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Southern Rust of Corn Found in North Dakota (09/10/20)

Southern rust of corn was identified in a NDSU hybrid performance trial two weeks ago. This article will review some of the commonly asked questions on southern rust and if management is needed.

Southern rust of corn was identified in a NDSU hybrid performance trial two weeks ago. This article will review some of the commonly asked questions on southern rust and if management is needed.

How to distinguish between common corn rust and southern rust?

Common corn rust will be brick-red in color and often have pustules on both the upper and lower leaf surfaces. Southern rust tend to be orange in color, often with bright yellow halos, highly concentrated on leaves, and pustules are usually only on the topside of the leaf. Differentiating between the two rusts can be difficult, but if you find southern rust, you will likely have common rust in the same field to help differentiate between the two (Figure 1).

How did southern rust get to ND?

Like other cereal crop rust pathogens, the disease does not overwinter in ND and spores are blown northward on storms and wind currents. As the name suggests, this disease can be very damaging in the southern states due to the lack of host resistance and early infections. This year, there was an early arrival of southern rust in Nebraska and levels are higher than normal. Similarly, southern rust prevalence in South Dakota is higher than normal. Southern rust has a short repeating cycle (about 9 days) compared to other rust pathogens, which means it can produce more spores in a shorter amount of time. The higher level of southern rust in states south of ND and ability to produce spores, likely contributed to why we were able to find southern rust in ND this year.

Is southern rust going to cause yield losses?

For ND producers, yield loss will be minimal. The biggest risks for southern rust epidemics include environment, host susceptibility, and growth stage of the corn:

  1. Environment: Southern rust prefers temperatures above 80F with high levels of humidity and dew. Temperatures will be much colder this week (below freezing), which will likely “shut down” this disease.
  2. Host: It is assumed that all northern corn hybrids are susceptible to southern corn rust, which would increase risk.
  3. Growth stage: The biggest risk of yield loss is driven by growth stage of the corn when rust is first reported. The earlier the disease is identified, the greater the chance of yield loss. Based on reports, most of our corn is beyond dent stages. This disease is most problematic if found prior to tasseling up until milk stage.

Do we need to manage southern rust?

The late arrival of this disease and cold temperatures over the course of this week makes management of this disease unnecessary. Fungicides can be used to manage southern rust, but are most effective between VT (tasseling) to R3 (milk stage). With most of the corn being at dent stages (and beyond), the crop will escape the window of potential damaging yield loss. Therefore, fungicides are not recommended for ND producers.

For more information on southern rust, please review this CPN publication.

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Andrew Friskop

Extension Plant Pathology, Cereal Crops

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