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Common rust is caused by the fungus Puccinia sorghi and occurs in temperate to sub-tropical areas. The pathogen overwinters in the southern United States and Mexico. During the summer, storms and winds blow the fungal spores north into the Corn Belt, which infect corn fields.
Once spores land on leaf tissue, 3 to 6 hours of moisture are required for spores to germinate and start infecting the leaves. Disease development is favored by moist conditions caused by rainfall, dew, or high relative humidity (95% or greater), and moderate temperatures between 60 and 77 °F.
Figure 1. Early development (left) and fully developed (right) of common rust pustules.
Infections begin as light green to yellow spots on leaves, which then develop into small (< 1/4-inch long), reddish brown, raised pustules (Figure 1 - left). Pustules rupture the leaf epidermis and contain small, cinnamon-brown, powdery spores and can become darker brown to black late in the season (Figure 1 - right). Pustules are often found in bands or patches indicating that infection occurred while the leaf was in the whorl. The “powdery” rust spores can be easily rubbed off and are found on both upper and lower leaf surfaces. Water balance within the plant may be affected due to water loss from the ruptured epidermis. Severely infected plants may show symptoms of moisture stress during hot, dry weather, even when soil moisture is adequate.
Common rust can be confused with southern rust; however, southern rust pustules are orange-colored, primarily located on the upper leaf surface and the pustules are less apt to rupture (Figure 2). Southern rust spreads more quickly under hot, humid weather conditions and can have a higher economic impact. Timely fungicide applications to control southern rust are more crucial compared to common rust.
Figure 2. Comparison of southern corn rust (left) and common rust (right).
Common rust usually appears to some degree in the southern United States and the Corn Belt every year. If infection occurs late in the season, the potential for economic yield loss is fairly low. However, if infection occurs at early vegetative growth stages and conditions are favorable for disease development, the potential for economic yield loss increases.
Because corn products differ in their level of tolerance to common rust, the rate of disease development may be different for each product that is grown. Late-planted corn is more likely to have higher levels of infection since it will likely have young, more susceptible leaves during the time when spores first arrive from overwintering locations.
The best management option is growing corn products with higher levels of tolerance to common rust. General plant tolerance reduces the number and size of pustules and the overall severity of infection.
Fungicides can effectively control common rust if initial applications are made while there are only a few pustules present per leaf. Use of fungicides is fairly common in sweet corn and seed corn production, but is rarely warranted in field corn because common rust rarely causes economically damaging yield loss in field corn. Scouting each corn field on a regular schedule will help determine if fungicide applications should be considered. If significant levels of common rust are present on the lower leaves prior to silking and cool, humid, or wet weather is likely to occur a fungicide application may be beneficial.
Sources: 1 Sweets, L.E. and Wrather, S. 2008. Corn diseases. University of Missouri Extension. IPM1001. https://extensiondata.missouri.edu/. 2 Shaner, G. 2000. Corn Rust - An epidemic? Purdue University.www.agry.purdue.edu. 3 Pataky, S. Differentiating common rust and southern rust. Plant Management Network. University of Illinois. https://www.plantmanagementnetwork.org/edcenter/seminars/corn/CommonRust/Differentiation/player.html. Web sources verified 5/9/2018. 140602060422