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Epidemiology of Stem Blight of Vinca minor Incited by Phoma exigua var. exigua. Gerald A. Paulson, Former Research Assistant, Department of Plant Pathology, University of Illinois; Donald F. Schoeneweiss, Associate Plant Pathologist, Illinois Natural History Survey, Urbana, Illinois 61801. Phytopathology 61:959-963. Accepted for publication 11 March 1971. DOI: 10.1094/Phyto-61-959.

The causal organism of stem blight of Vinca minor was identified as Phoma exigua var. exigua. Symptoms appear initially as lesions at the ground line on stems and runners and at the base of new shoots, finally including wilting, blackening, and death of affected stems. Leaf spots caused by the pathogen occur occasionally, but are not associated with stem infections. Production by the pathogen of a toxin, which might account for the rapid blackening and death of affected tissues, could not be demonstrated. Optimum growth of the fungus in culture occurred at 27 C; there was no growth at 30 C. Maximum infection occurred at 18 C, but dropped off rapidly at higher temperatures. High soil moisture content was necessary for infection. Stem lesions appeared 10 to 15 days after inoculation at 18 C. A high percentage of infection occurred on intact, vigorous stems, and neither stem wounding nor moisture stress increased disease incidence. Phoma exigua var. exigua should be characterized as a highly virulent, facultative parasite of V. minor. Attempts to control stem blight through the use of various mulches or by foliar applications of fungicides in 1970 were unsuccessful.