There is some late-planted corn, particularly in the northern part of Mississippi, which has not reached physiological maturity. This corn may have its yield reduced somewhat by severe problems or stress, depending upon the crop maturity and degree of problem. However, corn is not nearly as sensitive to stress at late-reproductive stages as compared to early reproductive stages, because plants can generally more readily compensate for photosynthetic limitations or other shortcomings, at advanced growth stages. Corn does so by reallocating energy and resources from vegetative parts to help fill grain, which may however reduce stalk integrity. Of course, once plants attain physiological maturity, plant stress has no effect on grain yield.
This chart may help you assess potential yield loss and risk associated with issues which may defoliate corn during late reproductive stages. It was adapted from the National Crop Insurance Services’s “Corn Loss Instructions” (1984). This chart was originally developed for estimating yield loss resulting from hail damage, but may have utility for other issues which may defoliate corn as well, such as a disease like Southern rust or premature herbicide application. However, if you use this for estimating potential yield loss associated with Southern rust infection, I believe you need to also allow some lag time anticipated for defoliation to occur. Here are a couple of references for more information:
Growth staging corn approaching maturity