Conidial Morphology, Axenic Growth, and Sporulation ofStegophora ulmea

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Abstract
Techniques were developed for axenic culture of the fungus S. 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 gound 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.degree. and 24.degree. C; growth was substantially reduced at 28.degree. C. Macroconidia were produced between 16.degree. and 24.degree. C with an optimum at 20.degree. C. Aeration moderately reduced growth and prevented sporulation. Growth was not influenced by absence of light; however, sporulation was 5 times greater in cultures grown in the dark. Temperature also had a significant effect on germination of ascospores (optimum at 8.degree. C) but was not a significant variable in macroconidial germination. Microconida were not produced in culture and did not germinate under the test conditions. Field studies confirmed the presence of 2 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). The spores of C. ulmeum may serve as spermatia.

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