The optimum number of leaves and terminals to sample to estimate the incidence and severity of Alternaria blotch, caused by Alternaria mali, of apple (Malus × domestica) and associated defoliation was derived from data collected between 1989 and 1992 in three locations in North Carolina. On the basis of estimates of variance and cost, the mean optimum number of leaves per terminal was 16.88 for evaluating disease severity and 19.37 for determining disease incidence. The optimum numbers of terminals per tree for assessing severity, incidence, and defoliation were 1.2, 1.18, and 2.04, respectively. The variation in disease among leaves and among terminals contributed equally to overall estimates of variance [...]
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Nenad Filajdić (1994) studied this question.