To characterize host specialization, we compiled data from four published empirical studies that tested S. hermonthica performance (specifically, “emergence” or the number of above-ground parasites per individual host plant at or near crop maturity) on sorghum, millet, and maize (King and Zummo 1977, Ramaiah 1983, Bebawi et al. 1986, Kim et al. 1994). These data include S. hermonthica populations from 27 locations spanning semiarid regions of East and West Africa between the equator and 14º N (Fig. 2, 3). S. hermonthica populations from empirical studies were classified into regional groupings based on their location of origin coordinates. Populations from localities with longitudinal coordinates less than 0º were considered “Western”, greater than 15º E were considered “Eastern”, and greater than 0ºW but less than 15ºE were considered “Central”. Studies of these populations include both pot experiments and field trials (Table S1) and to our knowledge, represent all studies that have quantified emergence of S. hermonthica from geographically distinct populations on multiple genotypes of multiple host species. To account for differences in inoculation pressure and experimental design between studies, for a given S. hermonthica population we considered the mean relative emergence on host species k as
\[Mean\ Relative\ Emergence_k=1/n\sum_{j=1}^nE_j/E_{\max}\]
where Ej is the average emergence on host genotype j, Emax is the maximum emergence of the parasite population observed on any host genotype of any host species tested at the same location, and n is the number of genotypes tested of host species k. Mean relative emergence ranges from 0 to 1 where, for example, a mean relative emergence of 0.5 on maize would indicate that for a given parasite population, on average half of the individuals emerge on genotypes of maize compared to the maximum emergence in an experiment. Similarly, a mean relative emergence close to 1 on sorghum would indicate that a parasite population specializes on sorghum.