Epidemiology of the aster yellows phytoplasma: the influence of non-crop hosts on geographic distribution and movement of the pathogen
Our survey of Wisconsin carrot fields revealed the presence of multiple aster yellows phytoplasma (AYp) strains among locations in the landscape. These results begin to illustrate the relative contribution and importance of local sources of inoculum. Moreover, these data enable us to evaluate the epidemiological importance of off-crop habitats on the distribution and spread of the pathogen and potentially determine the relative importance of in-bound inoculum in migratory insects. Furthermore, our goal will be to determine if AYp genetic variability relates to either prevalence or infectivity potential of the pathogen, and/or unique pathotypes. An improved characterization of the strain composition in Wisconsin carrot is warranted to improve our understanding of the epidemiology of aster yellows.
The primary focus of this project is to:
1) accurately identify primary inoculum sources of AYp of greatest epidemiological significance in non-crop habitats surrounding carrot fields, and
2) to compare the genetic structure of the population of AYp from reservoir hosts to that within carrot and to determine if genotype variability relates to either prevalence or infectivity potential of the pathogen.
Expected short-term outcomes:
Additional research planned for 2009 and 2010 includes a more comprehensive sampling of non-crop habitats outside of AYp-affected field studies. At each study site, we will quantify:
1) plant species abundance in non-crop areas,
2) AYp prevalence in non-crop plant species,
3) vector phenology in crop and in non-crop areas, and
4) disease progress and seasonal incidence of AYp in affected carrot. From our sampling at each study site we will characterize AYp variability in non-crop hosts and within affected carrot. We have also established a collection of AYp isolates that are currently maintained in planta and will be used to measure leafhopper transmission efficiency.
In the summer of 2008, we conducted preliminary studies to determine and characterize the range of AYp strains present in the Wisconsin carrot crop and also to aid in refining our methodologies for use in future studies.
• Our survey revealed significant diversity in the population structure of AYp present in Wisconsin carrot fields.
• The relative abundance of AYp strain types extracted from carrot varied significantly by location.
• Primer-typing of AYp isolates further indicated considerable variation in AYp genotypes present in carrot fields associated with specific symptoms present in those fields.
• We are currently attempting to determine if the AYp genotype profiles in affected carrot change throughout the growing season. A profile change could be indicative that the aster leafhopper is selectively transmitting predominant AYp strains or that the sources of AYp or inhabiting leafhopper populations change throughout the growing season.
Impacts and Contributions/Outcomes
Specific contributions include:
1) Research presentation and proceedings summary submitted at the 2008 Midwest Food Processors Associated, Inc. Convention and 82nd Processing Crops Conference, Madison, WI, 10-11 December 2008.
2) Research presentation and proceedings submitted to the Wisconsin Muck Farmers Association, Annual Conference, Stevens Point, WI, 3-5 February 2009.
Potential long-term impacts include:
These data will provide a basis to assess a field’s aster yellows risk in terms of the inoculum potential and leafhopper population sizes in field and field edges. Ultimately, this will provide accurate, new information about the relative importance of AYp sources in the habitat surrounding carrot fields so that management tactics can be developed and adopted to decrease the accumulation and persistence of the pathogen near susceptible carrot fields.