A COMPARISON OF SAMPLING METHODS FOR SURVEYING ALFALFA FOLIAGE DISEASES

Abstract

Two surveys of eastern Ontario alfalfa (Medicago sativa L.) fields were conducted in 1973 and 1974 to study the effects of the size of sampling area, the pattern of sampling paths, and the spread among sampling sites in relation to the incidence of common leaf spot (Pseudopeziza medicaginis (Lib.) Sacc.), Stemphylium leaf spot (Stemphylium botryosum Wallr.), and downy mildew (Peronospora trifoliorum de Bary). Results of the 1973 survey of 17 fields by three methods indicated that sampling along a 90-m diagonal of a square area of size 4,047 m2 (1 acre) within a quadrant of a field was adequate when disease incidence was either below 20 or above 80%; however, when disease incidence was intermediate (20–80%), sampling along a W-shaped path covering the entire field was more appropriate. Results of the 1974 survey of 60 fields suggested that the total number of sampling sites per field could be reduced from 40 to 15 without reducing precision, provided they were well distributed throughout the field.