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Current Research

Ecological significance of aflatoxin production
One of the main research goals in my lab is to understand the ecological significance of aflatoxin production in the ecology and life history of A. flavus. Aflatoxin is a highly carcinogenic toxin produced by fungi in the genus Aspergillus and causes contamination of food supplies, especially in tropical countries. Strains of A. flavus that do not produce aflatoxin (nontoxigenic strains) are fairly common in nature, strongly suggesting that this polymorphism is maintained by balancing selection, i.e., that aflatoxin production may be favored in some environments but selected against in others. Our goal is to understand how and/or when aflatoxin production is favored. We are approaching this question by comparing the fitness of toxigenic and nontoxigenic strains under experimental conditions, in particular, in soil and maize-kernel microcosms.

Population biology of Verticillium dahliae
The second major project, on the population biology of Verticillium dahliae, is being conducted in collaboration with colleagues in Spain, Israel and Penn State University. We recently used genotyping by sequencing (GBS) to describe the population structure of V. dahliae in a geographically diverse collection. As in previous studies with other genetic markers, we found that the V. dahliae population comprises a handful of clonal lineages. However, because of the large numbers of single-nucleotide polymorphisms (SNPs) generated by GBS we could determine that clonal lineages arose by recombination (see figure on sidebar), probably by sexual reproduction even though V. dahliae is thought to be an asexual fungus. We are currently developing a SNP-genotyping diagnostic method that can uniquely identify the clonal lineage of each V. dahliae isolate. The aim is to apply this method to studies on adaptation of V. dahliae to different host plants, and the effects of crop rotation on the diversity of lineages in soil populations of V. dahliae.

Previous research projects:

Population biology of grape powdery mildew
Until recently, our lab worked on the population biology of the grape powdery mildew fungus, Erysiphe necator (syn. Uncinula necator). Research was focused on genotypic and phenotypic variation of E. necator in the eastern U.S., where this fungus is thought to be native.  By developing microsatellite and mating-type markers we addressed several questions about the phylogeography and population structure of E. necator in North America. We collaborated with Dr. Lance Cadle-Davidson at the USDA-ARS Grape Genetics Research Unit in Geneva, NY, on a transcriptome sequencing project, and with Dr. Wayne Wilcox, Plant Pathology and Plant-Microbe Biology Section, Cornell University, Geneva, NY on mechanisms of resistance to demethylation-inhibiting fungicides (DMIs) in E. necator.
Publications on grape powdery mildew

Biology and genetics of the chestnut blight fungus, Cryphonectria parasitica
The main research focus of our lab from the early 1990s until 2005 was the genetic and biological constraints on biological control of chestnut blight using fungal viruses, in a phenomenon known as hypovirulence. Some viruses found in C. parasitica have been responsible for the biological control and almost complete recovery of chestnut trees from blight in southern Europe and a few places in North America. Our research addressed questions about the spread of viruses—or the lack of spread in the U.S.—for biological control.  We conducted extensive research in three main areas relating to this question:
1) The mating system of C. parasitica, because recombination generates greater diversity of vegetative compatibility types, and hence restricts virus spread.  We have studied the mating system both in the field and in laboratory, and we cloned mating-type genes for use as molecular markers; 
2) Population genetics of C. parasitica
3) Genetics and population genetics of vegetative incompatibility, which restricts the transmission of viruses from one fungal individual to another; 
4) Transmission and population genetics of viruses in C. parasitica responsible for hypovirulence.
 Publications on Cryphonectria parasitica and hypoviruses


Maize weevil on corn kernel. Click here to see short video

Aflavus sclerotium damaged

Sclerotium of Aspergillus flavus damaged by fungivorous invertebrates after six months in natural field soil. (Photo by Mickey Drott)



Verticillium dahliae

Neighbor-net network of clonal lineages of Verticillium dahliae based on SNPs generated by genotyping by sequencing. The extensive closed loops are indicative of recombination. (Published in PLoS One 9e106740)