COLUMBUS, Ohio — While all attention turns to soybean rust, a long forgotten issue is slowly creeping its way back into crop fields. And Ohio State University agricultural specialists say it's time growers start to take notice.
Corn diseases, such as northern corn leaf blight and diplodia ear rot — tucked away in the subconscious due to decades of effective control — are finding their way back into Ohio fields, mainly because some current hybrids lack resistance. And the re-emergence of these diseases has researchers concerned.
"This issue is concerning us because people are currently pre-occupied with soybean rust," said Peter Thomison, an Ohio State Extension agronomist. "High yielding hybrids are such a focus now that selecting for effective resistance to these diseases has been de-emphasized in the breeding process, and now they are showing up uncontrolled in certain fields. These are real issues that we hope seed companies and growers will start to address."
Northern corn leaf blight is the researchers' biggest concern.
Identifiable by cigar-shaped lesions on the leaves, the leaf disease can cause upwards of 40-50 bushel-per-acre yield losses in the most severe cases, as well as affect the feeding quality of the grain. Once a huge problem in the early 1900s, northern corn leaf blight was brought under control with the development of both partial resistance and a race specific resistance gene, called Ht.
"The incorporation of both types of resistance turned out to be a very usable, broad band type of resistance that became very effective in the United States during the ‘60s and ‘70s," said Pat Lipps, an Ohio State research plant pathologist with the Ohio Agricultural Research and Development Center in Wooster, Ohio.
By the early 1980s, however, the fungus had developed a new race, called Race 1, which was causing susceptible lesions on hybrids with the Ht gene. To counteract this, other specific genes, like the Ht2 gene, were evaluated for control of new races.
"Problem is," said Lipps, "is that not many hybrids have been developed with new resistance genes or improved levels of partial resistance, and Race 1 has become more common. So what's happening is hybrids that are relying solely on Ht resistance are now susceptible to the disease. These hybrids have little partial resistance, which, along with Ht resistance, is the best way to control this disease."
The presence of northern corn leaf blight was most evident in cornfields last year. To determine the level of susceptibility in corn hybrids, inoculation tests were conducted on certain hybrids in Ohio State's Ohio Corn Performance Trials.
"We found that two-thirds of 71 hybrids evaluated showed the susceptible lesion type," said Lipps. "What that is telling us is that a majority of the hybrids being used by growers in the state are likely susceptible to the prevalent race of northern leaf blight."
Diplodia ear rot is another disease that has become more aggressive in recent years.
"Diplodia, along with northern corn leaf blight, has now been evident in two years of our research. And in the 15-plus years that I've been at Ohio State, I've never seen it this bad," said Thomison.
Diplodia ear rot causes more cosmetic issues than anything else, but ultimately affects the grain quality and yield.
"What we want growers to start doing is to ask questions. Find out from their seed companies which hybrids they are getting that show resistance to these diseases. They are out there, you just have to ask for them," said Thomison.
Thomison does understand, though, why the disease issue may not be alarming for growers.
"When you get the growing conditions that we've gotten in Ohio the past two years, it's easy to dismiss the diseases and blame any reduction in yields on other factors," said Thomison.
Ohio growers have seen back-to-back years in record corn yields, with some fields pulling in 200 or more bushels per acre. Even with some yields only clocking in at 150 or 180 bushels per acre, the exceptional growing conditions are masking the damage being done by the diseases.
"Perhaps if and when we get a less-than-stellar growing season to express these diseases, it will finally get people's attention," said Thomison.
Until then, researchers continue to question what the potential arrival of soybean rust will do to corn production. "One alternative to soybeans is planting corn after corn instead of using a corn-soybean rotation," said Thomison. "But the downside to that is these diseases just get worse under that type of production management."
Note: Northern corn leaf blight and Diplodia ear rot aren't the only corn diseases becoming more evident in fields. Other diseases researchers have noticed an increase in throughout recent years include Stewart's bacterial wilt, common rust, and gray leaf spot.