Southern corn rust, caused by Puccinia polysora, often is a serious disease on corn especially in later planted corn. The wind-blown spores of this fungus fall on the corn plant, germinate and infect the plant. About nine days later, relatively small, circular-to-oval pustules develop primarily on the leaves but are also on the husks and sheaths of plants. These pustules erupt, revealing the orange-red spores that will be dispersed by the wind to cause another cycle of infection. As the level of infection increases, the leaves are essentially covered by these pustules and the leaves are killed. Often the plants look like it should be time to harvest the crop just because the rust has killed the plant tissue prematurely. These pustules differ from those of common rust (caused by Puccinia sorghi) in size and shape and the fact that the pustules do not normally grow through the leaf. The pustules of common rust are elongated and spores are a darker brick-red color. Common rust is much more prevalent in the spring and early summer.
In the tests grown at the Mississippi State location, the development of southern corn rust from natural inoculation was extensive by the time ratings were taken on August 9, 1994. Natural inoculation usually is not as uniform over an entire test area as when all plants are manually inoculated with spores at a given time. All plants had susceptible-type pustules, but not all plants were equally infected. Each two-row plot of corn was rated for relative severity of southern corn rust using a rather arbitrary visual scale of 1 to 5, with a 1 indicating the least and a 5 indicating the most severe infection and damage level. On this scale, primary considerations were for the amount of leaf tissue killed and the extent of pustule coverage especially on the upper leaves of the plant. Hybrid differences ranged from ratings of 3.3 to 5.0 for the early-maturing hybrids and 1.8 to 4.8 for the late maturing hybrids (Table 9). Interpretation of these data should be made with remembrance that these results are from one location in one year, and that the inoculum distribution may not have been uniform over the entire test site.
* Dr. Gene E. Scott, research agronomist, USDA-ARS Crop Science Research Laboratory, Mississippi State University
------------------------------------------------------------------------------- Early maturing | Late maturing Hybrid Brand Rating* | Hybrid Brand Rating* ------------------------------------|------------------------------------------ RX 801 Asgrow 4.3 | XP 9380 Asgrow 2.5 RX 897 Asgrow 4.0 | DK 743 Dekalb 2.8 RX 899 Asgrow 3.5 | S516 Dyna Gro 2.5 RX 907 Asgrow 4.0 | HB 847002 HyPerformer 3.8 RX 919 Asgrow 4.8 | HS 9944 HyPerformer 4.8 7997 Cargill 4.8 | HY 9899V HyPerformer 3.0 8327 Cargill 4.8 | HY 9919 HyPerformer 4.5 8936 Cargill 5.0 | SB 1860 ICI 3.0 DK 683 Dekalb 4.3 | N8656 Northrup King 4.3 4581 Deltapine 3.8 | N8727 Northrup King 2.5 4653 Deltapine 4.3 | N8811 Northrup King 2.8 4682 Deltapine 3.5 | 8460 Mycogen 2.8 8695 Deltapine 3.8 | 3085 Pioneer 3.0 HS 9843 HyPerformer 4.8 | 3146 Pioneer 3.0 8281 ICI 4.8 | 3154 Pioneer 2.3 N 8206 ICI 3.8 | 3156 Pioneer 1.8 N7707 Northrup King 5.0 | 3163 Pioneer 3.8 N7989 Northrup King 3.3 | 3165 Pioneer 4.5 N8020 Northrup King 4.0 | 3167 Pioneer 4.0 Oro 188 Mycogen 4.0 | X1183F Pioneer 3.5 3245 Pioneer 4.8 | TR702E Terra 3.3 X1183C Pioneer 4.3 | TV2930 Terral 3.3 E1168 Terra 4.5 | TR1167 Terra 5.0 TR1185 Terra 4.3 | TV 2543 Terral 4.3 | TVX 2642 Terral 4.8 | ------------------------------------------------------------------------------- Overall Mean . . . . . . . . 4.3 | Overall Mean . . . . . . . . 3.3 LSD @ .05 . . . . . . . . . 0.7 | LSD @ .05 . . . . . . . . . 1.1 ------------------------------------------------------------------------------- * Scale = 1 to 5, with 5 indicating the greatest disease severity.