VIEW ARTICLE

Resistance

The Effect of Leaf Developmental Stage on the Variation of Resistant and Susceptible Reactions of Phaseolus vulgaris to Uromyces appendiculatus. M. Shaik, Department of Plant Pathology, University of Nebraska, Lincoln 68583-0722, Present address: Department of Botany, Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario, Canada M5S 2C6; J. R. Steadman, Department of Plant Pathology, University of Nebraska, Lincoln 68583-0722. Phytopathology 79:1028-1035. Accepted for publication 6 April 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-1028.

The effect of leaf developmental stage of beans (Phaseolus vulgaris) at the time of inoculation on the expression of various reactions to rust (Uromyces appendiculatus) was investigated. The areas of uredinia, fungal colonies, and secondary uredinia were negatively correlated with leaf age or leaf length at the time of inoculation, in the most susceptible reaction (large uredinia). These three susceptibility parameters were all positively correlated with each other. In the reaction of smaller uredinia surrounded by necrosis, the percentage of uredinia surrounded by necrosis and fungal colony areas were negatively correlated, whereas uredinial area was positively correlated with leaf developmental stage at the time of inoculation. The effect of leaf developmental stage on uredinial area in this reaction was thus the opposite of that observed for large uredinia. However, the covariate (leaf age)-adjusted means of uredinial and fungal colony areas were significantly lower in the smaller uredinial reaction than those in the large uredinial reaction. In highly resistant reactions (immunity and necrotic spots), leaf age effects were not apparent. Based on these results, several recommendations are made for studying resistance manifested by small uredinia.