI) of
dry bean (Phaseoulus vulgaris) leaves in response to two rates of atrazine application. 

An alternative index using reflectance at 600 nm (Orange NDVI) was able to capture relatively small changes in reflectance of treated plants in this region.