WOOSTER, Ohio — Ohio's corn crop may be plowing its way through rapid growth and development this season, but diseases can still get the upper hand if conditions are right.
Pat Lipps, an Ohio State University plant pathologist with the Ohio Agricultural Research and Development Center, said that certain corn leaf diseases have already been detected and could become a serious problem under disease-favorable conditions and poor scouting.
"The corn crop in Ohio is progressing, but there is tremendous variability in the growth stage of the crop from one location to another," said Lipps. "In most of the more southerly counties, pollination was complete by mid July where in many northern counties that had significant rain through May and June many fields were not yet knee high or not yet tasseled."
According to the Ohio Agricultural Statistics Service, 85 percent of the corn crop is silking, 31 percent more than this time last year. Over 60 percent of the crop has been rated in good to excellent condition.
Despite the favorable report, Lipps said that now is the time to begin scouting for leaf diseases, which can greatly reduce yields in susceptible varieties and when the corn crop has yet to tassel.
"Growers should recognize and scout for three particularly important diseases that have been detected in fields already this season: northern leaf blight, common rust and gray leaf spot," said Lipps. "We have had reports of each of these diseases and our previous experience indicates that yield losses can be quite great if they become severe early. In most instances where yield losses have been excessive, the hybrids planted were known to be susceptible to the disease. This is why we always recommend obtaining hybrids with the best resistance available."
Northern leaf blight can be recognized as large spindle-shaped lesions on leaves. The lesions on susceptible hybrids are tan and may have a dark-colored border. The lesions increase in size as they age and spores in the center of the lesions can have a dark olive green-colored appearance.
"Northern leaf blight has caused significant yield losses in Ohio fields over the past two to three years and is a threat to the corn crop again this year," said Lipps.
The disease is most severe under moderate temperatures (65 degrees Fahrenheit to 78 degrees Fahrenheit) and frequent rainfall with prolonged dew. Moisture on the leaf surfaces for six or more hours is required for infection to occur. Lesions develop within seven to 12 days after infection and spores produced in lesions are spread to adjacent leaves by rain splash or wind causing rapid spread in the field. Plants severely affected by leaf blight are frequently prone to stalk lodging problems at harvest.
Common rust is seen as very small, dark rust/red pustules scattered over the leaf surfaces. Spores are carried up from the South and arrive in Ohio by about mid-June to early July. Spores germinate and infect the leaves in about six hours when water is present on leaf surfaces and the temperature is between 60 degrees Fahrenheit and 77 degrees Fahrenheit. Splashing rain, high humidity and moderate temperatures favor disease development
Gray leaf spot lesions are rectangular in shape with straight or parallel sides caused by the inability of the fungus to grow through the major veins of the leaf. Lesions can vary in color, but typically are tan in the center and some may have an orange or yellow border.
"The rectangular shape of the lesions is highly characteristic," said Lipps. "As the leaves age, the lesions may merge resulting in significant leaf damage."
Gray leaf spot development is favored by extended periods of high humidity resulting in heavy dew that keeps the leaves wet for 12 hours or more per day for several days. Lesion development may take two weeks or more depending on daily humidity in the field. The disease is more important in reduced tillage, continuous cornfields.
"Scouting fields now will help identify locations where these diseases are becoming important and will likely help explain lower yields in these fields later in the season. Excessive loss of leaf tissue now not only affects yield but the standability of the crop," said Lipps.