Biogeography of aster yellows phytoplasma: epidemiological significance of local versus long distance migrants

Each year, Wisconsin carrot growers produce a crop on approximately 4,200 acres grossing over $6 M in total revenues. Unfortunately, carrot remains threatened annually by the occurrence of aster yellows phytoplasma (AYp), which is obligately transmitted by the aster leafhopper (Macrosteles quadrilineatus Forbes). The spring migration of leafhoppers from the Gulf-states to Wisconsin has been documented to influence the potential for aster yellows epidemics in carrot. However, it has been suggested that the dispersal of migrant leafhoppers (after arrival) and locally produced leafhoppers have a greater influence on the likelihood of an AYp epidemic in a given year. In severe years, local AYp sources are thought to greatly contribute to epidemic development in Wisconsin carrot fields. Current control practices utilize insecticide sprays that exclusively target the aster leafhopper. Spray timing is guided by an aster yellows index computed from the proportion of potentially infective leafhoppers present in a field during and after annual spring migrations. Crop scouting and molecular diagnostic tools have decreased the lag between documenting inoculative leafhoppers and prescribed sprays. However, yield losses of 5-20% resulting from AYp are still commonplace. A more comprehensive and sustainable control strategy is warranted and the overall goal of our project is an improved understanding of where leafhoppers acquire the pathogen, when they move into susceptible fields, and when they spread the pathogen to crops. Specifically, our focus has been to compare the genetic structure of the population of AYp collected from long distance, or migrant populations of aster leafhopper to that which is present in affected carrot as well as reservoir hosts to determine if genotype variability relates to either prevalence or infectivity potential of the pathogen.