The change from urediniospore to teliospore production is a crucial de
velopmental stage in the life cycle of many rust fungi. Uromyces appen
diculatus has five different spore stages during its life cycle and al
ternates between asexual and sexual reproduction. Teliospore productio
n is a prerequisite to the formation of the spore stages associated wi
th the sexual cycle (basidiospores, pycniospores, and aeciospores) in
this rust. Many naturally occurring isolates apparently lack the abili
ty to produce teliospores and are restricted to asexual reproduction b
y production of urediniospores. A cross was previously made by using u
rediniospores from an asexual isolate to fertilize pycnia of a sexual
isolate. The parents and progeny from this cross were analyzed for the
ability to produce teliospores. When inoculated onto Phaseolus vulgar
is L. (cv. Top Crop), teliospores first appeared in uredinial pustules
produced by the teliospore-forming parent 14-16 days after inoculatio
n, compared with 20-22 days for the F-1 progeny of this cross. Teliosp
ore production relative to urediniospore production was quantified for
the parents, the F-1, and 67 F-2 progeny. At 38 days after inoculatio
n, pustules produced by the teliospore forming parent contained 68% te
liospores and 32% urediniospores, whereas pustules of the F-1 containe
d 33% teliospores and 67% urediniospores. The number of F-2 progeny th
at did not produce teliospores versus those that produced teliospores,
fit a single gene 1:3 ratio. Thirty-five backcross progeny produced f
rom backcrossing the F-1 to the teliospore-producing parent were also
analyzed for the ability to produce teliospores. The backcross progeny
segregated into two groups which fit a 1:1 ratio of intermediate vers
us high teliospore production. This supports the hypothesis that a sin
gle gene segregated for control of teliospore production in this cross
.