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SUMMARY
Ustilago bullata and U. trebouxii were hybridized by inoculating Hordeum jubatum var. caespitosum with 12 different monosporidial combinations.
The F1 sori had characters of both parents but, in being confined to the inflorescence, were more characteristic of U. bullata. The F1 spores were minutely echinulate, distinctly lighter colored on two sides, and intermediate in size.
Because of lysis, attempts to obtain complete sets of monosporidial cultures from hybrid spores failed; hence, hybrid teliospores were used as inoculum to produce succeeding generations.
Two of the F1 hybrids produced F2 sori, limited in each case to a single plant. In contrast to the F1, the F2 sori were striate and foliicolous, as in U. trebouxii. Except for being larger, the F2 spores resembled those of the F,. The F3 sori and spores were generally similar to those appearing in the F2. The F4 sori of one hybrid were striate like those of U. trebouxii; in the other hybrid, both parental sorus types were produced. The F4 spores were similar to those in the preceding generations.
The predominant type recovered in these crosses is sufficiently different from either parent to be regarded as a distinct species. Some of the hybrid spores exhibited other unique features, such as an unevenly thickened exospore or a tendency to develop polar “caps.” Spores expressing this latter character are suggestive of Ustilago spegazzinii var. agrestis, the stem smut of grasses.