Looking for disaster yellows

There has been few incidences this year, but aster yellows can cause high yield losses

Looking for disaster yellows

This year I have travelled extensively around Manitoba, Saskatchewan and Alberta and I have not seen a single canola plant with aster yellows (AY). A friend of mine said she found one plant only, in Saskatchewan in canola. Contrast this with 2012 when this phytoplasmic disease of canola, wheat, barley, flax, potatoes and probably all the rest of our field crops caused several billion dollars in grain crop losses.

How do you arrive at this billion-dollar figure? The 2012 year was by all accounts a very good growing season, yet national barley, wheat and canola production was fairly average at 8.0, 27.2, and 13.9 tonnes respectively. This was the year that the overall loss from canola was estimated at 20 to 25 per cent of yield potential with occasional fields exceeding 80 per cent grain yield losses. Wheat and barley losses were more difficult to estimate but prior to harvest in all three Prairie provinces between 15 and 40 per cent of wheat heads were observed to stand upright and empty. These figures across the Western Prairies were based on hundreds of observations by colleagues letting me know their crop loss estimates from Winnipeg to Dawson Creek, B.C. Previously these empty wheat heads, as in past years, were ascribed to common root rot stem. In point of fact the empty heads in 2012 were due to aster yellows.

In 2013, with record yields in the barley, wheat and canola crops, experts credited this with an exceptional growing season. In my books, the 2012 and 2013 growing seasons were pretty much the same so why the big yield jump? I will call it leftover nutrients.

The aster yellow-infected canola, wheat and barley seedlings in 2012 grew vigorously despite the aster yellows infection. They took up plant nutrients, but those nutrients were not translated into seed grain. Infected wheat and barley plants had empty heads and canola plants failed to pod out. So, in 2013 the Western barley, wheat and canola plants, bedsides a good growing season free of aster yellow, received a considerable amount of fertilizer from the decay of the phytoplasma-infected plants.

In 2012 the barley, wheat and canola yields (according to Statistics Canada) in millions of tonnes, were 8.0, 27.2 and 13.9 tonnes respectively. In 2013 the yield was 10.2, 37.5 and 18.6 million tonnes. That is over two million tonnes of barley, 10 million tonnes of wheat and about five million tonnes of canola, all ascribed to the better growing season in 2013. I think that much of this yield gain could be or should be accounted for by the leftover fertilizer from 2012 as a consequence of plant kill by aster yellows. In 2014 and 2015 barley, wheat and canola yields were not much different from the 2012 season. I am aware that this information could be argued but it is a backdoor way of attempting to calculate Prairie crop losses form this massive aster yellows epidemic.

What is aster yellows?

Prior to 2012, Phil Thomas (Mr. Canola) and I were in the Kamsack area of Saskatchewan where we encountered a quarter section of canola where 100 per cent of the plants were infected with aster yellows resulting in a 100 per cent crop loss. Wheat fields in this same general area ran 15 to 30 per cent with empty, upright sterile heads.

So what is aster yellows? The aster yellows organism is an extremely minute bacterium almost virus-like in size. Technically it’s called a phytoplasma when it infects plants and leafhoppers in its very many different stains. Mycoplasma is the name given to animals and humans infected by various types of this organism. These submicroscopic organisms can only survive in living tissues whether they be plants, animals or insects.

How and why do these sporadic destructive epidemics of these diseases occur? Phytoplasmas infect at least 300 species of plants (likely much more), from petunias to palm trees. Leafhoppers, particularly the six spotted leafhoppers, along with dozens of other leafhoppers are carriers of these destructive phytoplasmal diseases. Leafhoppers can overwinter as eggs in Eastern Canada but not likely in the West. Leafhoppers feed and multiply primarily on grasses and a range of broadleaf weed species. In most years, up to three or more generations of leafhoppers can result before laying overwintering eggs. Leafhoppers are not very good fliers (hence the name).

What happened in 2012 to cause the very big outbreak? In the South and Central U.S. States an early and severe drought took place in late April and early May causing countless billions of leafhoppers to migrate to “greener pastures.” Unusual warm air movements uplifted billions of leafhoppers from the southern U.S. thousands of feet into the air driving them north into the Canadian Prairies.

Normally only around two to three per cent of these leafhoppers are disease carriers but in 2012, 10 to 12 per cent of the hoppers were infected. I had friends in Manitoba describe the arrival of the hoppers somewhat like confetti at a wedding falling on crop fields in mid to late May.

Leafhoppers begin feeding as soon as they land and in the case of cereal crops they find suitable hosts as well as pastureland grass hosts. On these grasses and cereals, they can feed and multiply with the prospect of two more generations. When they land on canola crops the leafhoppers feed and infect the phytoplamal disease, but canola is not a good host so they move on infecting more canola plants. On cereal crop plants they generally settle down and stay put hence the likely difference in infection levels: very high in canola lower in cereals.

The damage from aster yellows

At this juncture it would be hypothetical to say that spraying canola with insecticide would be pointless but not so with cereals. On canola the insects do not multiply but they rapidly form new generations on cereals. These leafhoppers cause yield loss by sheer weight of numbers. Even if the plants are disease free, their feeding will cause yield loss.

A Swan River, Manitoba, grower said he got significant yield gains from spraying out the hordes of leafhoppers from his wheat crop in late June. I have to spray my grape vines for leafhopper control every year since the leafhoppers multiply astronomically and cause major reductions in grape yields.

In my cross-Prairie travels I did not find phytoplasma symptoms on corn, soybeans and only on a few potatoes. I figured that the answer was that most of these crops hardly emerge before the first of June, when the leafhoppers have long settled for the emerging cereal and canola crops. In other words, they miss the infectious leafhoppers since I know that all three crops are susceptible to aster yellows.

I did see all kinds of weeds infected by aster yellows, turf grass damage, lots of garden vegetables such as tomatoes, carrots and onions. Carrots have that typical hairy root symptoms when infected by aster yellows. Cosmos, delphiniums, petunias, asters, lilies and irises were examples of damaged flowering annuals and perennials. Other field crops with symptoms were flax, sunflowers, sugar beets, faba beans and peas.

Samples of peas that I sent for analysis to “phytodiagnostics” in Victoria come back negative despite disease symptoms. Having worked with phytoplasmas since 1963 it’s not surprising to me that while some plants are certainly infected with phytoplamas the disease can remain undetected in these plants to the best of our diagnostic procedures. It took 40 years to finally detect the highly destructive phytoplasmal organism that wiped out millions of coconut palms.

Not every person will agree with my findings but if we both agreed we’d both be wrong.

About the author


Dr. Ieuan Evans is a forensic plant pathologist based in Edmonton, Alta. He can be reached at [email protected]



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