Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2001
Publication Date: N/A
Citation: Interpretive Summary: Gray leaf spot is a serious disease of corn in the U.S. and other parts of the world. The development of corn hybrids with built in resistance to the disease is the preferred means of control. One problem encountered by corn breeders attempting to select for gray leaf spot resistance is the inconsistency in the reactions of hybrids from one location to the next. In our study, we took strains of the pathogen that causes gray leaf spot from four different locations in North Carolina where we screen hybrids for resistance and tested them side by side at a neutral location that doesn't have a history of the disease. We found that hybrids reacted to strains from those test locations similarly to how they reacted at the actual test sites. The most aggressive strains were most efficient at allowing us to discriminate among levels of resistance of the hybrids. Although strains varied greatly in their aggressiveness, we found no evidence that these strains represented distinct races of the pathogen. Hybrids that expresse resistance to one strain were resistant to all strains we tested. Our study shows that much of the inconsistency encountered in gray leaf spot trails can be eliminated if breeders and plant pathologists use the most aggressive strains available to screen hybrids for resistance.
Technical Abstract: The use of genetically resistant hybrids is the preferred means of control of gray leaf spot, caused by Cercospora zeae-maydis. One problem faced by maize breeders attempting to breed for resistance to gray leaf spot is the high degree of genotype x environment interactions seen in disease trials. In North Carolina gray leaf spot trials conducted at four locations in the western part of the state, we found consistent hybrid x location effects over the 1995 and 1996 growing seasons. Isolates of C. zeae-maydis from those test locations were evaluated on the same hybrids used in the multi- location testing at a neutral location in central North Carolina that does not have a history of gray leaf spot. The hybrid x isolate interactions seen in the isolate trial reflected that hybrid x location effects seen in the multi-location testing. Most of the interactions appear to arise from differences in the magnitude of differences between hybrids when inoculated dwith the different isolates rather than from any change in hybrid ranking. Analysis of ITS-RFLPs and mt rDNA RFLPs of those isolates and others revealed both Type I and Type II sibling species of C. zeae-maydis, as well as C. sorghi var. maydis are isolated from typical gray leaf spot lesions. Breeders should use the most aggressive isolates of the C. zeae-maydis to maximize discrimination between genotypes in gray leaf spot trials.