Vegetative phase change characteristics and resistance to common rust of corn cultivars developed in different eras
Since 1930, maize (Zea mays L.) yields in the United States have increased more than fourfold. Previous studies examining maize cultivars grown during that period have documented changes in yield, leaf angle, tassel size, drought tolerance, and insect resistance, but none have examined changes in vegetative phase change or resistance to common rust (Puccinia sorghi Schwein.). Vegetative phase change has been shown to affect both disease and insect resistance, and 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 eras and planted at two locations with two planting dates per location for 2 yr. 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 maturity group, percentage of leaf area infected by rust was found to decrease at a rate of 0.1% yr–1. Whole-plant rust resistance in the late maturity group was found to increase at a rate of 0.05% yr–1.