Southern rust is caused by the fungus Puccinia polysora and generally occurs in tropical to sub-tropic areas. Under favorable conditions, the disease can thrive in more temperate regions. The pathogen overwinters in southern Florida, the Caribbean, and Mexico and is transported north each year by winds and storms. Like common rust, southern rust requires a live host to survive. Disease occurrence is dependent upon wind dispersal of the pathogen, thus infection one year does not indicate that southern rust will occur the following year.
Disease development is favored by high humidity and temperatures between 80o and 90o F. Under favorable conditions, new infections are visible in about 10 to 14 days. Epidemics may occur when unusual weather patterns lead to mass air movements from the tropics where southern rust is endemic.
Pustules develop primarily on the upper surface of leaves and only sparsely on the lower leaf surface (Figure 1). Pustules are circular to oval in shape and light orange in color. These pustules erupt and expose small, dust-like, golden-colored spores, which are dispersed by wind. Unlike common rust, pustules may also develop on stalk, husk, and leaf sheath tissues (Table 1).
Southern rust has the potential to cause yield loss due to its ability to develop and spread rapidly. The effect of disease on corn plant health and yield depends on time of infection. Plants infected early in the season may develop significantly damaged leaf tissue. Heavy infections of southern rust can lead to early senescence and can limit the ability of the plant to produce carbohydrates for grain fill. This leads to stalk cannibalization and may predispose the plant to stalk lodging, stalk rots, and reduced grain quality.
If southern rust is identified, growers in the region should first focus their scouting efforts on irrigated and late-planted fields. Irrigated fields are more likely to have the high humidity required by southern rust to infect corn. Late-planted fields are at risk for developing more severe infections of southern rust because young leaves are more susceptible than older leaves. When scouting for southern rust, determine the growth stage and yield potential to help evaluate whether or not fungicide applications are an economically feasible option.
Resistant corn varieties are the most cost-effective means to manage southern rust in field corn.
Chemical control may be warranted if the weather forecast is for hot, wet, and humid conditions, pustules are present, and black layer is four or more weeks away. However, consider the following before applying a fungicide.2
Fungicide control is preventative rather than curative. Fungicides cannot restore the health of infected leaf tissues, but they can prevent new tissues from becoming infected. Thus, applications must be made before southern rust develops to severe levels.
Please consult with your local agronomist if you have concerns about southern rust in your fields, and review university recommendations for fungicide application timing.
1Robertson, A. 2007. Outbreak of southern rust on corn. Integrated Crop Management. IC-498(23). Iowa State University. http://www.ipm.iastate.edu (Verified 6/19/14);
2Koenning, S. 2005. Southern rust for corn. Corn Disease Information Note 2. North Carolina State University. http://www.ces.ncsu.edu (Verified 6/19/14);
Jackson, T. 2007. Rust diseases of corn in Nebraska. NebGuide G1680. University of Nebraska. http://www.ianpubs.unl.edu (Verified 6/19/14);
Lipps, P. et al. Common corn rust. Ohio State University Extension. FactSheet. AC-0031-01. The Ohio State University. http://ohioline.osu.edu (Verified 6/19/14).
Pataky, S. Differentiating common rust and southern rust. Plant Management Network. University of Illinois. http://www.plantmanagementnetwork.org/edcenter/seminars/corn/CommonRust/ (verified 6/19/14).