OPTIMUM SAMPLING SIZE FOR DETERMINING DIFFERENT ASPECTS OF ALTERNARIABLOTCH OF APPLE CAUSED BY ALTERNARIA-MALI

The optimum number of leaves and terminals to sample to estimate the i ncidence and severity of Alternaria blotch, caused by Allernaria mall, of apple (Malus X domestica) and associated defoliation was derived f rom data collected between 1989 and 1992 in three locations in North C arolina. On the basis of estimates of variance and cost, the mean opti mum number of leaves per terminal was 16.88 for evaluating disease sev erity and 19.37 for determining disease incidence. The optimum numbers of terminals per tree for assessing severity, incidence, and defoliat ion were 1.2, 1.18, and 2.04, respectively. The variation in disease a mong leaves and among terminals contributed equally to overall estimat es of variance. With the lower cost of sampling leaves compared to sam pling terminals, increasing the number of leaves sampled would improve sampling efficiency more than increasing the number of terminals samp led.