7/15/2019

Southern Rust of Corn

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Written by Madeline Henrickson, Pioneer Agronomy Sciences

Pathogen Facts

Southern rust is a disease caused by the fungal pathogen Puccinia polysora.

Southern rust does not occur as frequently in the Corn Belt as common rust (Puccinia sorghi), but can be more destructive when infection does take place.

Unlike other major foliar diseases of corn in North America, the rusts do not overwinter in the Corn Belt.

Rusts develop first in southern corn fields, and then may spread into primary corn-growing states.
Movement is by windblown spores that travel northward with prevailing weather systems.

Southern rust is favored by high temperature (over 77 ºF, 25 ºC) and high relative humidity, which tends confine it to tropical and subtropical regions.

Southern rust is generally more damaging to corn than common rust due to its ability to rapidly develop and spread under favorable conditions.

If economic thresholds are reached, a fungicide application may be warranted.

Yield impact depends on timing of infection, amount of leaf area damaged, and location of damaged leaves on the plant.

Damage

Photosynthesis is reduced as functional leaf area decreases, which can reduce kernel fill and yield.
Corn stalk quality can also be negatively affected as plants remobilize carbohydrates from the stalk to compensate for reduced photosynthesis.
Later-planted is generally at higher risk for yield loss due to leaf diseases.
If damage is confined to lower leaves or occurs after corn is well-dented, yield impact will be low.

Photo - Southern rust pustules on a corn stalk.

Southern rust pustules on a corn stalk.

Life Cycle

At the start of the growing season, urediospores from infected corn residue are spread by wind and rain on to growing corn plants (Figure 1).
Infection of these plants produces spores that serve as secondary inoculum and can be disseminated over hundreds of miles by wind.
In the U.S., southern rust usually appears later in the growing season and is more prevalent in the southern states.
In seasons with higher than average temperatures, southern rust can spread further up into the Corn Belt where it can impact corn yield.
Puccinia polysora is not known to have an alternate host.
Urediospores are the sole source of inoculum for both primary and secondary infection.
Secondary infection can occur multiple times in fields where southern rust is present.

Illustration - Life cycle of southern rust in corn.

Figure 1. Life cycle of southern rust (Puccinia polysora).

Identification

Both rust diseases of corn can cause substantial yield losses under severe disease pressure; however, southern rust generally poses a greater risk to corn yield than common rust, making proper identification important.
Southern rust looks very similar to common rust, but several characteristics distinguish the two, including the shape and color of pustules and their location on the plant.

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Southern vs. Common Rust

Southern rust spores on corn leaf.

Sunflower field with severe infection by Verticillium wilt and individual susceptible plants under stress from wilt.

Southern Rust Has small circular, pinhead-shaped pustules.

Coloration of pustules/spores is reddish orange.

Only infects the upper leaf surface, not the undersides of leaves.

Optimum temperature is warm-hot (over 77 ºF, 25 ºC).

Closeup photo - common rust on darker green corn leaf.

Closeup photo - common rust on corn leaf.

Common Rust. Has larger pustules that are more elongate and blocky.

Coloration of pustules/spores is brown to cinnamon-brown.

Infects both the upper and lower leaf surfaces.

Optimum temperature is cool-warm (60-77 ºF, 15-25 ºC). .

Distribution

In recent growing seasons, southern rust has occurred further north in the Midwestern U.S. earlier in the season than has been historically typical for this disease.
There have been confirmed detections in several counties in Indiana, Illinois, Iowa, Nebraska, and Kansas and even some cases in South Dakota and Wisconsin (Figure 2).
The increased prevalence of southern rust in the Corn Belt makes proper identification of this pathogen especially important.

Map - U.S. Midwest - confirmed detections of southern rust in corn through the first week of September during the 2018 growing season.

Sunflower field with severe infection by Verticillium wilt and individual susceptible plants under stress from wilt.

Figure 2. Confirmed detections of southern rust in corn through the first week of September during the 2017 and 2018 growing seasons. (Source: http://www.ipipe.org).

Photo - Southern rust symptoms visible in the upper canopy of corn in Johnston, Iowa.

Southern rust symptoms visible in the upper canopy of corn in Johnston, Iowa (Sept. 11, 2017). Foliar disease was extensive but infection occurred late enough in the growing season in this instance that yield impact was likely minimal.

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