Quantitative disease resistance

​Resistance to corn diseases is the result of the plant tissue recognizing the invasion of the pathogen and responding by turning on the resistance mechanism.  There are multiple corn plant genes involved and thus the term quantitative gene response is used to reflect the genetics. A recent research paper (Nat Genet. 2017 Jul 24. doi: 10.1038/ng.3919.) identified a single gene coding for an enzyme critical to lignin production.  Lignin is a component of cell walls, suggesting that presence of this gene causes the cells, at least in the region of the infection to produce a physical barrier to the pathogen.  This gene was found associated with resistance to the pathogens causing southern corn leaf blight, northern leaf blight and gray leaf spot.  It is assumed that this gene is not the only gene involved but does contribute to limiting the lesion size that develops from these diseases.
 
Most corn leaf fungal pathogens, other than those causing rust and smut diseases, receive primary nutrition from dead plant tissue. Their ‘strategy’ is to kill tissue, get nutrition, produce spores and then spread to new leaf tissue to repeat the cycle. Quantitative resistance in host plant strategy is to limit the area and slow the advance of the lesion. Corn varieties may differ in number and size of lesions when under identical disease pressure but even those with relatively good ratings for quantitative resistance can be damaged if pathogen pressure is high because of carryover diseased tissue and environment favors the pathogen. This is the reason that most disease resistance ratings need to consider pathogen pressure.
 
Although there may be some commonality among resistance to various pathogens, other genes participating in quantitative gene resistance can be more specific to a pathogen.  Perhaps it involves gene products associated with recognition of a specific pathogen. Although the gene sited above may be part of the resistance to three different pathogens, other genes may affect the response to each one of these.  Good resistance to the gray leaf spot pathogen does not assure good resistance to the northern leaf blight pathogen.
 
Physiological status of the corn plant also affects it ability to react to a pathogen. Leaf diseases often show greatest a few weeks after pollination, probably because leaf tissue is moving carbohydrates to the ear and cells begin to senesce.  Lower leaves of the plant, being the oldest and receiving less light are at a lower physiological state than upper leaves.
 
Varying disease pressure and plant maturities complicates establishing disease resistance ratings of hybrids.  The main attempt must be to distinguish those with extreme susceptibility from those with intermediate and very resistant levels to the level of each disease the hybrid could be exposed.