Race t of Helminthosporiummaydis(Bipolaris maydis) (Cochliobolus heterostrophus) spread across most corn growing areas in USA and elsewhere in 1970. The traditional version, race 0, of this pathogen was common in the Southeastern USA where temperatures and humidity favored the biology of the fungus. A related fungus Helminthosporium carbonum(Bipolariszeicola) (Cochliobolus carbonum) was a common pathogen of corn but tended to be more frequent in the northern part of the US corn belt. The summer of 1970 not only featured epidemics of race T of H. maydisbut spread of this pathogen to much of northern corn belt. This allowed the co-mingling of the two species.
The two species were distinguished by microscopic examination of their conidia, the asexually produced spores associated with spread of these fungi. H. maydisspores were consistently curved and appeared to be gray when viewed with a light microscope. H. carbonumconidia were darker in pigment and mostly straight. Both species had shown to have similar sexual reproduction structures and to have distinct sexual mating types.
Seed companies, including the one that I had just joined, were checking their inbreds and hybrids in the summer of 1972 to make sure there was no remnant susceptibility left among their materials. I was surprised to find a wide range of shapes and sizes of lesions naturally occurring among materials that looked like southern corn leaf blight in our central Illinois nursery. Examining the spores under microscope showed a range of spore shapes intermediate to H. maydisand H. carbonum. Other pathologists found the same thing. It had been shown previously that these two species could cross in lab experiments and now it appeared that the wide spread distribution of H. maydisinto regions where H. carbonumwas common allowed multiple opportunities for sexual crosses between the two. This apparently accounted for the range of conidia morphology seen in the summer of 1972.
The resulting population of these multiple crosses further sorted in virulence on corn. H. carbonumalready had been found, with one race (race 1) to produce a toxin affecting a few inbreds homozygous recessive to the toxin. Another group of this pathogen appeared to mildly pathogenic on corn leaves was defined as Race 2. After 1972, some inbreds and hybrids were found to be susceptible to Race 3, resulting in distinctive long, narrow lesions. In 1980, inbreds with B73 backgrounds, commonly used as female parents in seed production fields, were infected with a distinct race 4 of H. carbonum. Apparently, the crosses of the two related fungal species resulted in new genetic combinations.
This was the second experience for this novice corn pathologist of the interactive dynamics of pathogen and corn genetics. These dynamics continue.
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.