Sporisorium reilianum is a basidiomycetous fungus that causes head smut in sorghum and maize. Infection requires the formation of a dikaryon between spores of compatible mating type and leads to a change from yeast-like to hyphal growth within the host plant. This switch is controlled through mating type loci. Among forty four different sorghum isolates of S. reilianum (SRS), including five different pathotypes, only three compatible pairs were detected, leading to the establishment of haploid cultures with three different “a locus” mating types, as verified by mRNA expression. Amino acid sequence comparison showed that the pheromone gene components of the a mating type loci of sorghum isolates (SRS) are highly similar to those of maize isolates (SRZ). Genetic diversity was measured using amplified fragment length polymorphism (AFLP) to compare isolates collected from the same and different hosts. AFLP analysis showed polymorphism both within and between SRS and SRZ isolates, with isolates from each host tending to show greater similarity. However, the study did not reveal patterns that could be associated with pathotype of sorghum isolates. A recent outbreak of sorghum downy mildew in Texas has led to the discovery of both metalaxyl fungicide resistance and a new pathotype, P6, in the causal organism, Peronosclerospora sorghi. To identify sources of resistance against P6, a total of 336 sorghum lines (245 mini-core lines from ICRISAT, India, 67 elite accessions from KSU, Kansas and 24 accessions from Texas) were used in a greenhouse study. Fifty two mini-core and 20 accessions from Kansas and 13 from Texas were recorded with ≤10% infection and characterized as resistant for further confirmation. Out of the 52 resistant mini-core accessions, 20 were photo-insensitive and showed 0 % infection. Eleven out of 20 from Kansas showed 0% infection. To quantify the genetic diversity among resistant accessions a high throughput ABI Prism 3100 DNA sequencing system was used for DNA fingerprinting with 60 Simple Sequence Repeat (SSR) markers representing all 10 sorghum linkage groups. Analysis of SSR patterns showed high diversity among the resistant sorghum accessions that were collected from different geographical regions and include the five defined races of Sorghum bicolor. Accessions that are not closely related were most likely to represent unique sources of genetic resistance to P. sorghi.