In Manitoba, aster yellows (AY) disease, as transmitted by the aster leafhopper, Macrosteles fascifrons, Stal, often results in reduced yield and quality of celery and carrots. The incidence and severity of the disease, which may reach epidemic proportions, are directly related to spring influxes of migrant leafhoppers which represent the major source of disease inoculum. A critical situation thus exists, when large numbers of an efficient vector, a certain percentage of which are persistently infectious, invade an area when many susceptible crops are in the seedling stage. Due to the lack of adequate control programs, replicated field trials were conducted over a period of 3 years at Portage la Prairie. The efficacy of contact spray materials, as compared to several foliar and granular systemic insecticide treatments, for control of the aster leafhopper and aster yellows disease in celery and carrots was assessed... In individual trials, and over the 3 year period, foliar contact sprays, systemic foliar applications and systemic granular in-furrow treatments were found to have increasing orders of efficacy... Linear correlation analysis, of the trial variables, revealed the importance of early seedling protection. Early season leafhopper populations were better correlated with AY incidence at harvest and decreased yield than other variables... The major benefits of effective granular in-furrow treatments are: elimination of a critically timed spray program, early seedling protection, persistent activity and ease of application. The maximum yellows reduction expected as a result of in-furrow treatments is 60-75%.