Corn breeders strive to produce inbreds that are homozygous, mostly to allow repeatable increase of the parents of hybrids and therefore making the identical hybrid plants each season. Inbreeding causes both sets of each gene to be identical, whether it is a recessive or a dominant gene, ultimately causing the inbred parents of a hybrid to be much smaller than the hybrid. Successful hybrids have favorable genes in one parent covering up the unfavorable ones of the other parent. This is not only true of major agronomic characters but often susceptibility in one parent to a pathogen is modified in the hybrid by the resistance in the other parent.
Seed producers occasionally have problems in seed fields related to extreme susceptibility of one parent that can be severely damaging. A common pathogen of corn and other grasses is the species Bipolaris zeicola (formerly known as Helminthosporium carbonum). A variant of this fungus, known as race 1, produces a toxin that overcomes other resistance factors that corn produces to limit the pathogen. This toxin is destroyed by corn plant cells that have the dominant version of the Hm1 gene, because this gene results in production of the right enzyme to do the job. The problem comes when an inbred is homozygous recessive for the hm1 gene.
Such a case was found in seed fields in South Dakota, Illinois and Ohio in 2014 when a susceptible inbred was exposed to the race 1 of this fungus. How did this inbred get through the system to the point it was in seed fields? Traditional breeding methods in which an original segregating breeding population is selfed, then grown ear to row, from which individual selections are selfed again until 7-8 generations later the inbred was considered homozygous also meant that each generation it was grown, and selections were made. I recall seeing one row of one of those selections heavily diseased by this pathogen as it and its sisters were being evaluated in a nursery in Nebraska 25 years ago. That system was slow in developing new inbreds but it did allow for selections such as against normally minor traits, such as caused by hm1/hm1. Haploid-dihaploid breeding has many advantages in quickly producing new inbreds but it is possible that those with the rare homozygous recessive gene for susceptibility to race 1 of Bipolaris zeicola would not be caught until the wider exposure in the seed fields. Widespread testing of hybrids made from the susceptible parent would not show the problem because it is likely the other parent would have the dominant gene producing the anti-toxin enzyme. The distribution of race 1 apparently is widespread, probably maintained on grasses and only revealed when the susceptible inbred is exposed to it.
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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.