Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/1998
Publication Date: N/A
Citation: Interpretive Summary: Gray leaf spot is the most serious disease of corn throughout the USA with the potential to cause substantial economic losses in crop productivity. The increase in gray leaf spot severity during the past decade is associated with increases in conservation tillage practices and with monoculture of corn. The disease is caused by a fungus that lives in the debris from the previous crop and infects the leaves, destroying extensive areas of tissue. Disease control measures have been ineffective because most commercial hybrids of corn are susceptible to the disease. The possibility that the fungal pathogen is genetically variable and, therefore, has the potential to become more damaging to new corn hybrids was investigated in this research. Two closely related but distinct sibling species of the fungus Cercospora were identified by molecular fingerprinting methods, although they could not be distinguished by other criteria. One species was found to be distributed throughout the corn producing regions of the US; the other was found only in the eastern third of the country. The populations of each species were found to contain little genetic variation. No evidence was obtained to indicate that there are different races of the fungal pathogen. These findings are important because they indicate that breeding and selecting for resistance to gray leaf spot will not be confounded by local or regional populations of the fungus, and that, once resistant corn hybrids are developed, the resistance should be effective across geographic regions of the corn belt.
Technical Abstract: Isolates of the fungus causing gray leaf spot of maize were obtained from throughout the USA and analyzed for genetic variability by amplified restriction fragment length polymorphism (AFLP). Two substantially different AFLP profiles were generated, and cluster analysis revealed two distinct groups of Cercospora isolates. The groups were separated from each other by a genetic distance of 80%, and both groups were separated from C. sorghi by >67%. Characteristics of conidia and conidiophores were unreliable criteria for taxonomic differentiation of the two groups. Nucleotide sequences of 5.8S rDNA and the ITS regions were identical within each group but different between the two groups and from C. sorghi. RFLPs generated by digestion of 5.8S rDNA and ITS with Taq I readily distinguished each group and C. sorghi. Isolates in one group were generally distributed throughout the US; isolates in the other group were localized in the eastern third of the country. Both types were present in the same fields at some locations. The distinct AFLP profiles, different ITS nucleotide sequences, and the genetic distance between the two groups indicate that they are different species - the most prevalent, widely distributed species, C. zeae-maydis, and a sibling species. Both species are relatively uniform internally with an average genetic similarity of approx. 93 and 94%, respectively, suggesting that sexual recombination generates little genetic variation. The results suggest that the different pathotypes of the fungus does not exist and that resistant maize genotypes will be effective across regions of the US.