Epidemiology of the aster yellows phytoplasma: the influence of non-crop hosts on geographic distribution and movement of the pathogen
In 2009, we examined plant species abundance in non-crop areas surrounding carrot fields and also determined the prevalence of the aster yellows phytoplasma in select non-crop plant species. Our results begin to document and quantify the existence of aster yellows phytoplasma (AYp) inoculum in close proximity to susceptible vegetable crops in Wisconsin. 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. Our continued goals will be to determine if AYp genetic variability relates to either prevalence or infectivity potential of the pathogen, and/or unique pathotypes.
This project aims 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:
In 2010 we will continue to sample weed species in non-crop habitats surrounding susceptible crop fields. At each study site, we will continue to 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. Studies are on leafhopper transmission efficiency are ongoing.
In the summer of 2009, we measured relative abundance of weed species surrounding 10 carrot fields in Wisconsin. We also, randomly sampled stems of select plant species that have been implicated as weedy hosts from each field edge to be assayed for the presence of the aster yellows phytoplasma. Aster yellows incidence was estimated in the carrot crop.
Results from these studies indicated that members of the grass family were the most commonly found plant species in carrot field edges representing 45% of the cover. In field edges around the state, we found 15 plant species that represented the aster family and those species, on average, accounted for 13% of the cover. To date, a total of 829 plants (35 plant species) from our 2009 sampling have been assayed for the presence of AYp. However, we have only detected the AYp organism in a few select plant species, common horseweed (Conyza canadensis; 4.4%), ragweed (Ambrosia sp.; 1.4%), and prickly lettuce (Lactuca serriola; 1.4%).
Regional variation: Even though we are continuing to assay plants collected in 2009, the initial data suggests that the presence of the phytoplasma organisms might occur in localized “patches” surrounding carrot fields. For example, a single field edge currently accounts for 63% of the total plants testing positive for AYp.
Relationship to disease incidence: In mid-late August 2009, we did not measure AY incidence above 1% along any field edge despite the prevalence of AYp near some fields. It is likely that numerous factors, such as cool weather (and insecticide applications as well) leading to lower leafhopper population sizes, contributed to the lack of disease seen in 2009. In general, insect pressure was low in WI during the summer of 2009.
Impacts and Contributions/Outcomes
Specific contributions include:
1) Research presentations and abstracts submitted to the 2009 American Phytopathological Society’s annual conference, Portland, OR, 1-5 August 2009.
2) Research proceedings submitted to the Great Lakes Fruit, Vegetable and Farm Market EXPO, Grand Rapids, MI, 8-10 Dec. 2009.
3) Research presentation and proceedings submitted to the Wisconsin Muck Farmers Association, Annual Conference, Stevens Point, WI, 2-4 February 2010.
4) Article “Aster yellows index: What is it and how do you use it?” published in Carrot Country, Summer 2010.
Potential long-term impacts include:
As we move forward with this project we will start to calculate a risk index for each field edge by multiplying the relative abundance of a weed species and the estimated prevalence of AYp within that species. This index is a calculation of the AYp inoculum potential of the habitat immediately surrounding a carrot field and might be an indication of the AYp risk in the local environment. The localized nature and small number of plant species that appear to be contributing to local inoculum potential make source reduction a more viable management strategy for aster yellows.