Epidemiology of the aster yellows phytoplasma: the influence of non-crop hosts on geographic distribution and movement of the pathogen

This project is directed at enhancing our present understanding of the epidemiology and biogeography of the aster yellows phytoplasma (AYp) with a focus on factors that influence the pathogen’s geographical distribution and spread. Consistent with the objectives originally articulated in the SARE proposal, we developed molecular diagnostic tools to ensure AYp detection was accurate and precise, and characterized the phenology of the aster leafhopper (ALH), at multiple spatial and temporal scales to better understand which habitats surrounding carrot fields have the greatest epidemiological importance as pest or inoculum sources. We found that AYp titer, expressed as Log10 (copies/insect), ranged from 3.53 ( ± 0.07) to 6.26 ( ± 0.11) and increased approximately 100-fold in insects that acquired the AYp. ALH variability among and within years exceeded estimates of variation among farm locations and fields and time covariates explained the largest amount of variation of infectivity. Years in which high ALH abundance occurs co-incidentally with high infectivity can result in 1000-fold greater exposure of the carrot crop to infectious leafhoppers when compared to years in which low leafhopper abundance is co-incident with low infectivity. Similarly, the exposure of the carrot crop to infectious ALHs varies as much as 30-fold throughout the growing season. Thus, without information about insect abundance and infectivity for a specific field, the coincidence of these expected periods of high ALH abundance and infectivity represent a timing interval in which management of the insect could be focused to limit pathogen spread.

Introduction:

In Wisconsin, aster yellows management has focused primarily on controlling the insect vector, the aster leafhopper (ALH), and an AY risk index, known as the aster yellows index (AYI), was developed to describe the maximum allowable numbers of infectious leafhoppers during a discrete time period when plant protection is most needed. The AYI metric is the product of aster leafhopper infectivity, or percent of infectious aster leafhoppers, and the magnitude of the aster leafhopper population. Originally, the AYI was used to make insecticide spray recommendations based on a series of early season leafhopper collections, but following the observations that aster leafhopper abundance and infectivity in and around carrot fields varies, efforts were made to estimate the AYI for specific fields and dates. Contemporary tools (i.e. PCR) are currently used to detect the pathogen in the ALH. However, the relationship between pathogen presence in the vector and the vector’s ability to successfully transmit the pathogen is not known. In turn, many producers avoid risk of pathogen spread by using inexpensive, prophylactic insecticide applications, a management practice that circumvents the utility of the AYI.
Although successful from the perspective of managing insect pests in a cost-effective manner, this approach presents considerable risk, since these insecticides are broad-spectrum compounds with documented mammalian toxicity. The chemicals in this group are also harmful to aquatic organisms, are lipophilic, and in aquatic environments, tend to adsorb to organic sediments. These circumstances have prompted concerns about pyrethroid exposure to non-target areas, especially ecologically sensitive areas such as wetlands, which include the low-land, organic muck soils where the majority of Wisconsin carrot is grown. Thus, it has been our goal to reduce the nearly exclusive reliance on synthetic pyrethroid insecticides for the management of aster yellows in carrot. Specifically, we sought to improve grower adoption of reduced-risk (RR) insecticides by targeting insecticide applications to periods during which AY risk is high thereby reducing the number of applications of the more expensive RR insecticides necessary to control aster yellows improving the cost-efficiency of these newer tools. However, reliable information about ALH abundance and infectivity for specific fields and the coincidence of these expected periods of high insect population sizes and high rates of infectivity were not known.
The primary goal for completing the outlined objectives was to advance our understanding of the epidemiology of aster yellows in Wisconsin towards the development and implementation of a comprehensive disease and pest management plan for carrot. Our approach essentially tried to address the two problems described above by developing the molecular tools to accurately determine the rate of infectious individuals in a field population of ALH and analyzing historical ALH scouting data to identify seasonal trends in factors associated with periods of elevated AY risk.