Jason Jenkins
DTN Crops Editor
JEFFERSON CITY, Mo. (DTN) -- When farmers express interest in growing "short corn," this isn't what they have in mind.
Last week, Extension specialists in Oklahoma and Missouri announced the first confirmed cases of corn stunt disease in their states, marking the farthest north the disease has ever been detected in the United States.
A case of corn stunt was confirmed in Pottawatomie County in central Oklahoma, according to Ashleigh Faris, Oklahoma State University Extension cropping systems entomologist. Faris also reported that symptoms were observed in counties in the state's west-central, central, north-central and southwest districts.
In Missouri, the disease's presence was detected in Christian, Stone, Lawrence and Barton counties in the southwest region of the state, said University of Missouri Extension state entomologist Ivair Valmorbida.
Transmitted to the crop by corn leafhoppers infected with one of three pathogens, the disease literally stunts corn height. Plants struggle to reach 5 feet tall, and ear development is affected, eventually leading to yield loss.
Corn stunt has been previously found in Southern states, including Florida and Texas. In Central and South America, where corn stunt is considered a major disease, losses can be severe. For example, this past growing season in Argentina -- where a record corn harvest had been predicted -- corn stunt led to a significant production decline. While its effect varied by region and corn planting date, yield loss ranged from 5% to 100%, according to an April report from the Rosario Board of Trade.
DISEASED INSECT INVADERS
Aided by the wind, corn leafhoppers move northward from Mexico, where corn is in continuous production year-round. Valmorbida said the insects are 1/8-inch long and light tan or yellow in color. They can be distinguished from other leafhoppers by two dark spots located between the eyes of adult insects. These dark spots are visible using a 10X hand lens. Corn leafhopper nymphs are green to tan in color and have no wings.
They move rapidly within and among cornfields and fly or jump away when disturbed. They can be found in shaded areas of corn, resting and feeding in the whorl of young plants and hidden on the underside of leaves.
Corn leafhoppers affect corn health and yield in two ways. First, they feed on leaves by sucking plant sap. Corn leaves may appear shiny because the leafhoppers excrete honeydew as they feed. The honeydew can lead to black sooty mold, which impedes photosynthetic processes and negatively affects plant health.
Second, corn leafhoppers can be infected with one of three pathogens associated with corn stunt disease. These include both bacteria, corn stunt spiroplasma and maize bushy stunt phytoplasma, and a virus, maize rayado fino virus, Faris explained. She said that while corn leafhopper infestation usually begins at the early crop stages, transmission of the pathogens occurs in the vegetative growth stages. However, symptoms of corn stunt disease only appear later in the season, generally 30 days after leafhopper feeding, when corn has already reached the reproductive stages.
"In Oklahoma, we have observed corn plants that show reddening to purple streaks, red midribs and stunted growth due to the shortening of internodes," she said. "The fields where we first found the corn leafhoppers and symptoms of corn stunt disease had reached physiological maturity, so the incidence and severity of the disease could not be assessed.
"We have since found fields of later-planted corn that also contain corn leafhoppers," she added. "We are continuing to monitor these fields for symptoms of disease. Right now, we are in the process of working backward to try and estimate when the reported infections occurred."
Both Faris and Valmorbida said that corn stunt symptoms were not limited to field borders; the disease was present within the fields. Neither was able to estimate what effect corn stunt might have on yields in their respective states.
"There are a lot of variables to consider, including, but not limited to, the timing of planting, growth stage at time of infection and environmental factors," Faris said. "By continuing to monitor the late-planted cornfields, we may be able to make a yield estimate once the growing season ends."
"It is also important to note that the extent of symptom expression and impact on yield are heavily influenced by corn hybrid," Valmorbida added.
Corn stunt disease is not reduced by insecticide, Faris said. If disease symptoms are observed in the field, the plants have already been infected. Her best suggestion for managing corn leafhopper damage is "planting corn early in the season to allow corn plants to develop and mature before the insect vector and pathogen arrive in the system and impact yield."
While corn leafhoppers only reproduce on corn, they are known to overwinter in grasses such as wheat, alfalfa, Johnson grass, sorghum, sugar cane, soybean, millet and gamma grass. It is commonly thought that these leafhoppers cannot survive Missouri winters, but Valmorbida and other Extension specialists plan to continue monitoring for the pest throughout the winter in southwest Missouri.
The University of Missouri Plant Diagnostic Clinic can help diagnose suspect samples for a small fee. Ideally, samples will include stalks and leaves. For more information, go here: https://extension.missouri.edu/….
Jason Jenkins can be reached at jason.jenkins@dtn.com
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