Stop the Clock on Crop Disease
Disease moves fast to destroy crop yields. You need a fungicide that works just as fast.
Discover Aproach® Prima fungicideKey Points
- Southern rust (Puccinia polysora) is a foliar disease of corn common to the Southeastern U.S. that is now occurring with increasing frequency in the Corn Belt.
- P. polysora requires a living host to survive, so it does not overwinter in the Corn Belt. Spores are carried north each year from tropical areas by prevailing winds.
- Southern rust has the potential to be much more damaging to corn than common rust due to its ability to rapidly develop and spread.
- Southern rust is favored by high temperatures (over 77 ºF, 25 ºC) and high relative humidity.
Pathogen Facts
- Southern rust is a foliar disease of corn caused by the fungal pathogen Puccinia polysora.
- Southern rust does not occur as frequently in the Corn Belt as common rust (P. 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 temperatures (over 77 ºF, 25 ºC) and high relative humidity, which tends to 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.
- When conditions favorable for disease development persist for an extended period, severity can quickly reach epidemic levels.
- Yield impact depends on timing of infection, amount of leaf area damaged, and location of damaged leaves on the plant.
Crop 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 corn 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.
Figure 1. Southern rust (Puccinia polysora) pustules on a corn leaf.
Life Cycle
- Urediniospores are the primary infective propagule and are spread northward via the wind from living hosts in tropical areas.
- Spores will infect corn and cause symptoms within 3-4 days. Within 7 to 10 days, more urediniospores are produced and new infections continue to occur as long as conditions remain favorable, which can rapidly lead to an epidemic.
- In the U.S., southern rust usually appears later in the growing season and is more prevalent in the southeastern states.
- In seasons with higher than average temperatures, southern rust can spread further up into the Corn Belt where it can impact corn yield.
- P. polysora is not known to have an alternate host.
Figure 2. Life cycle of southern rust.
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.
Southern Rust
- Has small circular, pinhead- shaped pustules.
- Coloration of pustules/spores is reddish orange.
- Infects the upper leaf surface, as well as stalks and husks.
- Favored by higher temperatures: (over 77 ºF, 25 ºC).
Common Rust
- Has larger pustules that are more elongate and blocky.
- Coloration of pustules/spores is brown to cinnamon-brown.
- Infects the upper and lower leaf surfaces.
- Favored by cooler temperatures: (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.
- Southern rust is now routinely observed in Indiana, Illinois, Iowa, Nebraska, and Kansas and has been detected as far north as South Dakota, Minnesota, and Wisconsin.
- The increased prevalence of southern rust in the Corn Belt makes awareness and proper identification of this disease especially important.
Figure 3. Southern rust on corn; Johnston, IA; August 2024. Southern rust outbreaks often begin with isolated patches of disease in the middle or upper canopy along field edges.
Figure 4. Later in the season, P. polysora forms darker pustules called telia that contain teliospores.
Corn and Soybean Scouting Calendar
Download your copy of Pioneer's Corn and Soybean Scouting Calendar. It's your guide to the diseases and pests that threaten your yield and when to look for each one.
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Aproach® and Aproach® Prima are not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions.
The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as disease and pest pressures. Individual results may vary. Pioneer® brand products are provided subject to the terms and conditions of purchase which are part of the labeling and purchase documents.