Early Detection of Herbicide-Induced Stress of Annual Bluegrass Using Hyperspectral Radiometry and Derivative Analysis.

Annual bluegrass (ABG) is considered both a desirable turfgrass species and problematic weed on golf putting greens. ABG suffers from numerous biotic and abiotic stresses during the summer, resulting extensive management inputs. Advanced stress detection using hyperspectral radiometry (HSR) could be highly beneficial in timing key management events, such as fungicide applications. Detailed derivative analysis (DA) of the spectral signature of ABG is being examined for unique features triggered by plant stress. Herbicides known to injure ABG were used as an initial screen for stress detection prior to symptom development. A replicated field evaluation was conducted on a mixed stand ABG and creeping bentgrass research golf putting green at the Virginia Tech Golf Course, Blacksburg, VA, USA in October 2011. Bispyribac-sodium (74.1 g ai/ha) and amicarbazone (241 g ai/ha) were applied on October 3, 2011, and compared to an untreated control.  Treatments were randomized in a complete-block experimental design, with three replications and ten subsamples per plot. Reflectance data were collected daily between 11:00am and 1:00pm, using a hand-held spectroradiometer (350 to 2500 nm spectral range) fitted with a plant probe (10 mm spot size). 300 spectrum data were pooled and analyzed to provide one spectral signature per treatment per day. Daily reflectance and its first and second derivatives were examined for unique features useful for temporal separation. While numerous features were found, the most promising was a shift change in the second derivative at 715nm. Exploration of this feature revealed a temporal gradient with each herbicide following application, with first detection of stress at 2DAT. Mild chlorosis was first visually noticed 5DAT in bispyribac sodium treated plots, and a less pronounced thinning at 7DAT in amicarbazone treated plots. Results from this experiment provide evidence that physiological responses of ABG may be detected using HSR and DA as early as five days prior to visible symptom development. Evaluation of the second derivative at 715nm and other features is ongoing for stresses caused by various biotic and abiotic factors.