Yes Diane, there are viruses of yeast.
Cytoplasmically inherited viruses include a raft of dsRNA genome viruses,
reviewed in several places in 1990-92 by Reed B. Wickner. Also, yeast has
retrotransposons that can exist transiently in intracellular virus-like
particles, and prions (recently reviewed by Wickner).
However, none of these things give a phenotype like that your describe. Some
yeast strains are flocculent or clumpy and those phenotypes may be due to
specific chromosomal genes, which have been extensively studied by folks in the
brewing industry who like to be able to filter yeast out of good-tasting media.
I don't know if that is indeed what you are seeing, but clumpy strains often
have strange morphology when colonies are allowed to overgrow.
UMDNJ-RW Johnson Medical School