Aster yellows caused significant damage and resulted in heavy yield losses in many canola fields across Western Canada in 2012.
With no economic threshold established for the disease, and no known way to effectively control the problem, it’s something that’s hard for farmers to get a handle on.
Aster leafhoppers have been arriving in Canada earlier and earlier every spring for the last five years. There is some speculation that drought conditions in the U.S. may have contributed to higher leafhopper infection rates and an increase in the numbers of leafhoppers in 2012.
Last year’s aster yellows outbreak, the largest since 2007, coincided with an unseasonably warm Prairie spring that followed a mild winter. This meant there was more vegetation growing earlier in farmer’s fields than usual, providing lots of food for the hungry adult leafhoppers, which arrived in record numbers. This year’s coller spring may help.
Aster yellows this year
This year could be a totally different story. It will depend on the arrival dates of the leafhoppers, their numbers and infectivity, the crop stage and weather conditions.
It’s basically a case of wait and see, says Dr. Chrystel Olivier, Agriculture and Agri-Food Canada entomologist and leading aster yellows researcher. The only advice she can offer farmers as they head out to do spring field work is to try and keep their fields as weed free as possible. “The more weeds you have the more food source you are providing for the leafhoppers and they will have a tendency to stay there,” she says.
Olivier is hoping to have a few more suggestions in a couple of years when new research into the effectiveness of seed treatments (funded by SaskCanola) determines which, if any, seed treatments are effective in providing protection against the leafhoppers.
“Most canola seed is treated for control of flea beetles. We need to know whether any of those seed treatments are effective in helping to prevent leafhopper feeding, and to see if this might explain some of the differences in aster yellows incidence and severity that we saw in different fields,” says Olivier. She will test different temperatures and moisture regimes to determine if seed treatments used to control flea beetles also control leafhoppers and reduce aster yellows infection in canola seedlings.
How aster yellows spreads
Aster yellows is a bacterial infection caused by phytoplasma, which lives in plant phloem.
- The aster leafhopper is the main vector to spread the disease.
- Leafhoppers feed on infected plants and then infect other plants when they feed on them.
- Generally only two to three per cent of leafhoppers are infected when they arrive in Canada from the southern U.S.
- Once infected, leafhoppers are infected for life.
- There is speculation that aster leafhoppers may be able to overwinter in small numbers in perennial weeds and grasses in some areas in the Prairies
- Migrant leafhoppers are the major source of infection. They generally arrive in early June.
- Leafhoppers need to feed for at least a few hours to infect a new plant.
- There is a delay of 10 to 28 days after a leafhopper becomes infected and is able to pass on the infection to new plants.
Identifying aster yellows
Symptoms of aster yellows vary depending on the infected crop. Infected cereals will have a red or purplish discolouration and become very erect. The main symptom is empty heads.
In canola and other flowering crops, flowers are replaced by sterile, leaf-like structures and pods are malformed. Plants may turn purple or blue-green and leaves will develop a red or purple tinge later in the season. Flowers and pods are malformed. Plants may become woody and are often taller than the rest of the plant canopy. Symptoms may be mistaken for sulphur deficiency or herbicide damage.
Identifying aster leafhoppers
The aster leafhopper is olive-green or straw coloured with six dark markings on the forehead. In flight they appear white.
The abdomen is charcoal and the wings are opaque. They fly between plants in short bursts and may appear to glide.
Nymphs resemble a wingless adult but are much smaller, ranging in size from 0.6 to 3.0 mm. They can be yellow to light brown or a pale, greenish-grey in colour. They scuttle sideways when disturbed.
Adult nymphs range from 3.5 to 4.0 mm. They are poor fliers and tend to glide along with wind currents.
The only way to determine whether leafhoppers are infected with aster yellows is to test their DNA. †