Stalk rot resistance rating

​Resistance to stalk rot fungi involves so much of the corn plant’s biology and the environment that it does not become an easy trait to express in hybrid descriptions.  On the other hand, there are differences in the tendencies to develop stalk rot when the plants are under certain environments.  Hybrids differ in reaction to favorable pre-pollination conditions, some committing to greater movement of carbohydrates to the grain at the detriment of carbohydrate availability to the roots.  Reactions to late season photosynthetic stress also varies among hybrids.
 
The gradations of these variables and hybrid reactions do not allow absolute stalk rot resistance ratings possible.  Expression of stalk rot rating is much like expression of corn yield- absolute values are not appropriate but are only meaningful in relation to other hybrids or acceptable performance.  It is in this regard that evaluation of stalk rot vulnerability of experimental hybrids by plant breeders needs to be done in hybrid yield tests.  Stalk rot vulnerability is a hybrid phenomenon that may be influenced by the inbred parents but it is mostly the product of heterosis, with the combination of the parent genetics affecting the probability of stalk rot problems.  Consequently, it is evaluation of the hybrid that is critical.  Also, just as with yield testing, commercial seed breeders are interested in predicting the stalk rot vulnerability in the field where the hybrid will be used.
 
Plot yields can be taken with accuracy but evaluation of stalk rot requires human observation. Counting lodged plants is relatively easily done from the harvest combine but this method does not consider the rotted plants still standing but ready to lodge with the next strong wind.  An alternative is to rate the stalk condition by walking each plot, giving it an acceptability rating.  If all plants are strong and with green lower stalk color, the plot is scored as excellent.  If too many plants are weak and ready to lodge, making it unacceptable, then it can be scored as extremely unacceptable.  Of course, some plots would be scored as intermediate.  This method should be applied at all plot locations.  The final summary can be expressed as the percentage of plots or location in which a hybrid had acceptable levels of stalk rot.  This should allow an estimation of the frequency of stalk rot problems expected for each hybrid.  Test plot locations will not represent all the environments that a commercial hybrid will need to balance yield, stresses and stalk rot but growers will evaluate annually in their field conditions.
 
Stalk rot resistance ratings should not be considered as absolute resistance by hybrids against a specific late season fungus but more of the balance of photosynthetic stress and translocation of carbohydrates under most field conditions.