VIEW ARTICLE

Disease Detection and Losses

Quantification of Foliar Plant Disease Symptoms by Microcomputer-Digitized Video Image Analysis. S. E. Lindow, Assistant professor, Department of Plant Pathology, University of California, Berkeley 94720; R. R. Webb, graduate student, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 73:520-524. Accepted for publication 12 October 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-520.

The area of leaf lesions caused by Alternaria solani on tomato and Ascochyta pteridium on bracken fern, as well as the area of mycelia of Microsphaera alni on sycamore and marginal leaf necrosis of California buckeye, were quantified by using computer-controlled video digitizing hardware. Leaves under fluorescent lamps equipped with a red (λ= 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 grey. The digitized picture elements were grouped into three 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 sec. Estimated total and necrotic leaf areas differed from areas measured with a planimeter by <1.2 and 2.0%, respectively.