‘New’ corn diseases

Resistance to most potential corn diseases involves at least 3-4 genes beyond structural resistance to pathogen invasion. These genes are activated after the invading organism is detected.  Often, it seems, one of the genes has a more major affect then the others.  At least, what has been observed that some inbreds, being homozygous, appear to be extremely susceptible to a disease whereas most inbreds are not and that this trait is recessive. That means it is largely modified when the other parent of a hybrid is more resistant.  In these cases, corn breeders will witness only a few inbreds among the hundreds in a corn nursery that show the disease.  I cited in the last post, the case of Race 1 of Bipolaris zeicola recently showing up on seed production fields in one inbred.  I saw that same race in breeding nursery 25 years ago in one of many closely- related, new inbreds.  Stewart’s bacterial blight has a similar history of extreme susceptibility in a few popular inbreds such as N28 and A632 during the 70’s, and Goss’s wilt in inbred A632 during the 70’s.  In each case, these diseases were less damaging in hybrids because the dominant version of the gene, often from the other hybrid parent, had modified the susceptibility.
 
The immense genetic diversity available to corn breeders and the large number of genes in corn, includes the possibility of inadvertently selecting recessive versions of genes that interfere with normal resistance to less common potential pathogens.  If the new inbreds are not immediately exposed to these organisms the vulnerability is not known. Wider exposure in seed production fields often are the first opportunity to find occurrence of new diseases, although common practice of fungicide spraying reduces fungal infection.  New diseases will show up in hybrid fields and it is important that observers be alert and report back to the seed company.  Genetic diversity among corn parent seed is great and seasonal changes are available but some lead time to make the changes is needed.  Corn has genetic diversity and so do it’s many potential pathogens.