Mar 19, 2012

Aster Yellows can distort carrot growth

Aster leafhoppers are insect pests of carrots and many other crops grown in the upper Midwest. The majority of the economic loss is associated with these insects’ ability to transmit a bacterial pathogen called the Aster Yellows phytoplasma (AYp).

Infections of AYp result in the disease condition described as Aster Yellows, which distorts plant growth and reduces both raw product quality and yield. Aster leafhoppers can overwinter on weeds or leftover crop residue in Michigan, but they can also migrate in large numbers on weather fronts from the southern United States in midsummer. Therefore, growers can experience high numbers of leafhoppers, which rapidly colonize their crops after substantial summer storms. Similar to its insect vector, AYp can overwinter in weeds in Michigan, and it can be carried into the Midwest with the migrating insects. Thus, both locally produced and migrating leafhoppers influence the level of Aster Yellows infection in the crop.

Synthetic pyrethroids currently represent the backbone of aster leafhopper and Aster Yellows management programs, because they are cost-effective and efficacious. Their drawback is their broad-spectrum activity, their concern for use in wetland areas and their relatively short longevity under field conditions, which mandates frequent re-application to assure that plants are protected. In commercial carrot production, foliar applications are sometimes made prophylactically (weekly), but more often after leafhopper samples have been sent to a diagnostic laboratory to determine the proportion of leafhoppers that carry the AYp. After the proportion has been determined, this number can be compared to an Aster Yellows Index threshold to decide if a foliar insecticide application is needed. To reduce the reliance on pyrethroid foliar applications, new and cost-effective, reduced-risk insecticides may be able to be integrated into carrot pest management in the future.

Researchers at the University of Wisconsin, Madison are testing if an in-furrow application of a neonicotinoid insecticide (thiamethoxam) could provide sufficient control of the leafhopper vector, and ultimately the incidence of Aster Yellows. They found that a single, in-furrow treatment of thiamethoxam provided significantly better leafhopper suppression and Aster Yellows control in carrots when compared to a season- long foliar pyrethroid program. The question remains how long the in-furrow insecticide effect lasts in the growing season, because this will determine the need for subsequent foliar applications; this will be the focus of future investigations. Moreover, further investigations are planned to evaluate the performance of the in-furrow application in muck soils, as well as using seed treatment technology. The potential advantage of such a strategy is not only the elimination of some foliar insecticide applications, but the ability to have consistent insect suppression throughout the early part of the growing season, when carrots are more susceptible to the disease. In-furrow treatments are expected to provide protection for carrots from leafhoppers that overwinter in the Midwest or arrive in our region following spring migrations.

Since both the aster leafhopper and the AYp use a range of Michigan plants as hosts, weeds and crops in or around the field can provide multiple sources for the leafhopper problem. A study done at Michigan State University tested the impact of the presence of different weeds on the numbers of leafhoppers in carrots. They found that grassy weeds and cereal cover crops, such as rye, attract leafhoppers to a carrot field, but the presence of broadleaf weeds does not change the number of leafhoppers in the field. This means that cereals and grassy weeds near the carrots may lead to higher infestations by leafhoppers and potentially higher incidence of Aster Yellows in the field.

By Zsofia Szendrei, Kenneth Frost, Russell Groves, Michigan State University