USDA/ARS Adjunct Professor of Plant Pathology

Purdue University
Botany and Plant Pathology, Lilly Hall
915 West State Street
West Lafayette, IN 47907-2054

Office:  LILY 1-339
Phone:  (765) 494-6076
FAX:      (765) 496-3452
E-mail:   

Area: Host-Pathogen Interactions - Mechanisms of pathogenicity in fungal pathogens of corn and sorghum; Mechanisms of resistance to fungal pathogens of corn

Education

Ph.D., University of Wisconsin, Botany

Dunkle has published over 100 articles, including research papers, book chapters, and conference proceedings. He has participated in numerous national and international symposia on topics dealing with biochemical and molecular aspects of plant pathogenesis. Dunkle is a Fellow of the American Phytopathological Society and is Research Leader of the USDA-ARS Crop Production and Pest Control Research Unit.

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Research Interests

Our research involves studies on the molecular and biochemical mechanisms of fungal pathogenicity and virulence and of plant disease resistance. The ultimate goal of the research is to obtain information that will expand approaches for controlling fungal diseases of corn and sorghum and lead to new and improved disease control strategies.

Research in the Corn and Sorghum Pathology Laboratory emphasizes gray leaf spot of corn, caused by Cercospora zeae-maydis, and northern leaf blight, caused by Setosphaeria turcica. During the past three decades, gray leaf spot has increased in incidence and severity and has become the major foliar disease of corn worldwide. Substantial yield losses occur under favorable environmental conditions and particularly with no-till or conservation tillage practices. Very little is known about the mechanisms of resistance in corn or about the mechanisms of virulence in the pathogen. A thorough understanding of the molecular basis for host-pathogen interactions will be important to achieve effective and durable control of gray leaf spot. Northern leaf blight remains a persistent disease problem throughout the world. Studies of this host-pathogen system afford the opportunity to explore aspects of gene-for-gene interactions in a cereal crop in which single genes in the corn host confer resistance to single specific races of the pathogen.

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Current Research Projects

Role of cercosporin in gray leaf spot of corn. Gray leaf spot is caused by two genetically and taxonomically distinct fungi, Cercospora zeae-maydis and C. zeina. Like many species of Cercospora, C. zeae-maydis produces cercosporin, a light-induced, photoactivated perylenequinone that is toxic to a diverse array of organisms. C. zeina elicits identical disease symptoms but does not produce cercosporin in culture, suggesting that cercosporin is not essential for pathogenicity or virulence. To address the role of cercosporin in pathogenesis, we initiated a project to identify, characterize, and disrupt genes involved in cercosporin biosynthesis. Analysis of a cDNA subtraction (SSH) library revealed transcripts that are up-regulated during cercosporin synthesis. We focused on sequences with high homology to genes involved in signal transduction. Northern analyses of gene expression in cercosporin-inducing and cercosporin-suppressing media implicated the involvement of a MAP kinase kinase kinase (designated CZK3). Targeted gene disruption of CZK3 generated a mutant that is unable to produce cercosporin and does not produce conidia but is unaffected in vegetative growth. Inoculation of corn leaves with mycelia of the czk3 mutant resulted in small chlorotic lesions with minimal colonization by intercellular hyphae following penetration through stomata. The results indicate that CZK3 regulates functions that are important for pathogenesis and suggest that cercosporin is required only after the early stages of infection for extensive colonization and lesion expansion. Current efforts are directed toward understanding the role of cercosporin by analyzing mutants affected in their ability to perceive or transmit light signals and other environmental cues that influence cercosporin synthesis.

Colony and lesions produced by wild type (L) and czk3 mutant (R) of Cercospora zeae-maydis	Conidia and conidiophores of the gray leaf spot fungus, Cercospora zeae-maydis

Light-regulation of conidiation and pathogenesis in Cercospora zeae-maydis and Setosphaeria turcica.  Conidia of the pathogens causing gray leaf spot and northern leaf blight are the primary inoculum, inciting leaf lesions and initiating the disease epidemic, and are the source of repeated infections via wind dispersal to other plants and fields. Sporulation in these two pathogens is inhibited by growth in constant light and regulated by nutritional factors. We are investigating the physical and nutritional factors as well as the biosynthetic activities that are essential for conidial development and pathogenesis in these fungi. Only wavelengths in the blue light range repress conidiation, which contrasts with numerous other fungal species in which blue light induces or substantially enhances conidiation. In a project led by ARS Research Associate, Dr. Burt Bluhm, our primary objective is to characterize the gene(s) encoding blue-light photoreceptors and identify light-regulated genes that influence conidiation and biosynthesis of cercosporin in order to discover potential targets for disease control. Preliminary results have indicated that mutants of C. zeae-maydis disrupted in the CWC1 gene encoding white collar-1, a putative blue light receptor in fungi, are light blind in conidiation and cercosporin synthesis but nevertheless are unable to cause disease symptoms beyond small chlorotic spots at the site of penetration.

