Gray Leaf Spot of Corn

by Don White, Professor of Plant Pathology, Department of Crop Sciences

• Introduction
• Procedures
• Findings
• Summary

Introduction:

Gray leaf spot of corn caused by Cercospora zeae-maydis has increased in incidence and severity due to the wide-spread use of conservation tillage in conjunction with the susceptibility of most commercially grown hybrids. Though gray leaf spot was first identified in 1925 from lesions on corn leaves collected near McClure, in Alexander County, Illinois, the disease did not become a problem until the mid-1970s. In the summer of 1988, we initiated a project to identify sources of resistance to gray leaf spot and incorporate this resistance into commercially usable inbreds. The following is a summary of that research, most of which has been done at Urbana.

Procedures

In the summer of 1989, 1,396 corn inbreds were evaluated for resistance to gray leaf spot. Evaluations were possible due to the purchase of a mist irrigation system that allows us to mist over two acres of plot area with very low amounts of water. This system has allowed us to create the high humidity environment and frequent dews that favor gray leaf spot development. Immature plants were inoculated five to eight times with a spore suspension of Cercospora zeae-maydis. Inoculations, in conjunction with controlled irrigation, result in the severe disease epidemics necessary for classification of resistant and susceptible genotypes.

Of the 1,396 corn inbreds evaluated in 1989, five were eventually determined to have extremely high levels of heritable gray leaf spot resistance. The mode of inheritance of resistance was determined for these five lines after extensive study between 1990 and 1996. One line in particular was found to have superior, highly heritable levels of resistance. During the last two summers we have evaluated segregating generations of this line in mist irrigated plots and at a plot in Andrews, North Carolina. The resistance identified in these evaluations has been associated with molecular markers which may be further used in a marker assisted breeding program. This research has been supported by the Illinois Council for Foof and Agricultural Research (C-FAR).

In a separate research project, also supported by C-FAR, we have crossed resistance into commercially usable inbreds.

Findings

Our research has identified an inbred in which gray leaf spot resistance is controlled by only a few genes. Five RFLP markers associated with resistance in this inbred can now be used in a marker assisted selection program to screen for improved disease resistance. Additionally, we have developed commercially usable inbred lines with agronomically acceptable characteristics. These lines would not have been developed had it not been for the support of C-FAR.

In 1998, 32 test cross hybrids made from crosses between newly developed inbred lines and a commercially used inbred were evaluated at seven Illinois and two Nebraska locations. Seven of our resistant inbred lines have been selected (based on yield and disease resistance) for advanced testing and potential commercialization this summer. In hybrid combination, the selected lines yielded between 189 to 206 bushels per acre averaged over all locations. These yields were competitive with elite commercial hybrids included in the experiment. In general, our resistant hybrids had 50%, or less, leaf blight than contemporary commercial hybrids. This level of gray leaf spot resistance, however, may not be great enough to prevent yield loss in all environments in Illinois. The source of resistance does have the very high levels of resistance necessary to avoid yield loss in all environments. Our use of traditional plant breeding methods to backcross resistance from our resistant inbred into the widely used line FR1064 may explain why the high level of resistance was not recovered. It is possible that all of the genes for resistance now identified by molecular marker analysis were not incorporated into FR1064. We are currently in the process of determining what additional genes need to be crossed into our newly developed lines in order to recover the very high level of resistance that is present in our source of resistance.

Summary

This research has resulted in the discovery, description and use of a previously unidentified source of resistance to gray leaf spot. Inheritance of this resistance has been determined, and molecular marker associations identified for use in marker-assisted breeding programs. Resistance from this source also has been backcrossed into commercially usable inbred lines. Future commercially usable inbred lines in hybrid combination will hopefully provide for control of gray leaf spot on Illinois farms. This research was possible because of C-FAR funding, and because of access to research plots at the University of Illinois.