Aster yellows: Seed treatment benefit; rating damage

Key result: The study found that seed treatment can reduce leafhopper feeding and therefore suppress aster yellows (AY). It also produced a five-point rating scale to score canola plants for AY damage and help predict
yield loss.

Project title, Principal investigators: “Seed treatments as an alternative method of controlling leafhoppers and aster yellows disease in canola,” Bob Elliott and Chrystel Olivier, Agriculture and Agri-Food Canada, Saskatoon

Funding: GF2, ACIDF (WGRF and Alberta Canola) and private industry

Aster yellows (AY) is caused by a phytoplasma that infects vascular tissue in the leaves, stems and roots of B. napus. Leafhoppers feeding on canola transmit this disease-causing phytoplasma. AY caused major production losses to canola in western Canada in 2000, 2007 and 2012. In 2012, the disease was found in 77 per cent of the canola fields surveyed, with yield losses estimated to average 10 per cent.

The experience in 2012 inspired this project, which had two parts: (1) Study the influence of leafhopper feeding densities on phytoplasma levels and symptoms in hybrid canola plants, as well as yield and 1,000-seed weight in dry and wet soil. (2) Evaluate seed treatments for control of aster leafhoppers, which transmit disease-causing phytoplasma.

Part 1 results

Investigators compared infection in dry and wet soil conditions, and under a wide range of leafhopper population densities. They also observed a wide range of symptoms. With their results, investigators came up with two valuable management recommendations.

(1) Not all leafhoppers are feeding, and the percentage not feeding tends to be higher in dry conditions. Therefore plant inspections to estimate leafhopper feeding densities will provide a more accurate indication of potential AY infection than estimates based on sweep nets.

(2) Investigators produced a five-point rating scale to identify AY symptoms more accurately in the field. (See the table.) The scale shows a wide range of symptoms associated with AY. As the study found, plants with AY ratings of three to five produced little or no seed. Also, if plants are showing just some bladder-like pods, AY will likely cause yield loss in all pods – even ones that look normal. Assessments done eight weeks after initial infection seem to provide the most accurate correlation between damage and yield loss.

Part 2 results

Since early infection seems to cause the most significant damage, part 2 of the project studied whether seed treatments provided some protection from leafhopper feeding and the resulting transmission of phytoplasma.

Laboratory and field tests from 2013-15 evaluated untreated seed, fungicide-treated seed, four neonicotinoid seed treatments, two diamide seed treatments, three diamide/neonicotinoid mixtures and two experimental seed treatments. Laboratory bioassays in 2013-14 focused on the effect of soil moisture on efficacy.

In laboratory and field tests, it was found that a number of seed treatments already on the market can manage leafhoppers enough to suppress the AY phytoplasma and AY symptoms.

Results were sent to chemical companies to support the registration of the treatments for leafhopper control and AY suppression. However, given that leafhopper and AY infestations are sporadic and unpredictable, canola growers are unlikely to select seed treatments based on their efficacy against leafhoppers and AY. Instead, they will continue to select seed treatments based on their efficacy against crucifer and striped flea beetles.

AY rating	Plant stature and height	Presence of swollen buds	Pale green or purple flowers	Presence of normal or bladder-like pods
1	Normal height	None to a few swollen buds on lateral branches (<10% of all buds)	None	None at bolting or flowering (<10% bladder-like pods at tips of a few branches at plant maturity; >90% normal pods)
2	Normal height Erect plant/pods	Swollen buds on lateral branches and some main stems (<50% of all buds)	None	Few bladder-like pods on main stem and lateral branches at bolting (<50% bladder-like pods; >50% normal pods)
3	Normal to slightly shorter height Erect plant/pods	Swollen buds on all lateral branches and some main stems (<80% all buds). Condensed inflorescence	Many pale green flowers at tips of all branches	Many bladder-like pods on lateral branches at bolting (<20% normal pods; usually on main stem)
4	Short erect plant Limited bolting	All buds swollen Condensed inflorescence	All flowers green or purple	100% bladder-like pods at maturity No normal pods. No seeds
5	Bonsai-like plant No bolting	All buds swollen Swollen buds emerging directly from growing point	Only small green flowers	Few bladder-like pods at maturity No normal pods. No seeds