Aster yellows kill barley
Prairie-wide this year, a couple of billion dollars were lost in expected crop yields to a tiny bacterium called phytoplasma, commonly known as aster yellows. In canola crops from Winnipeg, MB, to Dawson Creek, B.C., aster yellows disease showed up in canola as big, scruffy, ugly sterile plants that failed to form any seedpods. The canola crop infection was in the general range of 10 to 40 per cent, some as much as 60 per cent. The aster yellows infection showed up more than a month earlier than normal. A 25 per cent aster yellows crop infection was a 25 per cent yield loss.
This infection occurs when infectious aster leafhoppers (Macrosteles quadrilineatus) migrate from the United States into Canada annually, usually during late May to early June. These annual migrations of leafhoppers have two to three per cent of individual insects infected with the aster yellows organism. Normally, in most years from the countless millions that arrive in Canada, we see minor to trace levels of aster yellows in canola, carrots (hairy roots) and potatoes. Sometimes, we get isolated aster yellows hot spots with very high levels of disease, such as in east central Saskatchewan in 2007 in canola fields with up to 100 per cent infection. That year, we attributed the dead wheat plants in the same area to common root rot.
This year, the aster yellows problem was radically different. In mid- to late-April, Agri-Trend Agri-Coaches in Manitoba reported clouds of leafhoppers descending on bare fields. What was happening? Word from Agriculture and Agri-Food Canada indicated that 10 to 12 per cent of the leafhoppers were infected with aster yellows, around four times the normal rate of two to three per cent. By early to mid-May, Agri-Coaches reported leafhoppers widespread in all crops from its field-scouting operations from Winnipeg to the Peace Region.It is speculated that the primary cause of this 2012 massive migration was the severe drought in the Midwest United States that burned off the vast grassy feed pastures of the leafhoppers. Countless trillions of these bugs took to the air, thousands of feet up, drifted north and blanketed the Canadian Prairies.
What many others and I as a plant pathologist failed to realize from past outbreaks, was that aster yellows infection of small grain seedlings kills them at heading time. In other words, wheat, barley, oats, triticale, rye and flax were as badly affected as canola. Corn, sunflowers and most legumes (peas, beans, lentils) appeared unaffected or immune.
In late July, while field scouting at Swan River, MB, I was shocked to see some wheat fields at late anthesis with up to 40 per cent dead or dying plants. Dying wheat plants taken from several fields were forwarded to Phytodiagnostics Ltd. on Vancouver Island. All were positive for aster yellows. Back in Alberta, many similar samples were taken by Agri-Coaches from wheat and barley and oat fields, and all were confirmed to be infected with aster yellows.
Surveys conducted in northern, central and southern Alberta confirmed aster yellows generally ranged from 10 to 40 per cent in canola and 10 to 25 per cent in cereals. By early August in Alberta, you could walk into ripening canola and cereal fields, from north to south, and see aster yellow’s destruction.
Canola crop damage was very obvious, but you had to look much more closely on the cereal crops and look for the thinner, dead cereal heads. By late August and into early September, it was easy to see the damage in most wheat fields. The healthy, full grain heads bent over, whereas the empty diseased heads stood upright several inches above the crop canopy. This made it easy to count the 10 to 25 per cent infection levels that I encountered in most fields.
The aster yellows leafhoppers do not multiply on canola, only on cereals and wild grasses. While I didn’t do as extensive a survey on barley as I did for canola and wheat, I have no doubt that we lost 20 to 25 per cent of the barley crops on average. While many of us were looking for answers on lower than expected crop yields this year, such as heat and drought, I have no doubt that the elephant in the room was aster yellows. I might also add that the severe winds of early September did significant damage to the canola swaths and broke off many a cereal head, adding to big crop losses.
What will happen in 2013? Chances are that it may be hard to find aster yellows in any crop next year. They do not normally over-winter on the Prairies. Maybe the leafhoppers in the southern U.S. will have a population collapse, the wind directions may be unfavourable or the infection levels in the hoppers will be low to non-existent. At Agri-Trend, we will be working on a contingency plan much in the way that municipal fire departments get ready for fire outbreaks.