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Restriction fragment length polymorphisms (RFLPs) were used to characterize the genetic structures of three field populations of Phaeosphaeria nodorum from Texas, Oregon, and Switzerland. Data from seven nuclear RFLP loci were used to estimate gene diversity and genetic distances and to make indirect measures of gene flow between populations. Three of the seven RFLP loci differed significantly in allele frequencies across populations. On average, 96% of the total gene diversity was found within populations. There was little evidence for population subdivision, suggesting that gene flow was not restricted among populations. Based on an average population differentiation of 0.04, we estimated that the exchange of 11 migrants among populations per generation would be needed to account for the present level of population subdivision. Genotype diversity based on DNA fingerprints was at a maximum for the Swiss population, whereas populations in Texas and Oregon had lower genotype diversities. Many multilocus haplotypes were found in each population. Ninety-five percent of RFLP allele pairs were in gametic equilibrium. The data were consistent with random mating within each population.

Restriction fragment length polymorphisms (RFLPs) were used to study the population genetics of Colletotrichum graminicola (= C. sublineolum), the causal agent of sorghum anthracnose. Screening of 80 anonymous probes from a genomic library detected polymorphisms in 81% of 299 probe-enzyme combinations among nine international isolates. Seven single- or low-copy probes were used to study a collection of 411 isolates sampled during 1991 to 1993 from a sorghum disease nursery in Georgia. Nei's gene diversity was moderately high, with = 0.215 on average, while genotypic diversity was extremely low with an average genotypic diversity value of G = 1.513. Only nine multilocus haplotypes were identified, with one haplotype being present at a frequency of approximately 80% each year. Two other haplotypes were found at significant frequencies (4 to 10%). Allele and haplotype frequencies did not differ over the 3 years, indicating that this population was stable. Our findings suggest that genetic drift and gene flow were not major contributors to genetic structure, while asexual reproduction had a significant effect.