Very little is known about the disease or the factors that contribute to outbreaks and their severity
Last year not only produced the biggest outbreak of aster yellows in Manitoba in five years, but also demonstrated how little is known about the disease and the factors that contribute to its appearance and severity.
The disease is mainly transmitted by aster leafhoppers, which arrive in the spring on winds from the southern U.S. Aster leafhoppers feed on plants that are infected with phytoplasma, the bacteria which causes aster yellows, and then carry it within their bodies to infect other plants. Not all leafhoppers are infected with the pathogen when they arrive, but phytoplasma can overwinter in the roots of perennial grasses and weeds. Once infected, the leafhopper is a carrier for life, so an overwintering adult insect can carry the disease into the next year, but it’s thought that the migrant leafhopper population generally provides the major source of infection.
Infection rates across the Prairies in 2012 averaged 12 to 17 per cent in canola crops, but the range went all the way from three per cent to as much as 45 per cent in some fields.
But researchers found other, more troubling, data.
Leafhoppers have been arriving earlier and earlier each year for the past five years and are now showing up in early April, Chrystel Olivier, an entomologist with Agriculture and Agri-Food Canada, said at the Manitoba Agronomists Conference last month.
Researchers have also determined infected leafhoppers are modified by the phytoplasma, which increases their lifespan, enhances their ability to reproduce, gives them a better chance of survival and allows them to feed on plants they’re not normally attracted to. It’s estimated there are more than 250 plant species the leafhoppers can feed on. Preliminary research suggests they prefer cereals to canola, but more work needs to be done in this area.
Researchers also want to know more about how phytoplasma builds up in the root tissues of perennial grasses and weeds, and to what degree warmer winters will increase this reservoir of the disease.
Controlling the disease also presents many challenges. Typically, by the time the disease is spotted, the damage to the crop is already done. In fact, the only way to know for sure if aster yellows is the culprit is to have a lab test done. And the presence of leafhoppers isn’t helpful, either, as that doesn’t mean there will be a high infection rate. Spraying may do more damage than good because it will also kill beneficial insects and leafhopper predators. Moreover, multiple sprays would be needed to deal with each new wave of leafhoppers brought in by southerly winds.
Even assessing yield loss in canola isn’t easy. Pods that appear normal may contain significant numbers of shrivelled or misshapen seeds or no seeds at all. Olivier says that 2012 was the first time she has seen eight to 10 per cent of pods in some fields with no seeds. She anticipates that yield losses in canola for 2012 due to aster yellows may be significant although not all the data is in yet to determine this.
Similarly in cereals scientists have concluded that phytoplasma is often endemic in the crop — at levels anywhere from 29 to 60 per cent — but is largely undetected because symptoms are rarely expressed. In barley, the symptoms can easily be confused with barley yellow dwarf virus and root rot in wheat. In 2012, levels were high enough for symptoms to be exhibited and samples showed a five per cent infection rate in wheat, 25 per cent in barley, and 17 per cent in oats. An economic threshold for aster yellows has yet to be established.
Clearly more research is needed in many areas, including developing an early warning system, a method to estimate disease potential, and varieties with improved resistance.
In the meanwhile, Olivier says farmers should keep fields clean and manage weeds effectively, even in field margins and ditches, to try and reduce the reservoirs where phytoplasma may survive. Scouting for aster leafhoppers with sweep nets or sticky traps is important to make sure the species in the field are actually one of the two species that can transmit aster yellows.
Local agronomists can assist with identification and there are also resources on the Manitoba Agriculture, Food and Rural Initiatives website at www.gov.mb.ca/agriculture/crops/insects/fad51s00.html.
Farmers should also make sure that they confirm an infection in their fields before jumping to conclusions, as aster yellows symptoms are similar to other diseases.