Warming temperatures are expanding the range of a South American pest that has devastated corn crops in Argentina recently. Researchers at University of Missouri Extension have confirmed the first case of “corn stunt disease,” which is transmitted by the “corn leafhopper,” and follows confirmed cases in Oklahoma and Texas as well.
Corn leafhopper and corn stunt disease are not problems farmers in the US typically have to deal with. Leafhoppers and the corn stunt disease they spread has been a minor issue in the southern part of the US off and on for decades, though limited mainly to Texas and Florida, as well as areas of California.
The corn leafhopper, aka “Dalbulus maidis,” is light tan/yellow in color and about 1/8” long. The characteristic that distinguishes them from other leafhoppers is two dark spots located between their eyes in adults. They also have a smooth head, and the hind margin of the first thoracic segment forms a deeply indented ‘V’ shape. Corn leafhoppers move rapidly within and among corn fields, and they fly or jump away when disturbed. They can be found in shaded areas of corn, resting and feeding in the whorl of young, developing plants, and hidden in the shade of leaves.
The pests inflict damage on corn in two ways – feeding on plants and transmission of pathogens that cause corn stunt disease. Corn leafhopper nymphs and adults feed directly on the corn plant by sucking the nutrients. The insects excrete honeydew as they feed that can lead to black sooty mold, which can impede photosynthetic processes and negatively impact plant health.
Corn leafhoppers are also highly effective transmitters of three pathogens associated with corn stunt disease. The transmission of those pathogens (singly or in combination) has been reported in all corn-producing regions where the corn leafhopper is present. Although the infestation by the corn leafhoppers usually begins at the early crop stages, and the transmission of the pathogens occurs in the vegetative growth stages, 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.
Corn stunt symptoms begin with chlorosis and/or reddening of leaf tips, followed by the production of axillary buds, tillering, plant size reduction, and spike and grain deformation. The resulting disease causes severe stunting, with infected plants only reaching about half their normal height and developing small ears that don’t fill properly.
According to Oklahoma State University Extension Specialists, corn leafhoppers move Northward aided by the wind from Mexico, where corn is produced year-round. Adult insects can overwinter in grasses like wheat, alfalfa, Johnson grass, sorghum, sugar cane, soybean, millet, and gamma grass, but they only reproduce on corn.
Texas A&M AgriLife Extension Service pest management expert David Kerns says large numbers of corn leafhoppers were last seen in 2016 in the lower Rio Grande Valley. They have reappeared to Texas this year and have moved much deeper into the state than what’s been seen before. Kern says disease symptoms have been documented as far north as San Angelo, which is in the center of the state.
In early August, Oklahoma State University Extension documented and confirmed the first reports of corn leafhoppers and corn stunt disease in the state’s Pottawatomie County. And while not yet confirmed, corn leafhoppers and symptoms of corn stunt have been observed in several counties located in the West Central, Central, North Central, and Southwest districts.
A few days after OSU confirmed corn stunt disease, University of Missouri Extension Specialists announced it had confirmed transmission of the disease in Southwest Missouri. MU Extension Agronomist, Tim Schnakenberg was alerted by area farmers and agricultural professionals about symptoms similar to corn stunt in Christian, Stone, Lawrence, and Barton counties in Southwest Missouri.
In Argentina, farmers in the north have been battling the corn leafhopper for decades and have adapted production methods to manage it. However, after the warmest year on record, corn leafhopper populations exploded 10 times greater than usual while the bugs traveled more than 900 miles south of their historical range. The infestation has absolutely devastated corn crops in the southern part of the country this season.
Argentina’s 2023-24 corn crop has plunged from an initial estimate of 56.5 million metric tons to 46.5 MMT, according to the Buenos Aires Grain Exchange. USDA pegs it higher at 50 MMT. Meanwhile, the country’s production is expected to sink even further in 2024-25 with the Exchange estimating farmers will cut the corn sowed area by some -15% in order to prevent a reinfestation of corn leafhoppers. The Rosario Grain Exchange estimates that farmers in central Argentina may reduce corn area by as much as -30%.
There are currently no corn hybrids in the US that resist corn stunt disease due to the typically low occurrence of corn leafhoppers here. According to OSU, the 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. The good news is that corn leafhoppers can’t tolerate temps much below 39 degrees so they shouldn’t be able to survive winter in areas that get a good freeze.
Notably, corn stunt disease incidence will not be reduced by insecticide control. If you see disease symptoms in the field, the plants have already been infected and there is unfortunately no available treatment.
In South American countries, current practices for chemical control are to spray at the sight of a corn leafhopper, and often, a field is sprayed 5-6 times within a season due to reinfestations. However, this persistent use of chemical control is likely a contributing factor to the increasing spread of corn leafhoppers and corn stunt from the South into North America. Per a Corteva Agriscience study, as few as two corn leafhoppers per plant for one day was enough to compromise an entire cornfield. (Sources: OSU, MU PDC, Texas A&M Agrilife, Buenos Aires Grain Exchange)
