Corn seed companies have proven highly successful in screening new hybrids for disease resistance, and combined with chronic low prices of corn in the past, most Southeastern growers rarely sprayed their crop with fungicides.
With corn prices continuing to be well above the 10-year average, more and more growers are looking to fungicides as a way to get an additional yield boost. Whether the extra input correlates to extra dollars at season end depends on a number of variables, according to North Carolina State Plant Pathologist Steve Koenig.
The first key, he contends is to know what disease is — or isn't — the problem. A common corn disease in the upper Southeast is crazy top, which is a disease, but not one that is treatable with fungicides. Corn infected with crazy top has multiple tillers, excessive tassling and shriveled tassles.
Koenig says the disease is most frequently misidentified as herbicide damage. It is a disease most frequently caused by early season flooding. The only way to prevent the disease is to control moisture and/or drainage.
Southern rust and gray rust are the most common corn diseases for which fungicides are used in the upper Southeast. Both these diseases can reduce corn yield significantly enough to merit spraying, especially with corn being more valuable in today's market.
Triazole fungicides are most commonly used to control Southern rust and gray leaf spot in corn. “How well these fungicides work, we can't say, because we have limited research on fungicides in corn, because it was assumed these materials won't pay.
“The corn companies do such a good job of screening and eliminating most diseases through natural resistance that the need for fungicides is a hit and miss situation at best in North Carolina,” Koenig says.
In limited testing conducted by North Carolina State Corn Specialist Ronnie Heiniger, spraying fungicides paid off in about one field out of four. Koenig says tests using two applications of triazole and strobilurin fungicides applied at the R1 and R3 showed no yield difference between treated and untreated in three of the four counties in which the tests were conducted.
In the one county where yield was enhanced, only the R3 application of Tilt, Quadris or Headline provided a 10-12 bushel yield increase.
With a combination of strobilurin and triazole, in this test Stratego or Quilt, yield increases were as high as 20 percent.
It appears, Koenig says, that the fungicides applied at the R3 growth stage kept leaves on the plant longer, which probably accounts for most of the yield increase.
As of early August, Southern rust had been found in Georgia, but had not spread into North Carolina and Virginia. According to Koenig, the disease is found at economic thresholds in North Carolina about one year out of five. The North Carolina State pathologist says growers may want to look at moderately susceptible varieties to Southern rust and gray leafspot, because the varieties with high resistance to these disease may have as much as 30 bushel per acre reduction in yield potential.
“The main reason we had such a big problem with these diseases two or three years ago is that we had a lot 120 day corn planted after June 1, meaning maturity is late, creating more susceptibility to Southern rust. So, from a disease perspective a grower would be better off planting a shorter-season hybrid, but that is likely to cost yield,” Koenig concludes.
Southern rust of corn is caused by the fungus Puccinia polysora. The rust is considered to be tropical or sub-tropical in distribution. It does not over-winter in North Carolina. Spores travel on wind from more tropical regions depending on weather patterns.
The optimum conditions for rust development are temperatures from 80 to 90 degrees F and high humidity. Ordinarily, Southern rust of corn is of little concern to North Carolina growers because spores arrive so late in the season that little or no impact on yield occurs.
The symptoms of Southern rust are orange to brown masses of spores (urediospores) that erupt through the upper leaf surface. Leaves, stalks, and the husks on ears may be infected. Southern rust typically sporulates profusely on the upper leaf surface and only sparsely on the lower leaf surface.
In contrast, common rust (caused by Puccinia sorghi) produces spores on both surfaces, often in streaks.
Gray leaf spot is more common in no-till systems planted year after year to corn and is sporadic in the Southeast. It is caused by the fungus Cercospora zeae-maydis.
The fungus can infect leaf blades and, to a much lesser extent, leaf sheaths. The gray or pale brown lesions are long and narrow with parallel sides delimited by leaf veins. The ends are usually blunt, giving the lesions a long rectangular shape. Lesions commonly are about one quarter inch wide by about 1-inch long.
When the disease is severe, lesions merge into long stripes. Eventually the entire leaf may be killed.
While both gray leaf spot and Southern rust can be yield and profit robbing diseases of corn in the upper Southeast, the jury is still out on whether use of fungicides will enhance profitability.
In some cases, Koenig says, big yield increases have been documented in North Carolina, but when these increases will occur has not been verified by research.