- Biological control of chestnut blight with viruses is constrained by the genetic diversity of the chestnut blight fungus, Cryphonectria parasitica. Viruses can be transmitted between genetically identical strains of the fungus, but are inhibited by a self/nonself recognition system, called vegetative incompatibility. Variation in vegetative incompatibility, and hence a deterrent to virus transmission and biological control, is maintained by sexual recombination in most populations. In some populations, however, a considerable amount of reproduction occurs asexually, resulting in clonal populations with little genetic variation. Recombination in fungi without sexual reproduction, a process called parasexuality, has been described for many fungi in the laboratory, but not in nature. Parasexuality is initiated by the fusion of cells from genetically different individuals such that different nuclei inhabit the same cells. This fusion is controlled by the same self/nonself recognition system that controls virus transmission. We have documented the occurrence of parasexuality in C. parasitica from clonal populations in Wisconsin, and most recently in Macedonia. In both of these populations, we have shown that cellular fusion and parasexual recombination occurred between strains that are vegetatively incompatible, contradicting the prevailing model for self/nonself recognition mechanisms in fungi.