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. 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. TOP 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. 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 are therefore directed against ALH adults which then reduce aster yellows infection.