Common rust is a disease that occurs in temperate to sub-tropical areas, including the Midwest. However, it cannot overwinter in the Midwest and arrives by wind from Mexico and southern states. Common rust is rarely an economic problem and can usually be managed by planting tolerant corn products, although a fungicide application may be warranted from time to time.
What To Watch For Common rust is a “common” fungal disease that occurs in temperate to sub-tropical areas, including the Midwest. However, it is unable to overwinter in the Midwest; therefore, fungal spores are transported by wind from Mexico and southern states into the Corn Belt. Development is favored by temperatures ranging from 60 to 77° F and moist conditions caused by rain, dew, or 95% or greater relative humidity.1 Spores that land on leaf tissue can germinate if they are subjected to 3 to 6 hours of continuous moisture. Young leaves, particularly those around the whorl, tend to be more susceptible because of the potential for water to pool in the whorl.
Upon infection, light green to yellow chlorotic spots are observed. As time passes, raised cinnamon-brown pustules form on the upper and lower leaf surfaces that can turn blackish in color late in the growing season 1). The pustules may be in bands across the leaves indicating that infection occurred while the leaf was wrapped within the whorl and are less than 1/4-inch long. When the pustules rupture, small cinnamon-brown powdery spores are released.
Another rust disease, southern corn rust can be confused with common rust. The distinguishing characteristic for southern rust is that the pustules form mostly on the upper leaf surface and spores are more orange in color (Figure 2). Southern rust spreads much more quickly and has a higher economic impact when hot, humid weather conditions persist. Timely fungicide applications to control southern rust are more crucial than with common rust.
Impact On Your Crop If infection occurs late in the season, the potential for economic yield loss is fairly low. However, if spores arrive from the South early in the season and infection occurs when the plants are small, the potential for economic yield loss increases if the environment remains favorable for rust development.
Corn products differ in their level of resistance to common rust; therefore, the disease development rate can be different for each product that is grown. Late-planted corn may have higher levels of infection because the growth of new leaves coincides with spore arrival from overwintering locations. Severely infected plants may show water stress during hot, dry weather because the pustules allow for water to escape from the plant.2
Tips To Manage Planting corn products with higher levels of common rust tolerance is the best management option. General plant tolerance reduces the number and size of pustules and reduced 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. Though fungicides are commonly used in sweet corn and seed corn production, they are rarely warranted in field corn for common rust because the potential for economic damage is low. Scouting each corn field on a regular schedule can help determine if fungicide applications should be considered. If significant levels of common rust are present on the lower leaves prior to silking and the forecast is for cool, humid or wet conditions, a fungicide application may be beneficial.
1Sweets, L.E. and Wrather, S. 2008. Corn diseases. University of Missouri Extension. IPM1001.
2Shaner, G. 2000. Corn Rust - An epidemic? Purdue University.www.agry.purdue.edu
3Pataky, S. Differentiating common rust and southern rust. Plant Management Network. University of Illinois. Web sources verified 5/9/2018. 140602060422