Quantification of Foliar Plant Disease Symptoms by Microcomputer-Digitized Video Image Analysis

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Abstract
The area of leaf lesions caused by Alternaria solani on tomato [Lycopersicon esculentum] and Ascochyta pteridium on bracken fern [Pteridium aguilinum] as well as the area of mycelia of Microsphaera alni on sycamore [Platanus racemosa] and marginal leaf necrosis of California buckeye [Aesculus californica] were quantified by using computer-controlled video digitizing hardware. Leaves under fluorescent lamps equipped with a red (.lambda. = 620-700 nm) filter were scanned with a black-and-white video camera. BASIC and machine language programming in an Apple II computer equipped with a video analog-to-digital converter were used to locate and digitize each of 16,232 individual pixels into 64 values of gray. The digitized picture elements were grouped into 3 magnitude categories corresponding to black background, healthy and necrotic tissue. Algorithms utilizing a correction for background variation allowed reproducibility to .+-. 0.8% in successive measurements. Measurements of individual leaves, application of correctional algorithms, and printing and storage of the data required 4.1 s. Estimated total and necrotic leaf areas differed from areas measured with a planimeter by < 1.2 and 2.0%, respectively.

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