The population genetics of Colletotrichum graminicola from different ecosystems of Sorghum

Neutral genetic markers (nRFLPs) were developed and employed to characterize several populations of Colletotrichum graminicola isolated from sorghum (Sorghum bicolor) and johnsongrass (Sorghum halepense) to elucidate the population genetic structure of this plant-pathogenic fungus.;Diseased plant material was sampled from several agricultural sites (research stations, commercial fields, subsistence farmers fields). The johnsongrass isolates were derived from three locations in Texas. Geographically the focus was mainly on the Americas (U.S.A., Honduras), but a population of C. graminicola from Zambia was examined also. One population of C. graminicola from the experimental sorghum nursery at Griffin, Georgia was sampled for three consecutive years. In total, data was gathered from 1278 single-spored isolates.;Findings using seven, assumed neutral, polymorphic nRFLP markers include: (1) The population genetic structure of all populations of C. graminicola examined was typical of clonal reproduction. (2) All populations of C. graminicola examined were dominated by a one or a few haplotypes. (3) One specific haplotype (haplotype "A") was found at high frequencies in populations from grain sorghum from Georgia, Honduras and Zambia. This was taken as evidence for long-distance dispersal. (5) The time study showed little genetic variation over years. (6) Samples collected from grain sorghum 250 km along the Texas Gulf Coast were dominated by a particular haplotype (haplotype "TB"). This haplotype was not found in other areas examined. This suggests dispersal on a regional scale and also an adapted ecotype. (7) Sorghum and johnsongrass isolates from the upper coastal plains in Texas were found to be genetically isolated. Therefore, it was concluded that isolates from johnsongrass do not provide the initial inoculum for the sorghum crop. The three spatially separated populations from johnsongrass shared a specific haplotype ("JA"), indicating long-distance dispersal in isolates from a natural host. (8) Isolates of C. graminicola collected from maicillos (landrace cultivars) in Honduras were found to be genetically more closely related to isolates from johnsongrass, than to grain sorghum isolates. This indicates that genetic relationships between fungal isolates may not be easily predicted from the genetic relationship between hosts.