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Survey Methods for Assessment of Citrus tristeza virus Incidence in Citrus Nurseries

Monitoring of plant health takes place in citrus nurseries to prevent the distribution of infected plants to commercial groves. In this article, both analytical and simulation methods are used to characterize schemes by which such monitoring may be carried out, in the particular context of Citrus tristeza virus infection. Two aspects of such schemes are discussed in detail. The inclusiveness of a sample is an assessment of the degree of redundancy that occurs because, in some samples, the progeny of identically infected propagation material may appear more than once. The operating characteristic function shows the probability of reaching a decision, based on sampling, that a population of daughter plants has an incidence of infection less than or equal to some adopted threshold level for any actual level of incidence in the population. If the same proportion of the population is assessed at different population sizes, both the inclusiveness and the operating characteristic function vary with population size. However, sample sizes may be calculated so that a specified operating characteristic function is maintained as population size varies. The sample sizes required to meet the conditions specified on the operating characteristics do not increase proportionally with population size. Under such a scheme, fewer samples might need to be taken from large populations of daughter plants than would be the case if a constant percentage sampling scheme were adopted.