B. Lenzen and W. D. Hutchison
Department of Entomology, University of Minnesota
Introduction
The aster leafhopper (ALH), Macrosteles quadrilineatus, is a consistent problem for carrot growers in the upper Midwest because it serves as a vector for the mycoplasma-like pathogen aster yellows.
Biology & Life Cycle
Migration of ALH (and the potato leafhopper, Empoasca fabae) occurs via transport on northern jet streams created by high pressure systems developing east of the Mississippi, converging with low pressure systems from the western states. Several weeks after the arrival of the ALH adults, an additional (minor) source of the annual ALH population hatches from overwintering ALH eggs found in winter wheat or other grass species. Five nymphal stages are completed in about 2 weeks.
Primary susceptible vegetables that serve as hosts include: lettuce, celery, carrots, endice, and parsnip. During early to mid summer, as winter grasses senesce, newly emerged adult ALH move to these crops, as well as more succulent spring grain, weed hosts, and susceptible vegetable, ornamental and field crops. Common weed hosts for aster yellows include: thistle, fleabane, wild lettuce, sow thistle, chicory, wild carrot, galinsoga, dandelion, plantain, cinquefoil and others. Overall, 150 species of plants in 40 different families have been recorded as hosts of aster yellows vectored by the ALH.
To acquire and transmit the aster yellow mycoplasma, ALH must feed for a prolonged period on an infected host (either locally or in a southern state). Next, the pathogen must incubate within the leafhopper for about 3 weeks before it can be transmitted to another plant. Because of the extensive incubation period, the disease is rarely spread from plant to plant within a commercial field. Thus, the primary method for transmission of aster yellows to a host in Minnesota is by the migrant adults already carrying the pathogen.
Damage
Aster yellows symptoms in carrots include yellowing of younger leaves, progressing to red or purple discoloration, dwarfed and twisted petioles, and a dense growth of shoots. Primary damage is to the carrot itself. Infected carrots are slender, elongated, and covered with a dense hairy growth of secondary roots. The taste of the carrot is bitter, which may be attributed to an increase in the phytoalexin, isocoumarin. Any damage to the carrot root can result in unmarketable produce.
Management
Control programs for aster yellows include manipulation of both host plants (crop or weed species) and the aster leafhopper population. Except for the use of resistant varieties, there are no practical ways to control the pathogen directly, at least in large-scale commercial production. Most control programs consist of insecticides directed against ALH adults which then reduce aster yellows infection. To ensure proper use of insecticides, refer to the most recent edition of the Midwest Vegetable Production Guide. Timing is more important than the rate, which can only be fine-tuned by weekly monitoring of the crop, and applying insecticide only when ALH is present.
In addition to chemical control and resistant varieties, removal or reduction of infected carrot and lettuce plants, and potential weed hosts (listed above) will also aid in minimizing aster yellows damage.
An aster yellows index (AYI) was developed at the University of Wisconsin to incorporate ALH infestation level (based on sampling) and varietal tolerance to aster yellows in carrots. This system also accounts for differences among 3 major vegetable crops (carrot=most tolerant, to celery, and lettuce=least tolerant). In addition, over 5 years of data are now available for many carrot varieties grown in Minnesota (Dr. Dave Wildung). Information about the AYI system and Minnesota data are available in the Midwest Vegetable Production Guide. This guide, updated annually, includes information about sampling ALH and action thresholds that can be used for tolerant and susceptible carrot varieties.
References
Midwest Vegetable Production Guide
Cornell University. 2003. Vegetable Disease ID and Management http://vegetablemdonline.ppath.cornell.edu/
Ch.15 of "Vegetable Insect Management with Emphasis on the Midwest" Meister Publishing Co. Willoughby, Ohio.