Vegetative Phase Change Characteristics and Resistance to Common Rust (Puccinia sorghi) of Corn Cultivars Developed in Different Eras.

Since 1930, maize yields in the United States have increased more than four fold.  Previous studies examining maize cultivars grown during that period have documented changes in yield, leaf angle, tassel size, drought tolerance, and resistance to second-generation European corn borer, but none have examined changes in phase change, common rust resistance, or any other disease resistance.  Vegetative phase change is the transition from juvenile vegetative to adult vegetative tissues and is an essential process to the lifecycle of higher plants.  It has been shown to affect both disease and insect resistance.  Common rust can reduce yields in maize especially east of the Mississippi River.  Thirty six widely used northern Corn Belt cultivars spanning the years of pre-1930 to 2006 were evaluated for phase change traits and rust resistance.  Cultivars were divided into early and late maturity groups from different decades and planted at two locations with two planting dates per location for two years.  Timing of vegetative phase change has not changed linearly over the eras examined.  There was, however, a significant linear increase in rust resistance over that time in the early maturing cultivars.  Percentage of leaf area infected by rust was found to decrease at a rate of 0.7% per year on leaves below the ear and 0.5% on the whole plant.  No linear trend for rust resistance was found in the late maturity group, but the pre-1930 era was significantly more susceptible to both rust measurements than all subsequent eras.  Leaves above the ear were found to have increased linearly by an average of 0.1 leaves every 10 years in the early maturity group while having no linear change in the late maturity group.