Destroying those volunteer corn plants around San Joaquin Valley fields this fall — and, more importantly, again early next spring — will go far in managing corn leafhopper and the costly corn stunt disease it carries, says a University of California entomologist.
According to Charles Summers, corn ears in harvest residue provide an over-wintering site and reservoir for the bacteria-like microorganism, Spiroplasma kunkelii, the cause of corn stunt.
As its name implies, plants infected with it reach only half their normal height. They also produce multiple ears with blanks and loose kernels. Young leaves at the top of the plant yellow and later turn reddish to reddish-purple, according to variety.
There is no chemical control for the pathogen, blamed for multi-million-dollar losses to corn crops of SJV growers and dairymen.
The lurking Spiroplasma is picked up from volunteer plants later by the corn leafhopper, which easily survives SJV winters on corn, as well as on alternate hosts such as small grains and grassy vegetation.
The leafhopper, the sole vector of the Spiroplasma, also causes damage to the corn on its own by sucking out plant juices and depositing honeydew that causes sooty mold.
“Volunteers carrying the disease might be in or near newly planted alfalfa, fallow fields, cotton fields, or even worse, corn fields in the spring,” Summers said during a recent alfalfa and forage field day at Parlier, Calif.
“There is no way to kill the leafhopper during the winter,” he said, “because that would require spraying every acre of winter forage, and that would be impossible economically and a disaster ecologically.”
Therefore, he added, the only effective approach to management is to break the cycle by destroying corn volunteers that maintain the disease reservoir.
Corn leafhopper, which Summers noted most growers have become aware of in recent years, is tan in color and about 1/8-inch in length. A prominent feature is the pair of dark spots between the eyes. Adults tend to run rapidly across the underside of a leaf. Both adult and nymphal stages prefer to feed inside the whorls of young corn.
“We've done considerable work with the leafhopper and the Spiroplasma and learned about how they over-winter in the valley,” Summers said.
“Corn cobs fall to the ground during harvest and volunteer plants emerge after the first rain. They will remain until we get a hard frost, perhaps in December. The leafhoppers will survive and continue to reproduce as long as corn plants are there.
“But even if we do get a killing frost, many times it doesn't kill the entire plant and the whorl is left intact with leafhoppers protected in it.”
The last couple of years of research have also revealed that the Spiroplasma overwinters deep in the embryo of infected corn kernels.
“This is the first time we've been able to demonstrate that the disease can be soilborne. When the seed germinates in the spring, the disease moves up into the plant as a ready source of infection,” he explained.
Earlier, corn leafhoppers were associated with corn planted late, or after July 1. However, in recent years they have appeared in early-planted corn as well. They go to all corn, silage, grain and sweet varieties.
Corn stunt disease has been detected in the SJV since 1996 and has been confirmed in Fresno, Kern, Kings and Tulare counties. Corn leafhopper has been identified in those counties and several others to the north.
While the disease and its vector are significant threats, Summers said growers may be overly concerned about a group of aphids sometimes seen in field corn but not always a serious problem.
“Normally, in corn, aphids are not a problem that requires any sort of insecticidal intervention, with some exceptions.”
Aphids on leaves should not be alarming, he said, but when heavy infestations of them occur on cobs at pollination time, particularly on silks, their deposits of honeydew can be a problem.
“Feeding aphids are after high levels of amino acids and proteins, and to get them they have to take in far more plant sap than they can utilize, so they excrete carbohydrates as honeydew.”
If the honeydew or the aphids themselves are dense on the silks at pollination, they can interfere with development of kernels in the ears.
Another problem is if they get into tassels at pollination they can interfere with pollen development and shedding.
Occasionally, maize dwarf mosaic disease vectored by several aphid species may be a problem. Infections in corn appear as parallel streaks of discoloration.
The group includes corn leaf aphid, identified by its bluish-green color and relatively angular body. It appears late in the season, in late summer rather than in March or April after planting.
Green peach aphid, generally more associated with tomatoes, squash, melons and other vegetables, can also develop high populations on corn.
The oat bird cherry aphid, spotted by a reddish orange patch, goes to all forages during the winter and early spring before it moves to corn.
A less common species, the potato aphid, can be distinguished by its reddish color and larger size relative to other species.
Green bug is another aphid, identified by its color, a prominent dark stripe across its back, and dark antennae.