College of Agricultural and Life Sciences
Above ↑ Canada thistle is persistent and prolific, but it’s not invulnerable. Weed scientist Jerry Doll is finding ways to abet a natural enemy of this prickly pest.
The Only Good Thistle is a Sick Thistle
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Canada thistle poses a prickly problem for pasture managers. The spiny weed lowers pasture productivity: It displaces forages, and grazing animals avoid infested areas.
The roots of this perennial pest can spread 15 feet wide and more than 6 feet deep. One plant can release 5,000 seeds that can sprout after decades in the soil. A native of Europe and Asia, it was introduced to the United States in colonial times.
Mowing at bud stage controls Canada thistle temporarily, but the plants grow back, thanks to the hardy root systems. Herbicides aren’t an option on organic farms.
Pseudomonas syringae pv. Tagetis, a naturally occurring bacterium that attacks Canada thistle, has potential to control this problem plant according to weed scientist Jerry Doll. Canada thistle turns yellowish white when infected by PST and some infected shoots die.
“While PST cannot eradicate Canada thistle in the short run, it can weaken plants by reducing their root reserves, lessening competitiveness over the long run,” Doll says. “We hope that farmers who are already growing good pastures — by using managed grazing, choosing well-adapted pasture species, seeding pastures at recommended rates, and maintaining soil fertility — will find that their pastures can out-compete weakened Canada thistles.”
Doll and graduate assistant Ryan Tichich studied how best to infect Canada thistle with PST and what environmental conditions favored spread of the disease.
The researchers added naturally infected Canada thistle foliage to water, chopped it in a blender, strained it and mixed the solution with a surfactant to help it adhere to and penetrate into healthy thistles. After spraying plants at several southern Wisconsin farms with the PST mixture, they measured how many became infected and how severely they were affected.
Environment was key, they found. Rainfall and high humidity are good for PST; infections were less prevalent during drier years. July was the best month to apply PST. The disease likes undisturbed soil; Doll says he’s never seen yellowing thistles in corn or bean fields.
“If your thistles are not infected and you do what we did, you should be able to achieve infection,” Doll says. “The disease is fairly widespread. Find a patch of infected thistles along a roadside or other nondisturbed site; cut stems with the yellowish-white leaves. Add the amount of leaves and stems that will blend with a quart of water in a blender. Blend for a minute or two, filter through cheesecloth, add an organo-silicon surfactant and spray healthy thistles in your pasture until the leaves are wet. Within a month you should have sick thistles.”
“Don’t expect it to work rapidly,” he cautions. “Try to get an infection by applying PST when relative humidity is high and rain is expected. Let Mother Nature take it from there.”
There is a lot more to learn about this disease and its potential, Doll says. “We need to study the biological interaction between the disease and the plant. We did mostly applied work. We didn’t study the microecology and microbiology of the leaf/organism interactions. We know nothing about the genetic background of PST,” he points out. “We can’t change thistle genetics. But there may be different varieties of the disease organism, and some could be more infective than others.”