Corn disease resistance (A)

​Disease resistance in plants is not totally unlike that in humans.  A pathogen is detected, the host responds by producing hormones that ultimately result in production of pathogen-inhibiting chemicals that eventually stop the pathogen.
 
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.
 
Our company, Professional Seed Research Inc., offers a service of evaluating disease resistance in experimental corn hybrids for seed companies.  We culture the pathogens and then inoculate the plants to the fungal pathogens by applying a concentration of spores to the whorls of plants when at about V7.  It is common that this moist environment assures spore germination and infection in the leaf tissue that was in the whorl at the time.  We inoculate only one pathogen per plant to avoid the resistance mechanism triggered by another pathogen and therefore not representing the genetics of the hybrid.
 
A rain storm arrived the evening after we inoculated the nursery in 2016.  Symptoms for the southern corn leaf blight and northern corn leaf blight were very sparse on the respectively inoculated plants.  Instead there were common rust pustules in the areas of the leaves that should have shown the other two diseases.  The hypothesis is that the rust spores were distributed by the storm into the leaf whorls, and the rust fungus (Puccinia sorghi) infected the leaf tissue. This triggered a general resistance factor that inhibited the other two fungi (Bipolaris maydis and Exserohilum turcicum).  I recall this happening several years ago as well.  There is a published report of infection by either of the latter two fungi limiting the infection by the rust fungus.  Hopefully, in our case, inoculating much later with the intended pathogens will find that the general resistance factor will no longer be present.