Investigating the role of plant hosts in the outbreaks of the aster leafhopper vectored Aster Yellows (AY)
Date: January 2022
Term: 4 years, ending January 2022
Status: Completed
Researcher(s): Sean Prager, University of Saskatchewan; Tyler Wist, Chrystel Olivier, AAFC
SaskCanola Investment: $256,220
Total Project Cost: $256,220
Funding Partners: N/A
Grower Benefits
When given a choice between a crop (including canola and three cereals) and a non-crop (weedy plants common in field margins), aster leafhoppers preferred crop plants over weedy plants and this preference likely drives their movement into crops from field margins once crops have emerged. Leafhoppers preferred barley over wheat, and cereal crops in general over canola.
Aster Yellows Phytoplasma (AYp) infection status of aster leafhoppers only influenced one of the studied preferences, indicating that infection with the phytoplasma likely does not have a strong effect on the behavior or reproductive potential of the vector insect on the tested plants.
Canola was found to have higher phytoplasma levels than Arabidopsis and barley. This suggests that it is more susceptible to AY infection than other species; however, higher phytoplasma levels does not always translate to more symptoms.
Canola and sowthistle were poor reproductive hosts for aster leafhoppers, yet sowthistle was one of the plants identified as an AYp reservoir. Cereal plants and fleabane were very suitable for aster leafhopper oviposition and nymphal development. This suggests that prior to crop emergence, a plant species such as fleabane could act as a suitable reproductive and food choice for aster leafhoppers. However, in the presence of an additional plant species, fleabane and other weedy species are less preferred (for settling, probing, and ovipositing), indicating that little reproduction is happening on the dicotyledonous weeds in the field margins.
Each year, the percentage of migrant leafhoppers arriving infected with AYp was very low, but several perennial and biennial plants harbored AYp infections in the spring and summer and are likely acting as reservoir plants for AYp infection of aster leafhoppers (alfalfa, sowthistle, stinkweed, dandelion).
Several of the potential reservoir plants (such as dandelion and sowthistle) were tested in no-choice and two-choice assays. The presence of stylet sheaths and the settling of leafhoppers on these plant species strongly suggest that dandelion and sowthistle can be part of the AY disease cycle.
Project Summary
Aster leafhoppers are the main vector of Aster Yellows Phytoplasma (AYp) in the Canadian Prairies. AYp causes Aster Yellows (AY) disease in many plant species but can greatly affect canola in outbreak years. These insects migrate to Saskatchewan each spring on winds originating in the southern United States. Researchers wanted to find out more about what they do when they arrive, as their arrival date precedes the emergence of suitable crops in cultivated fields. Researchers wanted to know more about the leafhoppers’ food and reproductive host preferences on non-crop plants and compare plant preferences to know which plants sustained AY before leafhoppers moved into crops. Researchers also wanted to look at whether migrating leafhoppers were already infected with AYp or if they acquired AYp infection from local plants after arrival.
Several crop and non-crop plants were used in two types of bioassays- no-choice (Figure 1) and two-choice (Figure 2). Wheat, oat, barley, canola, spiny annual sowthistle, dandelion, fleabane, marigold, and Arabidopsis thaliana were used. For the no-choice bioassays, three pairs of either AY-infected or AY-uninfected leafhoppers were caged onto a plant and allowed to reproduce for one week. Growth indexes (GIs), the proportion of individual insects that survived to adulthood, were calculated for each plant species. The number of nymphs and adults were counted, and the adult sex was determined. Females were dissected to determine how many eggs they contained. The GIs showed that they are similar between uninfected and AY-infected leafhoppers, that canola and sowthistle are unsuitable reproductive hosts compared to barley, and the development of the leafhopper nymphs reared on Arabidopsis, barley, dandelion, fleabane, marigold, oat, and wheat is similar. When looking at the number of individuals, adult sex and female egg load, the results suggested that different plant species are not equally preferred for oviposition and in some cases, oviposition behavior may change depending on whether the leafhopper is infected with AY. Overall, more offspring were observed on barley, fleabane, oat and wheat, suggesting that these plants are more suitable for oviposition. Adult sex ratios were similar among all treatments. In Arabidopsis and barley, a higher egg load was observed in AY-infected compared to the AY-uninfected females. In fleabane, oat, and wheat, a similar egg load was observed when comparing both treatments.
