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Southern Rust Outlook 2019

The top two diseases causing economic impact in corn each year are Gray Leaf Spot and Southern Rust. While we generally get GLS each year, it is not as common place to have infections of Southern Rust every year. However, when infection does occur, the yield impacts can be quite significant. As Southern Rust makes its way across Kansas, it has the potential to be a yield limiting factor in Brown, Doniphan, and Nemaha counties this year. This impact will be even more pronounced with the variety of planting dates and late planting this year. This blog post will cover the progression of Southern Rust in 2019 and its possible impact, how to identify southern rust and its differences from common rust, and finally threshold levels and control methods.


2019 Disease Progression and Possible Impact



Generally we start hearing about Southern Rust in Kansas the first or second week of August. This varies from year to year, depending on levels of Southern Rust in the South and in Mexico, as well as weather conditions. This year, Southern Rust has been identified in 12 counties in Kansas, as far north as Pottawatomie County. It has also been identified in 12 counties in south central Nebraska. However, it has yet to be identified in Brown, Doniphan, or Nemaha counties. This pattern in not unusual in any given year. In fact, it seems like areas North of us in South Central Nebraska generally have infection before we do. It is likely that we could see infection in the county in about a weeks time as wind moves the spores north. Winds from the Southwest will assist with that movement. However, prevailing winds appear to switch to the East for the next 7 days or so and will slow down the movement of inoculum to our area.



We are at a somewhat greater risk this year for significant yield impacts. The wide range of planting dates means that some later planted corn will be at greater risk if infection does occur. More mature corn will not be impacted as much as these later planting dates; if corn is already at dent, no impact should be seen. Some yield impact is possible at dough stage, and yield reduction is very possible if infection levels occur at or prior to milk. The wide range of planting dates also means more care needs to be taken in precise scouting by planting date and corn maturity. With this variety of dates, the potential for various stages of growth and disease levels will result in a wide variety of target dates for fungicide control. The process will just be a little more complicated and time consuming this year.


Southern Rust Background and Identification

Southern rust is caused by Puccinia polysora. It shows up as small pustules that are small and circular or oval in shape and light orange in color. To be clear, pustules are these raised clusters of spores. They will rupture through the top layer of the leaf cells which causes a whole host of problems that we will discuss later.


Southern Rust needs moisture for infection. Generally, 6 hours of dew on leaves will be enough for infection. Southern Rust is favored by warmer temperatures in the 80s and 90s, making August a prime time for infection. Southern Rust is capable of asexual reproduction, meaning that as long as living corn is available, and there is moisture, it will reproduce indefinitely. In fact, each pustule creates thousands of spores that can infect neighboring plants in as little as 7 days.


So, in what way does Southern Rust impact the plant? First of all, as more spores appear on leaves, often due to secondary infection by neighboring plants and leaves, the surface area the plant can use for photosynthesis decreases. The less area for photosynthesis, the less “food” available to the plant. (Southern Rust is considered a parasite because it actually steals nutrients from the plant in addition to blocking the plants ability to make food.) To combat this issue, the plant will take sugars and energy reserves from the stalk to put towards grain fill. This can result in stalk rots and lodging as well as not enough energy to fill ears resulting in reduced yield. Some estimates put the yield reduction as high as 45%.


Another significant impact is through water loss. When the spores rupture the epidermal cells, this interferes with the plants ability to regulate water through its stomatal openings. Plants with high level of infection will often show some drought stress and wilting due to this phenomenon.


Southern Rust is often confused with Common Rust. There are some distinguishing features between the two, however, the best way to differentiate is to send in a sample to a lab for testing. Southern rust will only infect the top side of corn leaves while Common Rust will infect both sides. Generally a common diagnostic feature of Southern Rust is chlorotic lesions on the underside of leaves that show up as little halo’s around the pustules. While this is generally more unique to Southern Rust, it is also sometimes a symptom in Common Rust and shouldn’t be used exclusively as the identifying factor. Both Southern and Common will rub off on clothes and skin when bushed up against and can stain clothes red. Southern Rust pustules tend to be grouped more densely together, while Common Rust will be more dispersed across the leaves. Additionally, Common Rust pustules are usually larger and more elongated, sometimes more “brick-like” in appearance. With some symptoms so similar, sometimes the only way to distinguish between the two diseases is under a microscope.





Southern rust will start in “hotspots” throughout the field. Be sure to check areas that might be prone to extended dew such as low lying areas as they will be more prone to infection first. Often times these hotspots can be found along field edges initially. If you think you might have Common or Southern Rust, contact a member of the Pederson Seed Agronomy Team to check it out and send in some samples.


Thresholds and Control



There are no concrete defined thresholds for treatment of Southern Rust as it is very dependent on weather conditions and growth stage of the corn. Some general guidelines are if the corn is in grainfill, and the spores seem to be spreading rapidly, apply a fungicide. We cannot rely on genetic resistance by corn plants. Most resistance levels for hybrids will be below a 5, meaning there isn’t great control genetically for Southern Rust. This emphasizes the importance of using fungicides for control. The more advanced the disease in your field, the more important it is that you use a fungicide with both triazole and strobilurin modes of action. These include Approach Prima, Headline Amp and Priaxor. These will provide much better control and last longer than a single mode of action. The following table from the Crop Protection Network gives control ratings for various fungicides.




To keep track of spread of disease across the state and Midwest, use this link:

https://corn.ipmpipe.org/southerncornrust/


As always, if you have questions or comments, let a member of the team know, or comment below to get the discussion going!


References

https://cropwatch.unl.edu/early-southern-rust-15

https://www.pioneer.com/us/agronomy/southern_rust_cropfocus.html

https://www.extension.purdue.edu/extmedia/BP/BP-82-W.pdf

https://cropprotectionnetwork.org/resources/articles/diseases/southern-rust-of-corn

https://www.bookstore.ksre.ksu.edu/pubs/MF3016.pdf

https://agfax.com/2019/07/29/nebraska-corn-southern-rust-confirmed-2/

https://webapp.agron.ksu.edu/agr_social/article/southern-rust-detected-on-corn-in-kansas-346

https://crop-protection-network.s3.amazonaws.com/publications/cpn-2011-corn-fungicide-efficacy-for-control-of-corn-diseases.pdf

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