Effects of Plot Size and Border Width on Assessment of Powdery Mildew of Winter Wheat. P. E. Lipps, Associate Professor, Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691. L. V. Madden, Professor, Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691. Plant Dis. 76:299-303. Accepted for publication 7 October 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0299.

A study was conducted to determine the effect of plot size (2.9 and 26 m²), distance between plots (1.7 and 5.1 m), and pairing of cultivar plots (slow-mildewing cultivar paired with a susceptible cultivar or susceptible cultivar paired with a susceptible cultivar) on the development of powdery mildew of wheat in 1989 and 1990. Both plot size and cultivar pairing had a significant (P < 0.05) effect on area under the disease progress curve (AUDPC). AUDPC values calculated for the susceptible cultivar in the smaller plots compared with the larger plots were 10 and 17% lower over the 2 yr, respectively. The lower AUDPC values indicated that a net loss of inoculum affected disease development in the smaller plots more than in larger plots. The susceptible cultivar had the same AUDPC value whether paired with the susceptible cultivar or the slow-mildewing cultivar. AUDPC values from the slow-mildewing cultivar were about half that of the susceptible cultivar. There was a significant interaction of cultivar pairing with plot size in 1989 and with border width in 1990 on AUDPC. This interaction occurred because, in a given year, one of these factors affected disease development on the susceptible cultivar but not on the slow-mildewing cultivar. Thus, there was no direct evidence of positive interplot interference. Results indicated that negative interplot interference caused an underestimation of the effect of slow-mildewing resistance in small plots. The underestimation resulted from comparison of the slow-mildewing cultivar with the susceptible cultivar in small plots because the susceptible cultivar had lower AUDPC values in small plots than in larger plots. Regardless, small plots (2.9 m²) of the susceptible cultivar had AUDPC values only slightly less than larger plots. Likewise, the effect of border width was minor. Thus, small plots with narrow borders could be used to evaluate cultivars for slow-mildewing resistance if estimates of errors attributable to interplot interference were recognized.

Keyword(s): Blumeria graminis f. sp. tritici, epidemiology, Triticum aestivum.