Using Color Infrared Imagery to Detect Sooty Mold and Fungal Pathogens of Glasshouse-propagated Plants

Abstract

Fungi are major biotic constraints for optimum production and quality of glasshouse plants. When plants are infested with sooty mold (Capnodium spp.) or infected with pathogens, the reflected wavelengths of the electromagnetic spectrum are altered. Spectroradiometric measurements and color infrared (CIR) images of control, honeydew-coated, and sooty mold-infested saplings and individual leaves from trifoliate orange (Poncirus trifoliata), sour orange (Citrus aurantium), ‘Valencia’ orange (C. sinensis), and ‘Bo’ tree (Ficus religiosa) were obtained. Grapefruit saplings and individual leaves infected with Mycosphaerella citri (greasy spot) were imaged under glasshouse conditions. Similarly, muskmelon foliage showing low and high levels of powdery mildew (Sphaerotheca fuliginea) disease severity were analyzed. When examining individual leaves, all fungal biotic stressors generally resulted in variable spectral reflectance data, especially in the blue (450 nm) and green (550 nm) wavelengths; however, values in the red (650 nm) tended to increase and values in the near-IR (850 nm) tended to decrease with stress. Near-IR/red image ratios were significantly reduced (P < 0.05) in stressed whole plant foliage and individual leaves relative to healthy controls. The accumulation of insect honeydew (which occurs before sooty mold infestation) significantly increased (P < 0.05) near-IR reflectance values and near-IR/red ratios in ‘Valencia’ orange and near-IR/ratios in ‘Bo’ tree foliage and individual leaves. Image acquisition and enhancement techniques may prove useful in large-scale production greenhouses where existing infrastructure and high plant populations require high throughput data analysis and identification of biotic stressors.