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The relative virulence of strains of Acidovorax avenae to their reported hosts was investigated. Strains of A. avenae subsp. avenae were more virulent to sweet com than to maize or sugarcane, and only weakly virulent or avirulent to oat. Passage of weakly virulent strains of subsp. avenae through sweet com did not affect strain virulence. A. avenae subsp. cattleyae was not shown to cause symptoms in Cattleya sp. Strains of A. avenae subsp. citrulli were more virulent to cucumber than to melon. Conditions which favoured the expression of symptoms by subspecies of A. avenae were established. A. avenae subsp. avenae and A. avenae subsp. citrulli were most virulent in the young tissue of their respective hosts (sweet com and cucumber) at 27-30°C, with expression at higher temperatures being limited by the thermal tolerance of the plants. A general equation relating symptom expression to inoculum concentration was developed. This simple linear equation related the logarithm of lesion number to the logarithm of inoculum concentration. The intercept values in the equation for each strain can be considered to be an expression of the heterogeneity of pathogenicity of inoculum, whereas the gradient gives an expression of the relative virulence of infective cells. [ABSTRACT FROM AUTHOR]