VIEW ARTICLE

Cytology and Histology

Expression of Pathogen Virulence and Host Resistance During Infection of Alfalfa with Stemphylium botryosum. W. A. Cowling, Former graduate research assistant, Department of Plant Pathology, University of California, Davis 95616, Current address: Department of Plant Pathology, Cornell University, Ithaca, NY 14853; D. G. Gilchrist, assistant professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 72:36-42. Accepted for publication 16 April 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-36.

Infection of Medicago sativa by Stemphylium botryosum (cool-temperature biotype) was examined histologically to assess the sequence and form in which pathogen virulence and host resistance were expressed. Pathogen growth during the infection process and host reaction to infection were followed simultaneously in a set of nine host clone-pathogen isolate (h-p) combinations. The h-p set consisted of the factorial combination of three alfalfa clones and three pathogen isolates chosen to encompass the range of relative host resistance and pathogen virulence observed in the field in California. In all h-p combinations, leaf penetration by germ tubes occurred exclusively through stomata, while subsequent fungal growth (revealed by aniline blue dye) was restricted to the development of a bulbous mycelium in the substomatal cavity. The formation of the bulbous mycelium coincided with an effect on adjacent palisade mesophyll cells that resulted in retention of dye by walls of the cells, in contrast to unstained walls of cells remote from the site of penetration. Penetrations in which dye was retained by host cell walls located more than 0.1 mm from the bulbous mycelium (the radius of the smallest lesion formed on susceptible leaves) were recorded as “effective” penetrations. Differences in relative virulence of isolates (expressed as the relative disease severity produced on a susceptible alfalfa clone at a fixed inoculum concentration) were correlated with the frequency of stomatal penetration. In addition, isolates with high relative virulence produced a higher proportion of effective penetrations than isolates with low virulence. The frequency of stomatal penetration was not affected by host resistance. Differences in relative host resistance were expressed as the relative frequency with which host cells collapsed and formed necrotic lesions in response to effective penetrations. Therefore, relative virulence is expressed independently of and prior to the expression of relative resistance in this interaction.