Detect and resist

 ​Humans and plants have similarities in fighting pathogens. Structural characters such as skin on animals and tight epidermal cells on plant leaves prevent invasion by most micro-organisms capable of destroying internal tissues rich in nutrition for them.  But evolution favors some organisms able to avoid this outer defense system, requiring the host to detect their presence and turn on resistance systems.
 
Corn leaf epidermal cells are tightly connected except for the stomata. The vast majority of microbes surrounding corn plants cannot penetrate the plants. The few that do have the capability, perhaps by enzymatically drilling through the epidermal cells to enter the leaf tissue, set off the alarm. In some cases, the plant’s first response is increasing the production of salicylic acid in the area of the invasion. This turns on the genes for production of the protein, often an enzyme, with the capacity of stopping the pathogen from spreading.  This final product may be effective against several potential pathogens or specific to one species.
 
 
​There are genetics, of course, behind the chemical responses to attacks by pathogens.  The genetics must relate to detection of an attack, perhaps detection of the specific pathogen, production of a general or specific anti-pathogen material and speed of the response.
 
Although all resistance to corn diseases involves multiple genes affecting the biology for the processes mentioned above, in many diseases there is a single corn gene that has a drastic, critical affect on the pathogen.  This is called qualitative resistance.  In the case of rust diseases, the pathogen is killed almost as quickly as it invades a cell.  With Exserohilum turcicum the plant with an Ht gene stops the fungus as it enters the vascular system, inhibiting the fungus from producing usual northern leaf blight lesions and spores to further spread the disease.  Qualitative resistance is convenient for the corn breeder to select during the breeding process but, unfortunately, nearly always the population of the pathogen includes individuals with a single gene that produces a product to overcome the resistance product.  Eventually, natural selection results in increasing presence of those pathogens with this gene and the the corn single gene is less useful.
 
The more stable resistance involves strengthening the detection, and the speed and concentration of the anti-pathogen materials.  This is called quantitative resistance. It is controlled by several genes and therefore more difficult to identify by breeders.  It is usually reflected by fewer and smaller lesions but assurance of exposure to the pathogen comparable to potential for the commercial life of the hybrid is not simple.  Most diseases are heavily influenced by environments which vary by location and season.  Most susceptible genetics are eliminated in breeding nurseries, but we do get surprised occasionally either with increased intensity of a disease or new pathogen changes.