Spectral reflectance of healthy and diseased watermelon leaves

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Abstract
SUMMARY: Significant differences in reflectances were found between healthy and diseased watermelon leaves in the 500–640 nm visible region and 740–900 nm near infrared regions of the electromagnetic spectrum. Spectrophotometric reflectance measurements were made to determine differences between healthy and diseased watermelon (Citrullus vulgaris) leaves over the 500–2700 nm wave length interval. Plants with Fusarium wilt, downy mildew (Pseudoperonospora), and watermelon mosaic were observed during the growing season. Early symptoms of the diseases had significantly greater reflectance values than the advanced stages of the diseases. A ratio of two wavelength bands, 500–640 nm and 660–900 nm gave an accurate index for determining stages of disease, both early and advanced. There were greater differences in reflectance between all three diseases and healthy leaves in both early and advanced stages in the 900–1400 nm, 1500–1900 nm, and 2000–2400 nm middle infra‐red wavelengths of the electromagnetic spectrum than in the visible 450–740 nm range. All three diseases had significant differences in the 700–900 nm near‐infra‐red range.