Conidiation by Setosphaeria turcica: Conidiophores produced during incubation in constant light (L) and conidia produced during incubation in darkness (R).

Molecular analysis of resistance of corn to Setosphaeria turcica. The host-pathogen interaction in northern leaf blight is controlled by single dominant genes in the corn host that condition resistance to specific races of the pathogen. Thus, it presents a rare opportunity to study gene-for-gene interactions in corn and critically evaluate the involvement of defense mechanisms established in model plant species. The approach involves microarray analyses of genes expressed during stages of disease development in near-isogenic lines of corn differing in the presence of a specific gene for resistance. In addition, this genetically defined pathosystem can be exploited to determine the mechanism of virulence in the pathogen. Our project evaluates the potential involvement of phytotoxins with genotype specificity or, alternatively, of race-specific avirulence factors, e.g., extracellular peptides, that interact directly or indirectly with the individual resistance gene products to ascertain the phenotypic consequences of the host-pathogen interaction. ARS Postdoctoral Research Associate, Dr. Burt Bluhm, is leading this project.

Resistant (top) and susceptible (bottom) disease reactions of maize to Setosphaeria turcica race 0.

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Assistantships and Positions

Please contact me directly for information on assistantships and openings in my program. Follow these links for general information on graduate programs or employment announcements.

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Selected Publications

Brunelli, K.R., C. Athahyde Sobrinho, A.C. Fazza, L.D. Dunkle, and L.E. Camargo. 2008. Molecular variability in the maize gray leaf spot pathogen in Brazil. Genet. Mol. Biol. (in press)

Bluhm, B.H., X. Zhao, J.E. Flaherty, J.R. Xu, and L.D. Dunkle. 2007. RAS2 regulates growth and pathogenesis in Fusarium graminearum. Mol. Plant-Microbe Interact. 20: 627-636.

Nagy, E.D., T.C Lee, W. Ramakrishna, Z. Xu, P.E. Klein, P. SanMiguel, C.P. Cheng, J. Li, K.M. Devos, K. Schertz, L. Dunkle, and J.L. Bennetzen. 2007. Fine mapping of the Pc locus of Sorghum bicolor, a gene controlling the reaction to a fungal pathogen and its host-selective toxin. Theor. Appl. Genet. 114: 961-970.

Flaherty, J.E., and L.D. Dunkle. 2005. Identification and expression analysis of regulatory genes induced during conidiation in Exserohilum turcicum. Fungal Genet. Biol. 42: 471-481.

Shim, W.-B., and L.D. Dunkle. 2005. Malazy, a degenerate, species-specific transposable element in Cercospora zeae-maydis. Mycologia 97: 349-355.

Shim, W.-B., and L.D. Dunkle. 2003. CZK3, a MAP kinase kinase kinase homolog in Cercospora zeae-maydis, regulates cercosporin biosynthesis, fungal development, and pathogenesis. Mol. Plant-Microbe Interact. 16:760-768.

Lapaire, C.L., and L.D. Dunkle. 2003. Microcycle conidiation in Cercospora zeae-maydis. Phytopathology 93:193-199.

Shim, W.-B., and L.D. Dunkle. 2002. Identification of genes expressed during cercosporin synthesis in Cercospora zeae-maydis. Physiol. Mol. Plant Pathol. 61:237-248.

Wolpert. T.J., L.D. Dunkle, and L.M. Ciuffetti. 2002. Host-selective toxins and avirulence determinants: What’s in a name? Annu. Rev. Phytopathol. 40:251-285.

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Web Links of Interest

• USDA-ARS, West Lafayette, Indiana, Location
• Crop Production and Pest Control Research
• Corn and Sorghum Pathology Laboratory - Dunkle Lab