Bleached tops means bye-bye Canada thistle

Take a look in the ditches near you. A fungus may be infecting those weeds

The white thistles at the roadside and in the field near Edmonton in 2019 will either be severely weakened or they will die since they have no chlorophyll.

Canada thistle is an invasive import from Europe. It is technically called Circium arvense, a prickly member of the Aster family. In the U.K., it’s called creeping thistle; in New Zealand it’s called Californian thistle, perhaps derived from Canada thistle. Canada thistle is also known in North America by a range of other names but none are complementary.

It may surprise you that Canada thistle is an excellent nectar producer for honey bees and that the thistle occurs in the form of distinct separate sexes, male and female. These thistle clumps or clones have deep-rooted stem-like rhizomes that spread rapidly in good soil and also serve as food reserves for the plants.

The old method of control, particularly in pastures, was to repeatedly mow them down three to four times a year in an attempt to exhaust their food reserves. Digging out the rhizomes or cultivating them only served to multiply the thistle clones. Sheep and goats are used in Europe to control thistles but cattle will only eat young, freshly cut but wilted thistles.

Before the advent of herbicides, Canada thistles and quack grass were the top farm weed enemies due to their persistent, perennial nature. The only way to control these two dauntless weed enemies in the UK was to grow a well-fertilized crop of kale (a seven-foot giant of the cabbage family) that would shade and choke out these farm enemies over a season.

When you see a clump of thistles on your cropland, roadside ditch or headland, check out the thistle’s sex in late August or September. If you pull out the fuzz before it blows away, you will notice that the fuzz either has seeds attached or no seeds. No seeds mean that it’s a male clone; seeds indicate a female clone. Often in the seed heads of a female plant you will find quarter-inch maggots instead of seeds. These maggots are the offspring of flies. These flies lay their eggs only in female thistle flower heads and depend on bees to carry pollen from male to female thistle flowers. No pollen, no seeds since it’s the developing seeds that feed the fly maggots which number one to three per head. The maggots are about a quarter- to a half-inch long and make excellent bait for rocky mountain fish or trout. These maggots severely reduce the seeds available in thistle patches and thus act as a fairly good biological control for seed that otherwise wold be dispensed.

White flower-like growths

As you drive or speed by the local un-mowed highways from Winnipeg, Man., to Dawson Creek, B.C., do you notice those white flowerlike growths in the ditches? I have seen these white-to-cream flashes on virtually every roadside ditch in prairie Canada. What’s up?

Stop, get out of the vehicle, park on the road shoulder with flashers on and take a close look. What you see are bleached white Canada thistle tops. These thistles have been infected with a fungus disease called Phoma macrostoma. I first saw this sporadically on roadsides and ditches in Alberta in the 80s. I dug up and forwarded lots of specimens to researchers at universities and provincial and federal governments. Eventually, around the 90s, research on this biocontrol was initiated by Agriculture and Agri-food Canada at Saskatoon. The fungus, technically identified as P. macrostoma var macrostoma reproduces by means of conidia (asexually produced spores) released from pycnidia which are dispensed by wind and water. Its conidia are somewhat similar to those of the blackleg fungus on canola. One of the Phoma isolates identified as P. macrostoma 94-44B is an isolate now registered for broadleaf weed suppression in both Canada and the U.S. The fungus has no effect on grass species, including all cereals. The fungus is primarily intended for dandelion and clover control in lawns, particularly where herbicides are banned.

The infected thistles run out of energy, caused by an infecting fungus. photo: Ieuan Evans

As you can see from the photographs, the odd sighting of this fungus (Phoma macrostoma) along prairie highways has now exploded into massed infections where up to 80 per cent per cent of the thistles in some ditches and pastures are now bleached white. The distribution of this fungus all over Europe and North America and has recently been found on thistles in New Zealand.

While I do not expect this fungus to wipe out Canada thistle or even dandelions, I can see from its distribution in this area of Alberta that we have strong biocontrol of this most noxious of pests. Along with the seed maggots’ natural biological control, this fungus may significantly reduce the all-too-common dense stands of pasture and roadside Canada thistles that are still major problems for organic farmers.

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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