Author
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Dunkle, Larry |
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LEVY, MORRIS - PURDUE UNIVERSITY |
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Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 2/2/2000 Publication Date: N/A Citation: N/A Interpretive Summary: Interpretive Summary: Gray leaf spot 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 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 Cercospora zeae-maydis, 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. In our previous research, two closely related but distinct sibling species of the pathogen were identified by molecular fingerprinting methods, although they could not be distinguished by morphological 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. In the present research, isolates of the gray leaf spot fungus from Africa were found to share molecular and morphological characteristics with the eastern U.S population. Isolates in these two groups grow more slowly and do not produce a toxin known as cercosporin. However, they are as virulent and destructive as the faster growing, cercosporin-producing strains. The results of this research will impact genetic engineering approaches aimed at incorporating resistance to cercosporin as the mechanism protecting corn against gray leaf spot and are important in defining the genetic composition of the gray leaf spot pathogen so that effective control strategies can be devised. Technical Abstract: Two taxonomically identical but genetically distinct sibling species, designated Group I and II, of Cercospora zeae-maydis cause gray leaf spot of maize in the United States. Isolates of the gray leaf spot pathogen from Zimbabwe and Uganda were compared with isolates from the US by amplified fragment length polymorphism (AFLP) analysis and restriction digests of internal transcribed spacer (ITS) regions and 5.8S ribosomal DNA (rDNA) and by their morphological and cultural characteristics. The isolates from Africa were taxonomically indistinguishable from the US isolates in both groups, but, like isolates of Group II, they grew more slowly and failed to produce detectable amounts of cercosporin on a variety of media. Analysis of restriction sites in the ITS/rDNA regions indicated that the African population is identical to the C. zeae-maydis Group II population that occurs in the eastern US and distinct from the more prevalent Group I population that is generally distributed within the US and in other parts of the world. Cluster analysis of 85 AFLP loci confirmed the genetic relatedness of the African and eastern US populations and revealed limited variability within the two populations. Although gray leaf spot was reported in the US several decades prior to the first record in Africa, the chronology of occurrence of C. zeae-maydis Group II in Africa and the US could not be ascertained because neither population contained significantly greater genetic variation and because the populations were genetically indistinct. |
