Humans attempt to define each others race by a few recognizable physical characters that in reality represent, at most, 0.0001 of the total DNA of any person and that varies greatly among people from any one geographical or ethnic origin. And those physical features have nothing to do with any individual’s behavior.
Plant pathologists have even a greater problem in trying to characterize the genetic variation with a corn pathogen species. Not only do fungi offer few morphological characters to use for defining individuals, and those characters are mostly only observable with a microscope, the numbers of individuals are huge. It becomes very difficult to take samples in order to estimate the range of characteristics within a species. Usually the term race of a pathogen is defined as it’s ability to overcome a specific gene for resistance in the host.
Helminthosporium turcicum (sorry, I can’t shake the habit) was successfully controlled by a single gene in corn that was called the Ht gene. That single corn gene, first discovered in a popcorn variety, inhibited the fungus from plugging the vascular tissue in a leaf and kept the fungus from producing spores to spread elsewhere in the field. It was widely used in the 1970’s by most corn breeders as it appeared to be effective everywhere in controlling northern corn leaf blight. Then, in 1979, it was discovered that a seed field in Indiana with the Ht gene was heavily damaged by the disease. I had the same experience as other corn pathologists at that time to discover that the newly discovered variant of the fungus could be found in many midwestern fields. So now the original population of the fungus controlled by the Ht gene was called race 0 and the new one as race 1. Race 1 had a gene mutation that allowed it to overcome the resistant gene in the host. Since then at least 4 more single gene resistance sources for resistance to this pathogen have been found and likewise every combination of fungal genes to overcome these genes have been found in the fungal population have been identified. These are all given distinct racial identities. But just as with the limitation of human racial descriptions, these designations depict only a small, limited description of the variability among individuals of the population. With the outbreaks of northern leaf blight in much of the USA Midwestern corn belt in 2015, I was asked if it was a new race. Probably not, but most likely a population of the fungus favored by many environmental factors and many genes.
Visit us at the ASTA in Chicago, Dec 9-12 (booth G207)
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.