Carrot Country

MSU Researcher Discusses Aster Leafhopper Infectivity Testing Program

Fall 2006
Carrot Country

By Beth A. Bishop
Michigan State University

Aster yellows affects many different crops, including carrots. Symptoms of the disease in carrots include distorted growth and yellowing of foliage, and “hairy” roots that have a bitter taste. If infected young, plants simply die.

Aster yellows are spread by aster leafhoppers, a pest of many different plants. Aster leafhoppers acquire aster yellows after feeding on an infected plant. For the first several weeks thereafter, the pathogen multiplies in the leafhopper’s body. After this initial period, the leafhopper, when feeding, is able to infect other plants. A leafhopper with aster yellows remains infected for the rest of its short life.

Adult aster leafhoppers are not able to survive in Michigan winters. Some aster leafhoppers may overwinter as eggs in the Great Lakes region. Since they hatch uninfected, these “native” leafhoppers acquire the disease through feeding. Many more leafhoppers migrate into the area in the spring of each year. Migrants may or may be infected, depending on their feeding history before migrating. Thus, the proportion of aster leafhoppers carrying aster yellows can vary considerably from year to year, location to location and over the season.

Since the percent infectivity influences treatment threshold so strongly, having an accurate estimate is crucial to adequate aster yellows control. Since 2001, we have been testing aster leafhoppers to determine infectivity rates. Besides informing growers so they can make treatment decisions, our results provide a base of information about how the infectivity rate varies. In 2005, Diagnostic Services began testing aster leafhoppers. Leafhoppers from 18 different locations in Michigan were collected by crop consultants and extension personnel and were sent to Diagnostic Services. Leafhoppers from each site were
collected on multiple dates—from two to 17, depending on the site. Results were provided to growers, scouts and consultants.

There was large variation in percent infectivity between different geographic areas. Early in the season (June), leafhoppers collected from carrot fi elds in multiple locations in west central Michigan had a relatively high infectivity rate (10 to 15 percent). During that same time, leafhoppers collected from celery fi elds in southwest Michigan had extremely low infectivity rates (less than 0.5 percent).

Infectivity rates also varied between locations in the same geographical area. For example, infectivity rates of leafhoppers collected from carrot fi elds in west central Michigan during the last part of June varied from zero to 12 percent, depending on location.

The infectivity rate of leafhoppers collected from a single location varied considerably over the season. The seasonal pattern in infectivity rate was different in different locations.

Results of aster leafhopper testing in 2005 agree with previous years. Since the infectivity rate is dependent on so many factors (how many infected plants are available for leafhoppers to feed on, how many leafhoppers are present, how many migrating leafhoppers are already infected, etc.), this crucial piece of information can change over the season and different between locations and years. Testing aster leafhoppers to determine aster yellows infectivity rate provides useful information to growers making treatment decisions.

Editor’s note: This presentation was part of the Great Lakes Expo carrot session, held last December in Grand Rapids, Mich. Beth A. Bishop can be contacted at the Department of Entomology, Michigan State University, (517) 355-5154, bishopb@msu.edu.