Corn growers in Virginia and Carolina appear to have one of the best corn crops in years in the ground, but high yields and quality could be damaged by Southern rust, a devastating disease of corn and soybeans that rarely reaches the area in time to do significant damage to corn.
North Carolina State University Corn Specialist Ron Heiniger says the disease was found earlier than usual in South Georgia.
If the disease spores move rapidly from south to north on tropical fronts, the subsequent disease could cause major damage to the corn crop in the Carolinas and Virginia in the second and third week of July.
“We’ve got a really good corn crop, all across North Carolina. The one thing growers don’t need is for one of these tropical systems to move up the Coastline, much like Tropical Storm Beryl that occurred in late May,” Heiniger warns.
Beryl is likely the reason Southern rust was found in Georgia so early. University of Georgia Plant Pathologist Bob Kemerait says the disease was found in at least four counties in South Georgia, along the Florida line in mid-June. “That’s much, much earlier than we saw the disease in 2011 and earlier than it occurs in most years,” he says.
Kemmerait says it’s likely that protective spraying of corn has slowed the severity of Southern rust damage to. While the use of fungicides can diminish damage on corn from Southern rust, it has not been highly successful in preventing the disease from spreading rapidly from one field to another.
“If we get Southern rust in North Carolina by the second or third week in July, we’ve got big trouble with our corn crop,” Heiniger says.
In addition to being one of the country’s top experts on corn production, Heiniger is also known more so in North Carolina as a weather guru. His predictions of long-range weather trends have been quite accurate over the past few years. Unfortunately, he says tropical weather fronts are likely to occur this year.
True to his prediction, in late June, a tropical system formed off the Florida Keys, and could provide the south to north movement needed to push disease causing Southern Rust spores into the Carolinas and Virginia.
Southern rust is caused by the fungus Puccinia polysora. It can be recognized by the bright orange or golden brown, circular to oval pustules which give a rusty appearance to the leaves.
Disease pustules are about the size of a pin head and are filled with powdery masses of orange spores which can be rubbed off.
Though the disease spores cannot over-winter as far north as the Carolinas, they can be easily dislodged from host plants and blown long distances by the wind.
The spores can survive and infect plants after being transported hundreds of miles by the wind.
The fungus can multiply very rapidly on susceptible corn, and the amount of damage that occurs depends upon how early the first spores arrive.
Epidemics may result from unusual weather patterns that cause mass air movements from the tropics where the rust is present.
Resistance can break down
The best management tool for Southern rust is to plant hybrids with genetically built-in resistance to the variety. However, in the past few years new races of the disease have shown up in the Southeast, such as the one that popped up in 2008-2009 that can attack once rust-resistant corn varieties with the Rpp 9 gene.
For this year’s crop, the best hope is to treat with a preventative fungicide, but that too hasn’t always been a good management tool, Heiniger says.
A number of the strobilurin fungicides can do a good job of buying a grower some time, if he gets it on the crop in time, he adds.
If a grower waits until he sees the symptoms of the disease in a corn field, it’s too late for fungicides.
“A couple of years back, we had a lot of Southern rust in some of corn plots, and we tried everything we could find, but nothing did much good after the disease got going,” Heiniger says.
Southern rust starts in a corn field with a circular pattern of yellow corn plants. In bad cases, the disease will spread through a field and across a whole farm in a few days.
But basically, if you see that distinctive yellow circular pattern in a corn field, it’s too late to treat with a fungicide, the North Carolina corn specialist says.
Triazole fungicides and strobilurin fungicides all have good activity against Southern corn rust. The triazoles will have greater systemic activity and strobilurins a longer protective window.
At best, the protective window provided with strobilurin fungicides is three weeks.
Growers with high yield potential and with high likelihood of re-infections may need to spray more than once to get corn past the danger stage from Southern rust.
With the combination of a high yielding crop and a high value crop, it is critical for growers to do everything they can to protect the extra corn yield expected for the 2012 crop in the Upper Southeast.
Growers will likely have to make some difficult decisions on the value of the crop versus the cost of fungicides, and possibly multiple applications.
In addition to high value, with corn prices topping $7 in June, this year’s corn crop in one of the highest risk crops ever.
The high seed cost, combined with technology fees and high fertilizer and crop protection costs, make it essential for most growers to get everything possible from their corn crop.