By Bruce Barker
2007 infestations were higher than usual.
Across the prairies, aster yellows disease was widespread, looking like a scene out of Chernobyl. The distorted flower heads looked like a mutant disease gone wild, but fortunately, the looks are worse than the losses.
“We saw aster yellows in virtually every crop susceptible to the disease. Canola was definitely affected in higher than usual numbers,” says Philip Northover, plant pathologist with Manitoba Agriculture and Food at Carman. “It was a very unusual year.”
In Saskatchewan, the 2007 disease survey results indicated that 86 percent of the fields surveyed had aster yellows, but half of these fields only had trace levels (‘trace’ means that aster yellows was found in the field but was not part of the sample collected; less than one percent).
Penny Pearse, provincial plant pathologist with Saskatchewan Agriculture and Food (SAF) in Regina, says the incidence values in each field were variable, ranging from trace amounts to as high as 15 percent in one field. The highest levels were found in the northeast (average of 2.5 percent for that region) and the lowest in the north-central region (average of 0.7 percent). Based on the survey results received as of August 2007, the average incidence for Saskatchewan was 1.4 percent.
“The last time we saw aster yellows this high was in 2000, when the provincial average was 1.6 percent and some fields had as high as 10 percent infected plants. In other years, the average incidence of aster yellows for the province has been at zero to trace levels,” she explains.
The question for 2007, though, is whether significant yield losses occurred. Pearse says that due to the nature of the disease, the percent of infected plants may closely relate to percent yield loss. However, a plant that is only showing symptoms on one branch may produce seed in the other branches, so there will still be ‘yield’ from that plant; but because seed produced in an infected plant is not usually viable, that seed will likely not produce healthy seedlings.
Stunted growth and malformed flowers and pods identify aster yellows disease. During and after flowering, the plants stand out and above the crop, making the infestation look worse than it really is. Flowers can be replaced by sterile, leaf-like structures and pods are malformed or replaced by odd flattened structures. Infected plants can be blue-green in colour with leaves developing a red or purple tinge later in the season.
The aster leafhopper, also known as the six-spotted leafhopper, seems to be the main vector in spreading the disease from plant to plant. Aster yellows can overwinter in the living crowns of perennial crops and weeds or it can be brought into Saskatchewan by the aster leafhopper insect as it migrates from the US.
Leafhopper migrations start from Arkansas where they overwinter, travelling through Wisconsin and eventually arriving on the prairies where they start feeding on a wide range of host plants, canola included. The leafhoppers typically bring aster yellows with them and usually arrive in early to mid June.
The disease falls somewhere between a virus and a bacteria. As a phytoplasma-like organism, aster yellows inhabits the nutrient carrying vessels (phloem) of an infected plant and is carried from plant to plant by sap-sucking leafhoppers. Not all leafhoppers are infected with the pathogen. Additionally, leafhopper feeding, in itself, is not considered an economic threat to crops, so spraying the crop may not only waste money, but also kill beneficial insect predators.
Once infected, the leafhopper can continue to pass on the phytoplasma for as long as it lives, which is usually about a month. As a result, the insect can carry the pathogen over great distances if the winds are favourable. Dr. Christel Olivier with Agriculture and Agri-Food Canada at Saskatoon has been studying the aster yellow infestation in 2007, and says that the populations of leafhoppers were not higher in 2007 compared to previous years. Nor was the population more infected with the aster yellows phytoplasma. She speculates that the incidence of aster yellows that was observed may not be unusual, but what is unusual is that the symptoms are being expressed more than is typical. This may be because the hot weather received in July caused stress to the plants and amplified the aster yellow symptoms.
“We are working to try to understand why the disease was worse this year, but we really can’t explain it at this point,” says Olivier. She also reiterates that the leafhoppers are capable of
overwintering in Saskatchewan as she has found high populations over the last few springs. Hence the phytoplasma can overwinter in both the insect vector and in the roots of perennial plants, resulting in infection earlier in the season.
One more possible reason why there was more aster yellows disease in canola is that leafhoppers prefer to feed on weedy crops, and weed growth was higher in some crops this season because the wet spring delayed herbicide applications.
Aster yellows can infect a wide range of crops besides canola, including flax, sunflowers, alfalfa, cereals and several speciality crops including echinacea, caraway, coriander and carrots. Aster yellows can be confused with damage from Group 4 herbicides (containing growth regulators) caused by drift or residue in the spray tank. Drought or other environmental stress is also
sometimes confused with aster yellows. In canola, purpling is caused by anthocyanin production as a result of stress. Although aster yellows can cause purpling, a purple plant does not necessarily indicate an aster yellows infection.
Not a lot can be doneAn aster yellows index has been developed for vegetable producers in Ontario and the US mid-west to determine when leafhopper populations are high enough to warrant insecticide applications. For high value vegetable crops, spraying the leafhopper can be warranted if economic thresholds are reached. However, for canola, economic thresholds have not been established.
In addition, there is not an easy method of determining whether or not the insects are infected with aster yellows, so spraying is not a recommended control mechanism.
The easiest way to estimate yield loss would be to do random plant counts of 100 to determine the level of infestation, and possibly the loss of viable seed.
Ultimately, the best canola growers can do is to be aware of the disease, monitor the infestation levels, and report unusually high infestation levels to canola agronomists and provincial disease specialists. That will not help with yield loss, but at least agronomists and researchers will be aware of the problem when developing research priorities.