The genus Cochliobolus, related genera [Lewia (Alternaria), Pyrenophora, Setosphaeria] and their asexual relatives are a species-rich collection of taxa in the Ascomycetes. Cochliobolus includes both saprobic and pathogenic species that are significant monocot pathogens worldwide; individual species/races attack corn, rice, barley, sugarcane, wheat, and oats, all major cereal crops. As a natural pathogen of corn, Cochliobolus heterostrophus can be found in many tropical and subtropical areas of the world. In the US, the fungus is usually found in the warmer southern states, thus, the disease it causes is commonly known as Southern Corn Leaf Blight (SCLB).
C. heterostrophus and related taxa are notorious for their ability to produce host-selective toxins (HSTs), a group of chemically diverse, low molecular weight compounds that serve as virulence or pathogenicity factors. On two occasions, Cochliobolus species caused devastating losses to US agriculture. In the 1940’s, Cochliobolus victoriae caused widespread destruction of oat varieties containing the recently introduced Pc-2 gene for general crown rust resistance. C. heterostrophus race O was known as a mild pathogen of corn until 1969-70 when a previously unseen, super-pathogenic race (race T) caused the worst epidemic in U.S. agricultural history, which destroyed more than 15% of the U.S. maize crop. Race T was found to differ from race O in that it produces an HST called T-toxin, a family of linear polyketides, while race O does not. The combination of the fungus, producing T-toxin, and corn carrying T-cms, widely planted at that time and discovered to be exquisitely sensitive to race T, resulted in corn crop decimation. Although species of Cochliobolus are not, currently, among the most economically serious diseases, clearly they have proven their ability to cause extraordinary crop losses. As cereal crops mature as a source of biomass for biofuel and bioproduct production, factors that lower crop yield become even more economically important.
The appeal of C. heterostrophus as a model necrotrophic fungus is due in large part to the tractability of the organism for classical and molecular genetic analyses. The sexual stage has rarely, if ever, been observed in the field, but can be induced easily in the laboratory. C. heterostrophus is unsurpassed in terms of genetic tractability (conventional and molecular), undergoing very efficient homologous recombination between transforming DNA and target genomic sequences, which facilitates functional analysis by site-specific gene deletion.
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