Lack of genetic markers has hindered the study of the mating system of
Cronartium ribicola, an exotic forest pathogen infecting natural and
cultivated white pines throughout North America. Isozymes, randomly am
plified polymorphic DNA (RAPDs), and restriction length polymorphisms
(RFLPs) were used to study the mating system of this rust. Heterozygos
ity (outcrossing) in diploid telia was demonstrated by analysis of cul
tures derived from the meiotic products (basidiospores) of individual
telia. Families of basidiospores cultured from single telia were used
to test for Mendelian segregation and for conformance or loci to Hardy
-Weinberg equilibrium. A total of 18 polymorphic loci were identified
with three marker systems. All except for three RAPD loci showed Mende
lian segregation in the single-telium families. To quantify the level
of outcrossing, gene and genotype frequencies were calculated for fami
lies from a single population. Up to 24 families were surveyed with is
ozymes, 14 with RAPDs, and 18 with RFLPs. Except for one isozyme locus
(MPI) in one sample, all 14 loci tested with these families were in H
ardy-Weinberg equilibrium, indicating random mating. Further studies,
with a different sample from the same population, showed all three iso
zyme loci to be in Hardy-Weinberg equilibrium. The three marker system
s were consistent as to the amount of variation detected. Resistance s
election and breeding programs must consider the implications of genet
ic recombination that outcrossing affords the rust.