Distinct NIR Reflectance Spectra Associated with Foliar Symptoms of Beech Leaf Disease

Beech leaf disease (BLD) is an epidemic spreading among American beech (Fagus grandifolia) populations, with characteristic symptoms including dark green, yellow, or brown bands of thickened leaf tissue between secondary veins. Because near infrared (NIR) light is reflected by leaf tissue structure, we used multispectral NIR imaging reflectance to distinguish symptomatic from asymptomatic leaves amongst affected trees, as well as to distinguish between different expressions of symptoms (dark green, yellow, and brown bands) within symptomatic leaves and symptom severity (low, moderate high) at the whole leaf scale. Key findings include significantly higher NIR reflectance in dark green bands of symptomatic tissue compared to asymptomatic tissue amongst affected trees (λ = 840, 860, 900, 940, and 980 nm; ∼26.2%, 25.9%, 35.0%, 35.3%, and 32.5% increase respectively). A partial least squares regression model predicted 97% of variation in symptomatic vs. asymptomatic NIR reflectance was attributed primarily to the increase in spongy mesophyll thickness and overall leaf thickness in symptomatic tissue. Whole leaf symptom severity (asymptomatic, moderate, severe) was most clearly diagnosed at λ = 860 nm. Water infiltration of the intercellular airspaces in symptomatic dark green bands and asymptomatic leaves removed differences in NIR reflectance, supporting a mechanistic link between increased NIR reflectance and the foliar symptoms of BLD. Our results highlight the interaction of leaf internal architecture with NIR wavelengths and inform the development of targeted remote sensing tools.