Corn genetic diversity

One study of a single corn inbred (B73) indicated that it had 30,000 genes.  We benefit from a species with a huge genetic potential and a pollination system that encourages new mixes of genetics.  This has allowed the species to be used for food in a wide range of environments.  Some of those genes are turned on in response to the many microbes searching for the products of photosynthesis for their nutrition.  Microbes have genetics too! Some, like rust and smut fungi, survive by attacking living corn cells, drawing carbohydrates to the cell, and then moving on via spores before the host cells respond to the fungus.  Many fungal pathogens of corn simply kill a limited area of the leaf tissue, feed on the dead tissue, produce spores and infect new areas.  Corn varieties differ in how quickly and strongly they respond to the invasions.
 
One fungus that I find interesting is Bipolaris zeicola.  It was formally known as Helminthosporium carbonum.  There are genetic variants of this species that apparently feeds only on dead leaf tissue, often caused by insect damage or simply physical injury.  These variants apparently lack the genetics for either penetrating the live corn leaf tissue or overcoming the resistance system of most corn varieties.  At least one variant of this fungus produces a toxin that kills corn plant cells but most corn varieties have dominant gene that effectively blocks this toxin. However, very occasionally, a mutation of that dominant corn gene does occur while developing new inbreds.  If this mutation, now a recessive gene, becomes homozygous during the inbreeding process, the inbred is vulnerable to the toxin. The result is practically no defense to this variant (race 1) of B. zeicola.  The pathogen kills small leaf area of leaf, produces spores and spreads to new leaf tissue and eventually causes the whole corn plant to die early.  Because susceptibility is recessive, and the dominant toxin-resistant gene is present in most corn inbreds, this creates a problem for seed producers but not for hybrid growers.  Good that corn has genetic diversity.