Pest Update: Southern Rust
Corn Not Rust Proof
OMAHA (DTN) -- That bright orange color coating skin and clothing after walking cornfields isn't a trendy new type of tan. Southern rust has blown in for an unwelcome visit in several parts of the Midwest this week.
Recent rainfall and an overall wetter summer than last year have encouraged the development of some foliar corn diseases, particularly southern corn rust. While typically more frequent in the South, the disease has recently surfaced in Nebraska, Kansas, Illinois, Kentucky and Arkansas.
Nebraska has seen the most reports of southern rust, with 11 eastern and south-central counties confirming infected fields. Kansas has three counties with confirmed infections. Kansas State Plant Pathologist Doug Jardine told DTN he expects that number to grow. "The [rust] levels so far have been kind of light," he said. "But it's been perfect weather for it, so I'm expecting it to increase here."
University of Nebraska Plant Pathologist Tamra Jackson said the prevalence of late-planted corn fields in Nebraska and other states puts farmers at a greater risk for yield loss should the disease start gaining speed.
"Since it's been a little cooler, those temperatures have held southern rust back a little bit," Jackson told DTN. "If we get into a warmer weather pattern and continue to see the rainfall, we very well could have it blow up on us because we do have a lot of spores and infected leaves out there."
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This season might produce flashbacks to 2006 for some central Nebraska farmers, she said. A similarly cool spring that year delayed planting, and an epidemic of southern rust dropped yields significantly in late-planted corn fields. According to recent USDA crop progress reports, most of the nation's corn crop has entered pollination stages a little later than usual this year, which makes them more vulnerable to yield loss, Jackson said.
In general, the earlier southern rust comes, the worse, she explained. "And that's why several of us got kind of bent out of shape when it showed up a week ago when we've got corn that has just barely tasseled in some of these fields. Now we're just past pollination in some fields and beginning to get into grain fill, and so of course if you start losing leaf area during grain-fill stages, that's mainly when you have yield impact."
Southern rust can be easily confused with common rust, another rust disease that has been cropping up in Nebraska and other Midwestern states. Both diseases produce spore-filled pustules on leaves, but southern rust pustules are mostly confined to the top of the leaves, whereas common rust can be found on both sides of corn leaves. Southern rust pustules tend to be small and reddish orange. Common rust pustules are larger and darker, Jackson said.
The distinction can be elusive to the untrained eye, but fortunately good images of the diseases side by side exist, such as in this University of Nebraska publication: http://goo.gl/….
Common rust is just that -- common -- so most commercial hybrids have some resistance to it. Southern rust is a less frequent visitor to Midwest cornfields. While many seed corn companies screen for the disease, genetic resistance is limited.
Like many rust diseases, southern rust has the ability to increase exponentially very quickly. "In one pustule, you can have as many as 5,000 spores, and so if there're a few hundred pustules in a little patch on a leaf, you can see where you can get up to a million spores really fast on a single section of a leaf," Jackson explained. "It goes from just a little bit to a whole lot in just a matter of days under the right conditions."
Hybrids with little tolerance for the disease combined with opportunity for a speedy spread make scouting an important component of protecting yield this year. Unfortunately, scouting a field can be a bit grueling. "Rust is not going to be evenly distributed across a field, especially when it first starts, so people really have to do a good job scouting in a pattern across their fields repeatedly over the course of the next couple weeks, Jackson said. The disease is wind-borne and doesn't overwinter in the U.S. So previous infections won't put your fields at greater risk. Any field is fair game for the disease this season.
No solid spray threshold exists for southern rust at this point, Jackson said. Growers should take into account weather forecasts, since the disease spreads best during humid days with temperatures between 70 and 85 degrees.
Growth stage of the corn is also important. "Every field has to be evaluated on an individual basis," Jardine said. "We've got corn that's everywhere from early milk to getting ready to dent. So if you're at or near dent, just ignore it, don't worry about it. If you're still in milk and you've got a pretty susceptible hybrid out there, it could still do a lot of damage."
For a list of fungicides and information about their effectiveness against various foliar diseases, see this University of Nebraska publication: http://goo.gl/….
In general, farmers in states where the disease has been spotted should keep an eye on both their corn leaves and the weather forecasts for the next few weeks. "We're not in a widespread epidemic by any means at this point," Jackson said. "It just seems we've got quite a bit of it around, and we're on the edge of something that could happen if the weather turns right."
Emily Garnett can be reached at emily.garnett@telventdtn.com.
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