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Optimizing Plot Size for Field Studies of Phymatotrichum Root Rot of Cotton. T. J. Gerik, Blackland Research Center, P.O. Box 748, Temple, TX 76503; C. M. Rush(2), and M. J. Jeger(3). (2)Blackland Research Center, P.O. Box 748, Temple, TX 76503; (3)Department of Plant Pathology, Texas A&M University, College Station 77843. Phytopathology 75:240-243. Accepted for publication 27 August 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-240.

A study was conducted to determine the effect of plot size on the variance and mean percent disease incidence in field studies of Phymatotrichum root rot of cotton. The variance among sample areas (σ²) and within plots (?²), the coefficient of variation (CV), and the mean of percent disease were systematically evaluated. Measurements were made in 1983 at three locations in the Blackland Prairie that differed in respect to disease incidence. Plot sizes ranged from 5.5 to 88.0 m². The σ² and the respective CVs were reduced by increasing the sizes of the sampling area. Minimum plot sizes of 33 m² (ie, three rows x 16 m or two rows x 24 m) were necessary to estimate the true mean. However, when plot size was held constant, the σ² and associated CV was lower when plots had a greater number of adjacent rows compared to increased row length. This occurred irrespective of location or level of disease incidence. Because the σ² was not reduced further when plot size exceeded 33 m², we conclude that minimum plot areas of 33 m² (ie, three rows x 16 m) are required to give the best estimate of the true mean and to provide the lowest variance in field experiments of Phymatotrichum root rot on cotton.