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Etiology

Conidial Morphology, Axenic Growth, and Sporulation of Stegophora ulmea. G. H. McGranahan, Graduate research assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706, Current address of senior author: USDA-ARS, Department of Pomology, University of California, Davis 95616; E. B. Smalley, professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 74:1300-1303. Accepted for publication 30 May 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1300.

Techniques were developed for axenic culture of the fungus Stegophora ulmea [= Gnomonia ulmea], the cause of black spot of elm, previously reported to be an obligate parasite. The fungus was isolated by the surface-sterilized leaf-disk method. Radial growth was optimal on oatmeal agar. Growth was limited on malt, Czapek' s, dilute potato-dextrose, and elm leaf extract agars. Maximum sporulation was obtained in a medium composed of the water-soluble fraction of ground oats. Production of macroconidia was minimal on potato-dextrose broth. Temperature had a significant effect on both growth and sporulation in liquid media. Optimum temperature for growth ranged between 16 and 24 C; growth was substantially reduced at 28 C. Macroconidia were produced between 16 and 24 C with an optimum at 20 C. Aeration moderately reduced growth and prevented sporulation. Growth was not influenced by absence of light; however, sporulation was five times greater in cultures grown in the dark. Temperature also had a significant effect on germination of ascospores (optimum at 8 C) but was not a significant variable in macroconidial germination. Microconidia were not produced in culture and did not germinate under the test conditions. Field studies confirmed the presence of two conidial forms occurring in lesions of S. ulmea. Macroconidia (= Gloeosporium ulmicolum) were common from the time of leaf emergence through June (in Wisconsin) at which time a transition occurred within the lesion to the microconidial form (= Cylindrosporella ulmeum). It is suggested that the spores of C. ulmeum may serve as spermatia.