Figure 1. No-choice bioassay with different plant species (Fb= fleabane, Ca= canola, Ba= barley).
Figure 2. Two-choice bioassay. The asterisk indicates where the leafhoppers were initially released.
For the two-choice bioassays, ten pairs of leafhopper adults were released inside a large mesh cage containing two test plants at either end. Similar procedures to the no-choice bioassays were followed for the two-choice bioassays. Additionally, plants were analyzed for presence of stylet sheaths and eggs. Stylet sheaths are structures produced by piercing-sucking insects during feeding and can help determine feeding activity. In most crop-crop combinations, similar numbers of leafhoppers were observed on both plant species. In combinations where canola was paired with oats or wheat, there was a preference for settling on cereals over canola. The same preferences were observed with stylet sheaths, which suggests that for these crop-crop plant combinations, settling and probing behavior are associated with one another unlike that of crop vs non-crop. Canola was less preferred for oviposition in the no-choice biosassay but was preferred for settling over several non-crop species. The findings suggest that the plant species preferred for settling, may not always be the most suitable host for oviposition and nymphal development. For AY-uninfected leafhopper behavior, fewer eggs were found on canola when it was presented with either wheat or oat, but that was not the case when it was paired with barley. When the researchers looked at stylet sheaths and egg counts, there was similar probing and oviposition behavior observed in the AY-infected and AY-uninfected leafhoppers in most plant combinations.
Figure 3. Canola plants and disease development over 2-, 4- and 5-weeks following infection.
Disease development was evaluated on different plant species by caging ten leafhoppers for one week. Plants were maintained in growth chambers until AY symptoms were seen. Leaf samples were taken at 2, 4, and 5 weeks following the infection period to test for AY infection. Disease development was evaluated in Arabidopsis, barley, canola, dandelion and wheat and there were symptom differences observed between these species following AY infection. In canola, yellowing and malformation of buds was observed at 2 weeks post infection, while reddening and signs of phyllody (abnormal development of floral parts into leafy structures) were observed at 4- and 5-weeks post infection. Canola was found to have higher phytoplasma levels than Arabidopsis and barley, which suggests that it is more susceptible to AY infection than the other species. A comparison of Arabidopsis, barley, canola, dandelion, sowthistle and wheat showed that barley, canola and Arabidopsis were more susceptible to infection with AYp than dandelion, sowthistle and wheat.
Field surveys were conducted through each growing season of the project. The number of sites varied each year as did the type of survey. Plants surveyed include those from fields and field margins suspected of having AY symptoms, alfalfa reservoirs which were tested for the presence of AYp, and five to six perennial weed species identified as potential hosts. These plant species were visually inspected for the presence of leafhoppers prior to sampling. Insect samples were taken with sweep nets in field margins next to canola and wheat fields. Any leafhoppers that were found were screened for AYp presence. Several of the plants that were identified as potential hosts have similar life cycles- stinkweed, scentless chamomile and sowthistle all germinate into rosette stage plants in the fall, which after overwintering, makes them a potential green bridge for AY. Detection of AYp in some of these plants before the arrival of leafhopper migrants suggests that they may be acting as AY reservoirs. If migratory leafhoppers arrive in the spring when there are no crop plants to act as hosts, these leafhoppers could be acquiring AYp from these reservoir plants near the fields and then moving the infection into crops later. There may be two origins (migrant leafhoppers and local reservoirs) for the AY infections in canola each year.