DNA test helps growers identify virus-carrying leafhoppers before they infect vegetables
By Marni Katz
The aster leafhopper rarely is a vegetable pest. But when it carries the aster yellows disease, it can be devastatingespecially to susceptible crops such as lettuce, celery and carrots.
The good news is that the pest’s unique feeding habits lend themselves to integrated pest management and provide several treatment opportunities.
Aster yellows is caused by a phytoplasma, a plant-killing microorganism similar to a bacterium. A phytoplasma is not carried by soil, air or water; only the aster leafhopper can transmit infections into susceptible crop plants through prolonged feeding.
The aster leafhopper is a problem only when it shows up in significant numbers and is infected with the aster yellows microorganism. Whereas growers once guarded themselves against aster yellows by automatically spraying whenever leafhoppers appeared, today’s technology allows them to treat only when the pests are infected at rates high enough to put crops at risk.
An unusual relationship
The relationship between aster yellows and the aster leafhopper is somewhat unusual, says Edward Grafius, a Michigan State University Extension entomologist at Michigan State University in East Lansing.
While some viral diseasesare transmitted almost instantly when an insect begins feeding on a plant, it takes prolonged feeding periods before the leafhopper can pick up and transmit the phytoplasma from one plant to another.
“To pick up the phytoplasma from an infected host plant, the leafhopper must feed for up to six to eight hours on that host,” Grafius says. “Then the disease has to multiply inside the leafhopper’s body for two or three weeks before the leafhopper is able to transmit it.”
That means growers have an opportunity to interrupt infection by controlling infected leafhoppers. It also means typical disease parameters, including field history or environmental conditions, are not factors from one year to the next.
“Because they are migratory, aster leafhoppers arrive in different numbers from one year to the next,” Grafius says. “It depends on the weather systems. Some years the winds coming in create a lot of trouble with aster yellows in vegetable crops. Other years, different storm systems may not.”
Aster leafhoppers migrate in late spring from the Gulf of Mexico into the Great Lakes plains region, often carrying aster yellows phytoplasmas. Adult leafhoppers, which are narrow, pale-green, wedge-shaped insects up to one-eighth of an inch long, feed on the underside of leaves and hop or fly when they are poked.
Not a significant Western problem
While aster yellows has become a significant disease in the easternplains area because of leafhopper migration patterns and available host crops, it is seldom a problem in California’s coastal vegetable production areas, says Davis-based University of California Extension plant pathologist Mike Davis.
The disease sometimes shows up in isolated incidents in the Salinas Valley, but there is seldom enough economic damage to warrant treatment other than to prevent leafhopper feeding damage, Davis says.
Distorted plants and sometimes death
Throughout the Northeast, and particularly in the Great Lakes plains, aster yellows is a common and often significant problem, says Sally Miller, an Ohio State University plant pathologist in Wooster.
Lettuce is particularly susceptible, and there are no known resistant varieties. Symptoms range from mild plant distortion to plant death. Infected leaves typically are small, narrow and distorted.
“Lettuce may show symptoms of premature bolting or witches broom, like a hormonal dysfunction in which plants are twisted, discolored and stunted,” Miller says.
In infected celery, petioles also are distorted and twisted, and infected carrots typically have roots with a bitter taste and root hairs.
“If the plant is infected early, symptoms will be much worse, and it is more likely to die than if it is infected later,” Miller says.
Monitor, then treat if needed
Growers such as Bruce Buurma, president of Buurma Farms Inc. in Willard, Ohio, monitor early for leafhopper migration.
“Aster yellows is something that’s been around for a long, long time,” says Buurma, who farms lettuce and 40 different vegetable crops in north-central Ohio and Michigan.
Growers can’t treat the disease, but they can control the pest that carries it. Buurma says the severity of the problem depends largely on Gulf of Mexico storm patterns, which often carry leafhoppers infected with the aster yellows phytoplasma north into his lettuce fields.
“In the past when we had leafhoppers in good numbers, we automatically sprayed because we couldn’t tell whether the leafhopper had aster yellows or not,” Buurma says.
Now Buurma sweeps his fields regularly and sends leafhoppers to Ohio State University laboratories, where a DNA assay can determine whether the percent of insects infected with aster yellows is high enough to warrant treatment.
“If there is any kind of positive reading, growers will typically want to start spraying,” Miller says. “If what comes in in late May or early June is positive and leafhopper numbers are high, growers will usually spray in lettuce, because if they don’t they can get pretty significant infestation. If growers control them early, it helps a lot.”
Buurma says the DNA-based system has provided more information to help him manage leafhoppers within an integrated pest management program.
“Within three days time, they report back whether the leafhoppers are infected or not, so it’s become an integrated pest management situation where we don’t spray for leafhoppers any more unless they have aster yellows,” Burma says.
When infection levels reach more than 5 percent, he typically sprays every seven to 10 days, using a synthetic pyrethroid specific to beetles and leafhoppers but safe on beneficial insects. Eliminating unnecessary sprays for aster leafhoppers can save up to $15 per acre in materials and application costs, Buurma says.
Aster yellows index
The aster yellows index, developed by the University of Wisconsin, can be used to determine treatment thresholds for aster leafhoppers, which spread the disease.
Although growers can't treat plants once they are infected with the viral disease, they can reduce the potential for disease infection by reducing the number of leafhoppers in a field.
The index is based on the number of leafhoppers per 100 samples multiplied by percent infection. If, for example, growers collected 30 leafhoppers per 100 sweeps with 2 percent infection, the value is 60.
Wisconsin has assigned treatment threshold index numbers of 25 for lettuce, 35 for celery and 50 for susceptible carrot varieties. Less susceptible carrot varieties can get by with a threshold of 75.