Genetically pure lines of the mononucleate (haploid) stage of Cronarti
um ribicola can be grown by isolating mycelium from single spots on in
fected white pine needles or by culturing single basidiospores on defi
ned media preconditioned by cell-free diffusates of massed germinating
basidiospores. Although growth is initially slow, cultures can be inc
reased rapidly by alternately subculturing from solid to liquid media
and back again indefinitely. The ability to culture pure haploid clone
s has important implications for studying the genetic structure and se
xual behavior of rust populations and the nature of virulence.