Feasibility of detecting apple scab infections using low-cost sensors and interpreting radiation interactions with scab lesions

Abstract

ABSTRACT Apple scab is a disease caused by the fungus Venturia inaequalis (Cke.) Wint. which can spread rapidly throughout orchards diminishing tree productivity and causing huge losses in marketable fruit. Efficient orchard reconnaissance and early detection of infections can inform fungicide applications for effective disease control and a range of new low-cost sensors offer a means of imaging orchards as the basis of scab detection. This study evaluates the potential contribution of three imaging devices: a multispectral (VIS-NIR) camera, thermal camera and a 3D sensor, for the detection of scab on young apple plants. In a controlled experiment, apple seedings were infected with scab and disease progression was imaged daily under natural illumination conditions in a glasshouse with minimal image processing. Whilst the thermal and 3D sensors images were deemed unsuitable for scab detection, the high-resolution multispectral imagery was exceptionally effective, with the NIR band (800–1000 nm) permitting the earliest scab detection due to the substantially lower reflectance of the fungal structures of V. inaequalis relative to healthy leaf tissue. We offer a model of near-infrared radiation interactions with the fungus and leaf interactions to explain reflectance characteristics of scab-infected leaves throughout the growth cycle of the pathogen. The simple, low-cost remote-sensing approach developed here holds considerable promise for providing timely information on tree infection to improve the efficiency of apple scab disease management routines.