MANHATTAN, Kan. - Small, light brown, circular lesions covering corn leaves this time of year are not a welcome sight.
Southern corn rust, a fungal disease caused by the Puccinia polysora pathogen, begins to show up in Kansas around Aug. 1 each year, according to K-State Research and Extension plant pathologist Doug Jardine. Corn producers, particularly those who farm north of Interstate 70, should evaluate each field for the disease and determine if fungicide treatment is necessary and economical.
Jardine identified southern corn rust in Kansas for the first time in 2013 on Aug. 1 in McPherson County. He said he has since seen it in other north-central Kansas counties including Riley, Clay, Cloud and Washington, but there may be more cases other crop scouts have identified elsewhere.
The disease does not live year-round in Kansas, as it requires a live plant to survive. It lives mainly in southern Texas and northern Mexico, where temperatures allow corn to grow year-round, and it travels northward to the U.S. Corn Belt each summer. Because the disease is currently widespread in south central Nebraska, it would have had to blow over Kansas from those southern areas.
The negative impact of southern corn rust depends heavily on a number of factors:
- Weather forecast;
- Stage of crop development;
- Yield potential of the field;
- Amount of disease in the field; and
- Susceptibility of the hybrid.
Southern rust is favored by hot, humid conditions, which Kansas experienced in early August. However, mid-August temperatures cooled, which Jardine said slowed the spread of southern rust.
Although the recent weather has been a benefit to farmers, the stage of the corn might be a problem.
"The problem this year is corn was planted two to three weeks late," Jardine said. "This is a problem across the entire Corn Belt."
If corn is hit with southern rust and has not yet passed the soft dough stage - 35 days before maturity - it may be beneficial for farmers to spray fungicides. Products containing a triazole mode of action are recommended.
If the corn is already in the denting phase of maturity, Jardine said producers probably won't see an economic benefit to spraying, as the rust likely won't affect the yield too much. He said much of south central Kansas has reached that denting stage of development already and likely won't need to be sprayed.
Jardine said corn producers should evaluate each field separately. If southern rust has hit a field of younger corn and the corn has good yield potential, spraying could be economical.
"There is some good corn this year, and some of it has 200 (bushel per acre) yield potential," Jardine said. "A field with a reasonable amount of rust could have 10 to 15 percent yield loss."
A 10 percent loss on a 200-bushel per acre yield is 20 bushels. If corn is at $6 per bushel, that could mean $120 loss per acre. Therefore, Jardine said, it would be economical to pay $30 per acre to spray that field.
Jardine said the worst-case situation would be to see an epidemic of southern rust early on that is not treated, and producers could see as much as a 30 percent loss. With the current cooler-than-normal August temperatures, though, that is not a likely situation this year.