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Use of the Electron Microprobe to Measure Exchange of Materials between Host and Pathogen. J. C. Comstock, Graduate Assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48823, Present address of senior author: Department of Botany and Plant Pathology, Iowa State University, Ames 50010; R. P. Scheffer, Professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48823. Phytopathology 63:689-691. Accepted for publication 14 December 1972. DOI: 10.1094/Phyto-63-689.

Relative concentrations of Mg, K, Rb, P, and S in conidia of Helminthosporium carbonum and H. victoriae on corn leaves (resistant and susceptible to H. carbonum) were measured with the electron microprobe X-ray analyzer. All five elements decreased significantly during germination and penetration (0-16 hr after inoculation) of susceptible and resistant tissues. H. carbonum regained Mg from susceptible leaves (24-48 hr), but not from resistant leaves. H. carbonum conidia on susceptible leaves had 70% and 30% of the original K content at 16 and 36 hr after inoculation. Conidia on resistant leaves had 30% of the original level of K at 16, and almost none at 36 hr. Since Rb, like P and S, was lost from conidia on both resistant and susceptible leaves at equal rates, the results imply an uptake of K from susceptible leaves which is masked by a larger loss. The losses from conidia of H. victoriae on both types of corn were comparable to those of H. carbonum on resistant corn. Thus, there is an exchange of certain materials between a pathogen and a susceptible host soon after infection, but probably only losses from conidia to the highly resistant host. These observations indicate the potential usefulness of the microprobe for studies of infection and disease development in plants.