Disease resistance vs tolerance

​Rating corn leaf reactions to a pathogen carries some ambiguity because of complications of the timing of infection, intensity of the pathogen and relative maturity of the plants.  An exception can come with single gene resistance such as the Ht1 gene.  In that case, the single gene inhibited the fungus from developing normal lesions and, more importantly, stopped the spore production and consequent spread of the pathogen within the field.  The problem, as with most single gene resistance systems, is that within 10 years of widespread use of the gene, a race of the pathogen could overcome the resistance mechanism, returning the effective resistance to the multi-genetic system and its rating system.
 
We are interested if a leaf disease is likely to cause a performance problem for a hybrid.  This involves the tolerance concept- how likely will an intense infection by the pathogen cause a reduction in grain yield or harvest difficulty because of stalk lodging. Tolerance is real but even more difficult to evaluate and express.  We participated in an experiment testing the hypothesis that yield loss from heavy gray leaf blight infection occurred only if the plant died early, such as with the sudden wilt associated with root rot and stalk rot. A known susceptible hybrid was inoculated with the fungus and compared to plants of the same hybrid not inoculated.  Individual plants that died before normal black layer were noted for date of death. Ears were hand harvested, dried, kernels counted and weighed.  Heavily infected plants that died early had lighter kernels and less total weight than those that equal leaf damage but stayed green until black layer.  Yield loss appeared to only occur if the plant died early.  This hybrid would be classified as very susceptible based upon leaf damage from the disease but at least somewhat tolerant if based upon yield loss or stalk rot.  If the season had been one that encouraged higher kernel counts causing more translocation to the ear, the reduction in photosynthesis from the leaf damage could have resulted in more yield loss compared to the potential for that season.
 
It is probable that a hybrid that tends to maximize the translocation of available carbohydrate to the grain is likely to be classified as less tolerant to a leaf disease than would be indicated by the lesion number or leaf damage resistance rating.  It is best to know the hybrid’s tendency to stay alive until completion of grain fill under many conditions (tolerant to late season stress) and its disease resistance ratings as separate concerns.