Aster yellows is a minute phytoplasma bacterium that causes losses annually to a wide range of farm and horticultural crops. Most years, it is usual to see little in the way of aster yellows damage to canola, the most obviously affected crop. When you check canola fields in full bloom you can often pick out one to two per cent of odd green petal free flower-like shoots.
The spring of 2012 was an unusually early warm spring on the Prairies. I saw monarch butterflies north of Edmonton in mid-April, and crop producers had devastating losses from the aster yellows bacterium.
I keep hearing that sources in Saskatchewan estimated the losses at five per cent. Later they upgraded the losses to eight per cent, Prairie-wide. How did they get this data?
That year I travelled extensively across the Prairies. The growing season was unusually good but the crop losses from aster yellows were devastating. Not only canola crops were damaged, but so were wheat, barley, oats, rye, peas, flax, sugar beets, fava beans and pasture grasses. In some instances, wheat fields lost more than 50 per cent of expected yield.
Since the leafhoppers arrived on the Prairies in late April and early May in their countless billions, late-seeded crops such as potatoes, corn, soybeans, flax and dry beans missed out since these crops do not emerge until late May or early June, after the infecting leafhoppers settled on emerged grain and canola crops.
The major culprit that carried this infectious bacterial disease organism was the six-spotted leafhopper. This bacterium can only live inside the gut of insects like leafhoppers or inside living plants. I worked on aster yellows in 1963 at the University of Florida, when we all assumed that the aster yellows organism was a virus.
The aster yellows organism has a wide host range from ash trees to lilies and kills off lots of garden plants. The host preferences of these six-spotted leafhoppers are flax, wheat, rye and carrots. When you pull up thin, hairy rooted carrots in the garden with yellowish leaves, they are infected with aster yellows.
These leafhoppers can have two or three generations a year on favourite food crops such as wheat. They are unable to multiply on canola since its not a food host, despite the fact that it gets infected.
In 2012, an enterprising wheat grower in Swan River expecting 70-bushel yields sprayed his wheat fields with insecticide. On the sprayed fields he got 50 bushels. On the one unsprayed quarter the leafhoppers were so numerous that it looked like snow in summer. That field yielded only 30 bushels of sample wheat. So much for the chorus that says there’s no point in spraying leafhopper-infected cereal crops.
I believe actual yield losses from aster yellows in 2012 were not eight per cent, but closer to 20 per cent. How many canola fields did I see that year with massive infections of 50 to 80 per cent of the crop? How many wheat fields had 60 to 80 per cent of the grain-filled heads drooping over and 20 to 40 per cent upright dead heads standing above the rest of the grain-filled heads.
The canola yield for 2012, a good growing year, was 13,869 thousand tons. For 2013 that yield jumped to 18,551 thousand tons — an increase of over 20 per cent with much the same acreage and growing season.
The wheat yield for 2012 was 27 million tons. In 2013 that yield jumped to 37.5 million tons. A ten-million-ton difference in comparable seasons.
Now, that you have seen the math and smelled the coffee do you believe that aster yellows only caused a five to eight per cent yield loss in both canola and wheat?
Since wheat normally alternates with canola, some of the 2013 yield gains could be attributed to leftover fertilizer from 2012. That is, aster yellows-infected wheat and canola plants broke down very quickly and the following crops benefited.
Of course, there are variations in wheat and canola acreages but the big difference points to aster yellows infection. So, the record wheat yield in 2013 is likely owed to the extra nutrition caused by the damage to the canola crop from the aster yellows infection in 2012.