Regional population diversity of Pyricularia grisea in Arkansas and the influence of host selection

MGR586 DNA fingerprinting has been widely used to characterize population d iversity of the rice blast pathogen, Pyricularia grisea. However, the frequ ency and distribution of particular-haplotypes (individuals) within MGR-del imited lineages has not been examined in the United States. MGR586 DNA fing erprinting, mitochondrial DNA (mtDNA) restriction fragment length polymorph ism (RFLPs), and virulence phenotyping were used to examine genetic diversi ty of P. grisea in Arkansas. A total of 470 monoconidial isolates were reco vered from eight rice cultivars in 18 commercial fields in nine counties in Arkansas. All isolates were examined for nuclear DNA RFLPs with the MGR586 DNA fingerprint probe, and both the MGR lineage (isolates with >80% simila rity) and the haplotype frequencies were determined. Four distinct MGR586 D NA fingerprint lineages (designated A, B, C, and D) were identified among t he 470 field isolates. All four lineages were found in 9 of the 18 location s. Three lineages were found in four locations, two lineages in three locat ions, and only a single lineage was found at two locations. In all, 10, 19, 16, and 13 haplotypes (isolates which had MGR586 DNA fingerprints which di ffered by 1 to 20%) were identified within lineages A, B, C, and D, respect ively, among the 470 isolates examined. Within each lineage, a single haplo type (clone) predominated, representing 51 to 71% of the isolates collected for each of the four lineages. Overall, 60% of the 470 isolates belonged t o one of only four haplotypes (A1, B1, C1, and D1) and these four predomina nt haplotypes were recovered from between 7 and 14 of the 18 locations samp led, indicating a widespread distribution of these four clones. These data indicate an exceptionally low level of genetic diversity in the regional ri ce blast pathogen population in Arkansas relative to several other populati ons of I! grisea examined from tropical environments. In addition, no mtDNA RFLPs were detected among representative haplotypes within each of the lin eages, indicating a single mtDNA haplotype was present in the population. E xamination of virulence indicated that two races predominated in the region al collection. All 30 isolates in lineages A and C tested had an IB-49 viru lence phenotype. Out of 30 isolates in lineages B and D, 29 had an IC-17 vi rulence phenotype. One isolate in lineage B, isolated from a highly suscept ible cultivar (L201), had an IG-1 virulence phenotype. The frequencies of t he four lineages varied among the locations sampled and may have been due, in part, to the cultivar from which isolates were recovered. A single linea ge was recovered from two cultivars, Mars and Millie. Although only a singl e field of each of these cultivars was sampled, the data indicate that cert ain cultivars grown in Arkansas may serve as a "bottleneck", selecting out specific lineages in the regional population. To test this hypothesis, an a dditional 283 isolates were recovered from replicated plots of cvs. M204 an d Mars located within commercial rice fields at two locations during two se asons. All four MGR586 lineages were recovered from each location. However, there was a strong bias for lineage B on cv. M204 (79% of all isolates) an d a strong bias for lineage A on cv. Mars (95% of all isolates), indicating some cultivars were effective in excluding certain lineages.