Optimum Sample Size for Detecting Virulence Differences in Leucostoma Isolates from Peach. G. C. Adams, Assistant Professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. S. A. Hammar, and A. Iezzoni. Research Assistant, Department of Botany and Plant Pathology, and Associate Professor, Department of Horticulture, Michigan State University, East Lansing 48824. Plant Dis. 73:754-759. Accepted for publication 23 February 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0754.

A wide range in virulence was found among 28 isolates of Leucostoma persoonii and L. cincta after inoculation of each isolate on one branch per tree in 10 peach trees. An optimum experimental design for detecting differences in virulence among isolates of Leucostoma spp. was developed on the basis of measurement of variance in susceptibility of inoculated trees and branches. Inoculating more than one branch per tree had a minimal effect on reducing experimental error, but increasing the number of trees inoculated to more than six significantly reduced the statistical variance. Inoculating six to nine trees, one branch per tree, gave high precision in the detection of differences in isolate virulence while utilizing few trees and reducing labor. The experimental design could detect a minimum difference of 4.45 cm (P = 0.05) in canker size (virulence) between two isolates with 90% assurance that the difference in virulence was genuine. Criteria for differentiating L. cincta and L. persoonii in culture were examined, and a system based on colony margin and color, size of pycnidia, and growth at 37 C was found to be suitable. Isolates of L. cincta and L. persoonii were comparable in virulence, and the geographic origin of isolates tested was not correlated with the level of virulence.