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Sampling for the Incidence of Aphid-Transmitted Viruses in Snap Bean

Data collected in 2002 and 2003 on Alfalfa mosaic virus and Cucumber mosaic virus incidences of infection in commercial snap bean fields in New York State were used to develop relationships between disease incidence (p_low) and sample size while accounting for the inherent spatial aggregation of infected plants observed with these two viruses. For a plan consisting of 300 sampled plants (N = 60 quadrats, n = 5 plants per quadrat), estimating p_low from the incidence of positive groups (p_high; testing of N = 60 grouped samples) provides the same precision in p_low as testing 200 plants individually, up to about p_low = 0.2. Above that, the confidence interval width for p_low obtained via group testing becomes markedly larger than the width obtained by testing individual plants. Our results suggest using group testing until p_high is in the range [0.35, 0.59], which corresponds to p_low in [0.1, 0.2]. Results indicate that group testing can be more economical than the testing of individual plants without loss of precision, at lower incidences of infection. The approach presented provides a general framework for sampling and the estimation of incidence of other aphid-transmitted viruses in snap bean.