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Abstract
Aster yellows (AY), a disease caused by phytoplasmas, is a limiting factor to carrot production in Manitoba, Canada. The pathogen is transmitted by insect vectors, Macrosteles fascifrons (leafhoppers), and causes serious losses in many other crops. Traditional methods for estimating AY incidence in carrots and the proportion of AY-vector insects are based on visual symptoms and on bioassays of leafhopper populations. In the present study, techniques based on polymerase chain reaction (PCR) were adapted for the examination of insect and carrot tissues. A more accurate determination of infected plants and of virulent leafhoppers was achieved over two growing seasons. Surveys were carried out in five carrot fields in southern Manitoba throughout the 2001 and 2002 growing seasons. Large numbers of leafhoppers were captured weekly, and both direct and nested PCR were performed on individual samples in a population of more than 1000 insects and on 250 carrot samples as well as on several potential hosts around the carrot fields. According to PCR testing, visual assessments appear to overestimate the true incidence of AY in carrots. The percentage of leafhoppers carrying phytoplasma differed between the two years (18% and 31% in 2001 and 2002, respectively). However, higher proportions of leafhoppers carrying phytoplasmas were found early in the growing season in both study years. Therefore, implementation of management decisions would be more valuable early in the season, when the presence of the first leafhoppers is confirmed.