Disease resistance ratings

​Rating the tendency of a corn hybrid to get stalk rot is complicated because of the biology of the corn plant interacting with the environment.  Describing resistance to corn leaf disease may be a little simpler but it still is with complexities. Fungi causing diseases such as northern leaf blight, southern leaf blight, gray leaf spot and eyespot exist in dead leaf debris between corn seasons. Moisture and temperatures favoring early corn growth also stimulate these fungi in exposed leaf debris to produce spores that are carried by wind to the young corn plants.  Most spores require a few hours of moisture to germinate and grow special hyphae for penetration of the corn leaves.
 
Resistance to the pathogen begins with the recognition of the pathogen, often associated with a protein component of the fungal cell wall.  Corn genetics are involved in efficiency of that process.  After getting established in the leaf, the pathogen attempts to grow to surrounding cells.  Host genes are turned on to produce anti-fungal products to limit the pathogen.  Hybrids vary in the efficiency of limiting the number of lesions that develop from initial invasion by the pathogen.
 
We attempt to characterize the differences among hybrids for their successful limit of damage by each leaf disease.  Researchers have used number of lesions, leaf area damaged, number of dead leaves and size of lesions in efforts to quantify the resistance to a leaf disease.  Every method requires some control of the pathogen such as virulence of the pathogen isolate, number of spores as well as environmental influences such as moisture.    None of these methods are perfect in terms of determining the expectation of the hybrid performance in many environmental interactions that a hybrid may actually face during its commercial life.  Disease pressures vary each season in timing and intensity.  Fungal isolates vary in their genetics affecting their spore production and virulence.  Leaf resistance also is influenced by the biological state of the leaf, often reducing with senescence beginning a few weeks after pollination as sugars are moved to the ear.
 
We at PSR inoculate plants with spores in a water suspension into the whorl of corn plants at the V8 stage.  This attempt to give each corn genotype about the same intensity of pathogen initial pressure.  Hybrids vary in the number of lesions successfully established in these leaves a few weeks after inoculation.  Further spread of the disease will be influenced by host genetics and environment.  Resistance ratings determined reaction to this primary infection can be a fair indication of the relative resistance level expressed on more mature corn plants, at least according to a study with northern leaf blight that I did many years ago (unpublished).  It is more assuring, however, if the season favors spread of the disease to more leaves.  Resistance ratings always carry some influence of relativity to resistance expression by other corn varieties and to disease pressure.
 
We prefer that things are simple and easily characterized but there are always qualifiers.