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Sampling Procedures for Determining Severity of Alfalfa Leaf Spot Diseases. Wayne M. Thal, Graduate assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; C. Lee Campbell, Associate professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 77:157-162. Accepted for publication 9 June 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-157.

Alfalfa fields in three North Carolina counties were sampled during spring and summer, 1982, to estimate leaf spot severity. Three stems per quadrat were selected from 1- 1-m quadrats arranged in grids of 64 or 256. Two lower and two upper leaves were chosen randomly from each stem and percent diseased leaf area was estimated for each leaf. Leptosphaerulina briosiana was the most commonly isolated leaf-spotting organism. Variance components were estimated to partition total variation into variation among leaves within a stem, stems within a quadrat, and quadrats within a field. Variance components were used to determine optimum sampling rates based on linear cost functions. Three to four leaves per stem half was optimum for most samples. Spatial autocorrelations were estimated for quadrat mean disease severity and these were used to estimate optimum quadrat size and shape based on a cost matrix. Autocorrelations among neighboring quadrats were generally low, indicating that several quadrats should be grouped into rectangular-shaped sampling units.