Pyricularia grisea causing gray leaf spot of perennial ryegrass turf: Population structure and host specificity

Gray leaf spot is a serious disease of perennial ryegrass (Lolium perenne) turf in the United States. Isolates of Pyricularia grisea causing the disea se in perennial ryegrass were characterized using molecular markers and pat hogenicity assays on various gramineous hosts. Genetic relationships among perennial ryegrass isolates were determined using different types of transp osons as probes. Phylogenetic analysis using Pot2 and MGR586 probes, analyz ed with AMOVA (analysis of molecular variance), showed that these isolates from perennial ryegrass consist of three closely related lineages. All the isolates belonged to a single mating type, MAT1-2. Among 20 isolates from 1 6 host species other than perennial ryegrass, only the isolates from wheal (Triticum aestivum) and triticale (x Triticosecale), showed notable similar ity to the perennial ryegrass isolates based on their Pot:! fingerprints. T he copy number and fingerprints of Pod! and MGR586 in isolates of P. grisea from perennial ryegrass indicate that they are genetically distinct from t he isolates derived from rice (Oryza sativa) in the United States. The pere nnial ryegrass isolates also had the same sequence in the internal transcri bed spacer (ITS) region of the genes encoding ribosomal RNA as that of the wheat and triticale isolates, and exhibited rice isolate sequence polymorph isms. In pathogenicity assays, all the isolates of P. grisea from Legacy II perennial ryegrass caused characteristic blast symptoms on Marilee soft wh ite winter wheat, Bennett hard red winter wheat, Era soft white spring whea l, and Presto triticale, and they were highly virulent on these hosts. An i solate from wheat and one from triticale (from Brazil) were also highly vir ulent on perennial ryegrass and Rebel III tall fescue (Festuca arundinacea) . None of the isolates from perennial ryegrass caused the disease on Lagrue rice, and vice versa. Understanding the population structure of P. grisen isolates infecting perennial ryegrass and their relatedness to isolates fro m other gramineous hosts may aid in identifying alternate hosts for this pa thogen.