Virulence and Molecular Genotyping Studies of Sporisorium reilianum Isolates in Sorghum

Head smut, caused by the fungal pathogen Sporisorium reilianum, has been reported with increasing frequency in the grain sorghum growing areas of Texas. To facilitate analysis of changes in pathogen virulence, four inoculation techniques were examined: soil and teliospore mixture, seed coating, media placement, and syringe injection. Of the four, syringe injection was determined to be the most effective. Inoculations of sorghum host differentials BTx643, BTx7078, BTx635, SC170-6-17 (TAM2571), SA281 (Early Hegari), and Tx414 showed 23 of 32 Texas isolates were race 4. Two isolates from College Station, TX, were classified as race 1, but no race 2 or 3 isolates were found. New, virulent races 5 and 6 were identified among isolates from south Texas. Using 16 amplified fragment length polymorphism (AFLP) primer combinations, genetic diversity was assessed in DNA samples from 49 S. reilianum isolates, including 44 sorghum isolates from Texas, two from Uganda, and one from Mali; and two maize isolates from Mexico. Single-base extensions with EcoRI and MseI primers in the selective amplification increased the number of informative polymorphic bands. High genetic dissimilarity (50%) was observed between isolates originating from maize and those originating from sorghum. The resultant dendrogram, made using cluster analysis, grouped the Texas S. reilianum isolates into four small clusters with ≥82% similarity. Other than for two race 6 isolates from Weslaco, TX, no evidence for geographical or other restrictions on gene flow was evident.