A rank test for distinguishing environmentally and genetically induced disease resistance in plant varieties

Many species of plant pathogens vary genetically in ability to cause disease on a given host plant. Conversely, plant populations often have variability with respect to a gene or series of genes that provide resistance to a given pathogen (Leonard, 1987, in Populations of Plant Pathogens: Their Dynamics and Genetics, 163-181; Christ, Person, and Pope, 1987, in Populations of Plant Pathogens: Their Dynamics and Genetics, 7-21). These dynamics have become apparent, as there have been many cases in which crop plants bred for homogeneous high-level genetic resistance to a disease have lost the resistance, sometimes catastrophically (Day and Wolfe, 1987, in Populations of Plant Pathogens: Their Dynamics and Genetics, 3-7). This has led plant protectionists to seek ways to develop stable resistance to plant pathogens that are variable by nature. A nonparametric procedure that tests for variation among populations of a specific pathogen at different-locations is described. A randomisation approach is used to compute critical values and p-values. A small simulation study is carried out to examine the performance of the test. The same procedure can be used to determine the reaction of a specific crop variety to a pathogen under different environmental conditions or in different locations. The procedure is demonstrated using data on the fungus Colletotrichum graminicola (Ces.) Wilson, which is a pathogen on the cereal crop Sorghum bicolor (Moench). Disease reactions of genetically different sorghum varieties to specific strains of the fungus in different environments are recorded. From these, varieties are selected that do not exhibit interaction with environment. These varieties are then used to identify which. locations have dis tinct pathogen populations. Once a set of locations has been identified as having distinct pathogen populations, the breeding materials are planted and selected for insignificant genotype by location (pathogen population) interaction, i.e., for genetic stability with respect to a given disease.