I first worked with aster yellows (AY) as a graduate student at the University of Florida in 1963, some 62 years ago. At that time, we thought this disease agent was a virus of some kind. In 1967, Japanese scientists defined it as a minute bacteria, then classified as a mycoplasma — a submicroscopic infectious bacterial disease that affects man and animals. These mycoplasma-infecting plants were renamed phytoplasmas.
Many strains of phytoplasmas have been identified as causing disease in a very broad range of crops in Canada. Phytoplasmas are known to destructively infect some 300 ornamental, fruit, vegetable and field crop diseases.
The real AY story
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I am writing up this story because many Canadian scientists and agronomists seem to think AY is only really destructive on canola. I differ very, very strongly.
Back in 2007, while working with Agri-Trend, an organization made up of scores of agronomists, specialists and scientists, we set out to grow 100-bushel wheat crops for a major wheat and canola grower in the Kamsack area of eastern Saskatchewan. The best wheat variety was selected and the fertilizer inputs, following soil tests, were adjusted to a target yield of 100 bushels. The 2007 season had good rainfall and crop yields looked good in the Kamsack region. In July of that year we noticed many canola fields were showing from 10 to 30 per cent aster yellows infection. A colleague and I actually found one quarter section with 100 per cent aster yellows in the canola crop.
When it came time to combine the grower’s wheat crops, we expected close to, if not, 100 bushels of wheat from the many seeded quarters. We had inspected the wheat crops in August and, based on head count and grains per head in a proscribed area, the wheat crop was on target for that 100-bushel yield. The yield monitor in the combine, though, showed 75 to 80 bushels an acre for the wheat consistently.
What was wrong? Aster yellows. It was estimated that 15-25 per cent of the wheat heads were empty. They had been killed off by AY at anthesis. Plant pathologists on the Prairies had called this problem “early wheat root rot” — a total misdiagnosis.
In subsequent years we had a hard job explaining to Prairie agronomists that if they saw aster yellows in a canola crop, they would be sure to find it in an adjacent wheat crop. They would find it when the wheat was mature: full grain heads would be bowed over and AY-infected wheat heads stood straight and tall with empty heads. Wheat plant failure was not due to root rot or even take-all.
Then along came 2012, the year we had a huge outbreak of aster yellows from south of Winnipeg up to Dawson Creek in British Columbia.
AY and crop losses in 2012
This was the year I travelled extensively from Quebec to British Columbia and, in particular, to all three Prairie provinces. The first inklings of the AY outbreak that year was the fact that I captured a monarch butterfly in my garden on April 12. Normally monarch butterflies get to southern Alberta only in August of any year. Later on, to the end of April and very early May, I got reports from agronomists in Manitoba that leafhoppers were falling on cropland like confetti at a wedding.
As the season progressed into June and early July, it was obvious all over the Prairies that every canola field that I visited had some degree of AY infection. By August I saw canola fields with 20 to 80 per cent aster yellows infections. Looking at and counting the wheat heads adjacent to those infested canola fields in August, I could see 30 to 60 per cent AY-infected dead and empty grain heads. The wheat crops were every bit as damaged by aster yellows as the canola crop. The heaviest AY infections in 2012 were in the canola and wheat seeded the earliest on the Prairies, in late April and early May. Entomologists were able to show that year that up to nine per cent of leafhoppers were infected with the AY phytoplasma.
AY will infect canola and many other crops and ornamentals, but canola plants can be unsuitable as hosts in order for the phytoplasma to subsist and multiply. Cereals, on the other hand, are the perfect hosts — particularly wheat, in which these bugs can multiply in huge numbers and grow for two generations or more. I saw fields of wheat in late July in the Swan River area of western Manitoba where the clouds of leafhoppers in the wheat looked like a light snowfall that would swarm up when the wheat plants were disturbed. Not only were these leafhoppers introducing AY to the wheat plants but the sheer vast numbers in the wheat crop were obviously draining the food reserves of the growing wheat. One grower in the Swan River area used insecticide on one quarter in early July and another unsprayed for comparison. His target yield was 70 bushels of wheat per field. The sprayed field went 50 bushels and the unsprayed field went 30 bushels of sample grain.
—> READ MORE: Wheat disease: Is it aster yellows or fusarium?
Now let’s get down to the nitty gritty of the actual yield losses on both wheat and canola on the Prairies in 2012 compared to 2013. Both growing seasons in that region were very similar with good timely rains and little or no excessive heat.
From federal sources, the canola yield was as follows: 2012 produced 13.869 million tonnes of canola; 2013 produced 17.966 million. There was an increase of 4.097 million tonnes — a record yield.
In 2013, there was no AY. I found maybe the odd AY plant in a few canola fields.
Let’s look at that number again: 4.097 million tonnes. At $600 a ton that would be $2.4582 billion.
In 2012 according to Statistics Canada, the country produced 27.205 million tonnes of wheat. In 2013, also according to StatCan, Canada produced 37.53 million tonnes of wheat — a staggering 10.325 million-tonne difference in the annual wheat yield, amounting to a difference of $2.581 billion at $250 a tonne.
Now in 2012, where the AY was heavy in both wheat and canola fields, it would have left behind nutrients that could have boosted both the wheat and canola 2013 yields. Even so, I will challenge anyone to provide me with alternative explanations for that combined total loss of some $5.04 billion to AY in 2012. This is not the “rare disease” that cost some $400 million as reported by some research scientists.
A host of other hosts
Aster yellows has a very, very wide host range and in 2012 we had reports of hay pastures thinning out. Grasses and forbs were killed off. Even golf course grass specialists complained of turf thinning. How much haylage did we lose?
The early-emerging wheat, barley, hay and grasslands took the brunt of the AY damage, but its later spread in 2012 was also destructive.
I sent many samples of suspected AY for confirmation to a diagnostic facility on Vancouver Island for confirmation. Overall, I and my colleagues identified AY on many weed species, barley, oats, rye, flax, fava beans and potatoes. Infection levels on these later-emerging crops ranged from trace to 25 per cent or more. These were no confirmations on peas, corn, soybeans or lentils. In southern Alberta we did see a sugar beet field with around 25 per cent damaged beets, but I did not survey more than the one beet field.
Horticulturally, AY did significant damage to flowering plants provincewide in 2012 on lilies, iris, delphinium, cosmos and many others. In the vegetable garden, damage was done to tomatoes, carrots, onions, leeks and parsnips. I also suspect some of my fruit trees, such as plums, apples and pears, that died off in 2013 were possibly infected with AY.
Not to panic on this information, but I could not resist placing the huge damage to Prairie crops in 2012. Aster yellows is highly sporadic in its incidence on Canada’s Prairies and unfortunately there is little that we can do except perhaps for the spraying of cereal crops with insecticide early in the growing season if we detect high amounts of leafhoppers that have migrated from the U.S. in any given spring. Perhaps we will have AY next year, or maybe not for many years — but if there’s one message I want you to remember is that this disease is as destructive of cereals, particularly wheat, as it is in canola.
Final thought: “Stay away from negative people, they have a problem for every situation.”
