October 2, 2003
From: Western Farm Press, 17 Sep 2003 Online Exclusive [edited]

Disease vectoring corn leafhopper again threatens corn crop in California

As the corn leafhopper continues to spread the corn stunt pathogen to California's Central Valley, several evaluated insecticides have only a short-term effect, suggesting that other measures, including a corn-free period, have more potential for success.

Charles G. Summers, University of California entomologist at the Kearney Research and Education Center, Parlier, says a corn-free period, from Oct. 31 to April 1, would help in management of the leafhopper.

The most important step for the time being is for growers to destroy  volunteer corn plants that provide the insect, capable of overwintering in the valley, a bridge to infest the next crop.

Several commonly used insecticides provided control in his trials, but only for relatively brief periods ranging from 10 - 30 days. Developing plant resistance to the corn stunt pathogen is another potential defense but will take time.

Taking yet another approach, Summers and others found in preliminary - but as yet inconclusive - studies that corn plants grown over a mulch of wheat straw for a month after planting had reduced populations of corn leafhoppers. Mulch trials also showed one-third the incidence of corn stunt disease and 25 percent more yield compared to plants on bare soil. Summers suggests that the mulch might be used to repel aphids and whiteflies, as well as leafhoppers.

Summers and a team of several farm advisors and UC specialists surveyed valley counties with sticky traps and D-vac sweeps in 2002 and confirmed the leafhopper can successfully overwinter in these counties. Observations are continuing this year.

According to Summers, while leafhopper damage can impair yield and quality, the disease poses the greater threat.

Corn stunt is caused by a bacterial organism, _Spiroplasma kunkelii_ [Sk]. Beyond stunting of plants, it causes production of multiple ears that fail to fill. In the fall, leaves in the upper portion of infected plants take on a reddish color.

The corn leafhopper [_Dalbulus maidis_] is common throughout the southeastern and southwestern U.S. Brownish or tan in color and less than one-eighth of an inch long, it is the only vector of corn stunt disease in California.

Summers said the leafhopper, or something very like it, was first reported in Fresno and Tulare counties in 1942 and a disease fitting the description of S. kunkelii was recorded a few years later.

Corn stunt is now found from Los Angeles County to Yolo County. Outbreaks once lasted only 1 or 2 years and were blamed on leafhoppers migrating from Mexico, but since 1996 they have been annual events with increasing severity in the southern San Joaquin Valley.

The disease formerly was seen mostly in corn planted after July but later showed up in fields planted in April or May. "This suggests to me that both the insect and the disease were overwintering in the San Joaquin Valley," Summers said.

The leafhopper has no diapause period and during winter cold it is inactive, only to resume flight when temperatures reach about 55 F. Leafhoppers are present from October Into March on riparian areas, alfalfa, weeds and triticale.

Summers said a "real key" to managing the leafhopper is destroying volunteer corn that provides the bridge between seasons. After corn has been cut, thousands of volunteer seedlings emerge, serving as hosts for the insect vector over the winter. In August and September, as chopping intensifies, younger fields catch the full impact of the leafhopper movement.

Even though some corn was frozen back, significant numbers found shelter enough in remaining intact whorls. They were discovered in Kings County in February on residue ears that had started to sprout.

Global imaging system surveys analyzed by University of California scientists in 2001-2002 showed heaviest and earliest infestations around the Lemoore area of Kings County, confirming earlier observations on the ground.

However, the same surveys from 2002 through July of 2003 identified a shift of hot spots of leafhopper populations to Tulare County as pressure continued in Kings County.

Tulare County farm advisor Carol Frate said corn stunt appears to be scattered throughout the county and she expects that since much of the acreage is late corn, the problem will worsen by the end of the season.

"It's been increasing since 1996-97 and some fields had a 50 percent reduction in yield. We can see the insect in corn but we don't know what the loss will be. Unfortunately, we have a lot of late corn this year, and those fields are most at risk," she said.

[Byline: Dan Bryant]