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Methods of Preservation of Corynebacterium insidiosum Isolates in Relation to Virulence and Colony Appearance on a Tetrazolium Chloride Medium. R. B. Carroll, Graduate Assistant, Department of Plant Pathology, The Pennsylvania State University, University Park, Pa. 16802, Present address of the senior author: Plant Science Department, University of Delaware, Newark, Delaware 19711; F. L. Lukezic, Associate Professor, Department of Plant Pathology, The Pennsylvania State University, University Park, Pa. 16802. Phytopathology 61:1423-1425. Accepted for publication 6 July 1971. DOI: 10.1094/Phyto-61-1423.

Three major colony types were detected when dilutions from single-cell isolates of Corynebacterium insidiosum, maintained for 1 year by three different methods, were streaked on a 2, 3, 5-triphenyl tetrazolium chloride medium. These were (i) a large butyrous colony with deep red center and a smooth, wide, white border; (ii) an intermediate pink-to-white colony sometimes showing a small, dark pink center and somewhat irregular border; and (iii) a small, green, round-to-oblong colony. Tests conducted on DuPuits alfalfa seedlings, grown under sterile conditions, showed that isolates derived from the pink-type colony were highly virulent, whereas those derived from the red- and green-type colonies were avirulent. A relationship was found between the method of storage and type of colonies produced on the tetrazolium chloride medium. A much higher percentage of the virulent pink colonies were recovered from isolates maintained in a sterile soil mix than from the same isolates maintained in sterile distilled water and on agar.