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Watch for threats from the south in the corn belt.

Agronomy Bulletin 72 ─ Southern Corn Rust

Southern rust pustules are primarily found on the upper leaf surface, brought on by extended periods of stifling heat and humidity.

Situation

Southern rust is a common threat to corn crops across the southern United States. The disease damages crops in the Corn Belt only about once every five years, but when it does yields may be cut by more than 40 percent.

Factors to Consider

  • Identification
  • Management strategies
  • Life cycle
  • Hybrid selection
  • Causes
  • Fungicide treatments

Action Plan

  1. Identify southern rust. Caused by the fungus Puccinia polysora, southern rust is identified early by clusters of small circular lesions about 0.2 to 2 millimeters wide. As lesions mature, the fungus erupts through the leaf surface and the lesions become more elongated, shadowed by a yellow halo. The end result is a brownish-orange oblong pustule. Unlike common rust, pustules are found mostly on the upper leaf surface, although fungi also can attack stalks and husks. Wind-blown from the south, urediniospores cyclically infect the plant every seven to 14 days. Following infection, black teliospores are produced within the lesions during which one lesion may produce both tan-red urediniospores and black teliospores.
  2. Understand the causes. Southern rust thrives in temperatures above 80 F with high humidity, especially if corn has been planted late. In the Corn Belt, southern rust tends to develop later in the season when humidity is high, urediniospores are abundant and overnight temperatures favor dew formation. Rain will spread fungus development. Note that southern rust rarely overwinters in the Corn Belt.
  3. Consider your region when selecting hybrids. Hybrids with southern rust-resistant characteristics are more common in the Southeast than the Midwest for two reasons: The disease is not an annual threat to Corn Belt crops and yield potential of adapted hybrids can be less than non-resistant hybrids, which are better adapted.
  4. Know management practices to avoid infection. Younger leaf tissue is most susceptible to fungal infection. Delaying disease development until crops are more advanced reduces the likelihood of yield loss. In regions where southern rust is a more consistent problem, some growers minimize damage by planting earlier or by using shorter-season hybrids.
  5. Determine if fungicides make financial sense. Many fungicides labeled for use on corn can effectively control rust with timely application, but weigh the economics. A fungicide typically costs $15 to $20 per acre, including the $5 per acre cost of application. Corn within two weeks of the black layer stage and fields projected to yield less than 150 bushels per acre are unlikely to benefit from treatment. Still, fungicides should be carefully considered if more than 10 percent of the leaf area is affected. Before applying fungicides, monitor weather forecasts to determine if high humidity and hot temperatures will persist.

Summary

If southern rust is a frequent threat to your fields, identifying the disease, selecting the appropriate hybrids and using fungicides can help you manage the issue and avoid yield loss. For more information, contact your local Mycogen Seeds customer agronomist or trusted agronomic adviser.

Resources

Published on Monday, August 05, 2013