UC entomologist Charlie Summers and his research team are educating growers from Madera, Sacramento and Yolo counties about corn leafhopper and corn stunt disease in a series of meetings in March.
Corn stunt disease damages crops in a dozen California counties with losses estimated at nearly $55 million in the San Joaquin Valley.
Summers is partnering with seven UC Cooperative Extension farm advisors to stress the importance of keeping newly planted corn fields free of “volunteer corn,” or plants that grow from seed produced by the previous year’s crop, to reduce the spread of corn leafhoppers and corn stunt disease. The project is funded by the UC Statewide Integrated Pest Management Program through its IPM Demonstration Grants Program.
Corn leafhoppers are carriers of the corn stunt spiroplasma, Spiroplasma kunkelii, a bacteria-like organism that causes corn stunt disease.
"Infection with the disease can result in even more significant yield losses than those caused by leafhoppers feeding alone," says Summers.
The entomologist will also teach growers and pesticide control applicators how to sample for the presence of the insect and stunt diseased plants, as well as how to make informed decisions about the need for insecticide treatments. Insecticides, except as a seed treatment, are generally not effective in controlling either the leafhopper or the disease.
In 1996, corn leafhopper populations reached extremely high levels on late-maturing field corn (silage and grain) and sweet corn in Kern, Fresno, Tulare and Kings counties. In addition, many fields had a high incidence of corn stunt disease.
Following the outbreak in 1996, the corn leafhopper and the disease continued to increase annually. The problem continues to become more severe, and leafhopper populations and the incidence of corn stunt disease are now a yearly problem in the southern San Joaquin Valley.
While most damaging in Fresno, King, Kern and Tulare counties, corn leafhoppers have been found in Madera, Merced, Stanislaus, San Joaquin, Sacramento, Yolo, Contra Costa and Solano counties. The spiroplasma has been identified in Kern, Tulare, Kings, Fresno, Stanislaus, San Joaquin and Sacramento counties.
"Damage is most severe on corn harvested late in the season, from September on," says Summers. "Severely infected corn has a much lower nutrient value, and many dairies refuse to accept it to feed their cattle."
In a separate three-year study in 2004 funded through the UC IPM and UC Exotic/Invasive Pests and Diseases research programs, Summers determined that corn leafhoppers could successfully survive over the winter. Volunteer corn is present long after all harvest operations are completed and extends the corn season by as much as two months, thus shortening the period of time the leafhoppers are forced to go without a food source.
"We designed an experiment to determine if the adult leafhopper would feed on winter cereal plants such as wheat, barley, oats, triticale and alfalfa, and we demonstrated that adult leafhoppers fed extensively on all of these crops," explains Summers.
"In a second experiment, we determined that corn leafhopper adults could even survive without access to any food sources, only water, for up to 86 days. Adult leafhoppers can survive for up to 90 days on alfalfa and up to 170 days on triticale and wheat. All of these crops can successfully serve as a winter bridge to carry adult leafhoppers through between corn crops."
Red leaves sometimes signify the presence of the disease. Other commonly observed symptoms include stunted plants, shortened internodes, and multiple corn ears that fail to fully fill with grain.
Summers and his research team will hold classroom and field meetings. Classroom lectures will introduce growers, pesticide control advisors and seed salespersons to the leafhopper and the disease. Information will include identification, biology, sampling, history, and proposed management strategies. Local farm advisors will be on the front line at field meetings to answer questions as the season progresses. The meetings will include hands-on exercises on sampling and identification, as well as discussion about the likely outcome based on the pest populations they are encountering. Meeting locations and times will depend on where and when the corn leafhopper and the disease are found. The team will organize a final fall classroom meeting to discuss the outcome of the season.
Summers will stress the importance of keeping newly planted fields clean of volunteer corn as these plants serve as an early point of buildup for the leafhopper and may also serve as a source of inoculum for the bacteria.
"Kings County has taught us that spraying for the leafhopper to control the disease does not work," says Summers. "We'll stress this point to the growers and PCAs of the northern San Joaquin Valley to reduce the number of insecticide treatments that might be applied to control the disease."
Summers is also developing photo guides for growers, pesticide control applicators and seed company representatives to help identify the leafhopper and symptoms of corn stunt disease.
Spring classroom meetings are this month. Field meetings are planned for June and will continue on a monthly basis until harvest is complete.
In addition to Summers, the scientists are: UCCE farm advisors Carol Frate, Tulare County; Shannon Mueller, Fresno County; Tulio Macedo, Madera County; Mick Canevari, San Joaquin County; Marsha Campbell-Mathews, Stanislaus County; Kent Brittan, Yolo, Solano, and Sacramento counties; and Carol Collar, Kings County.
Meetings are open to the public. For more information, contact Charlie Summers at firstname.lastname@example.org.