Submitted to: Corn and Sorghum Seed Research Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: December 11, 1998
Publication Date: N/A
Interpretive Summary: Gray leaf spot (GLS) is the most serious disease of corn throughout the USA and other parts of the world with the potential to cause substantial economic losses in yield. The increase in GLS severity during the past decade is associated with increases in conservation tillage practices and 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. Two closely related but genetically distinct groups of the fungus Cercospora were identified by molecular fingerprinting methods, although they could not be distinguished by standard taxonomic criteria. One group is distributed throughout the corn-producing regions of the US; the other occurs only in the eastern part of the country. We evaluated the ability of isolates from each group to cause disease by inoculating corn in field plots. Although each group was internally variable in aggressiveness on corn, no evidence was obtained to indicate that there are different races of the fungus. Although more testing is required to evaluate a range of corn germplasm for disease reaction to each pathogen group, these findings are important because they indicate that breeding and selecting for resistance to GLS will not be confounded by local or regional populations of the fungus.
Technical Abstract: The fungus causing gray leaf spot of was analyzed for genetic variability by a variety of molecular fingerprinting methods. Two substantially distinct groups of Cercospora isolates were separated from each other by a genetic distance of approximately 80%, and both groups were separated from the sorghum pathogen C. sorghi by >67%. Characteristics and dimensions of conidia and conidiophores were unreliable criteria for taxonomic differentiation of isolates comprising the two groups. Nucleotide sequences of 5.8S rDNA and the ITS regions confirmed the genetic separation established by other analyses. Isolates in one group were generally distributed throughout maize-producing regions of the U.S.; 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 molecular data indicate that the two groups comprise sibling species of the same taxon. Field inoculations of corn hybrids displaying a range of disease reactions indicated that isolates in each group vary in their aggressiveness but that neither group is more or less damaging to specific hybrids than the other. The results suggest that the potential for the presence or emergence of different pathotypes or races of the fungus is negligible. Although more trials with a broad range of maize germplasm are necessary, the present results suggest that resistant maize genotypes will be effective across geographic regions of the U.S.