Potatoes in Canada

Features Agronomy Crop Protection
Not glyphosate ready

Dr. Pam Hutchinson found that greatest injury to the Ranger Russet mother crop was from glyphosate applied at hooking. No injury was found at the mid-bulking stage.. Phot courtesy of Dr. Pam Hutchinson.

December 11, 2014
By Rosalie I. Tennison



When everything has been done correctly and there is no visible reason why a potato crop is substandard, it could be glyphosate damage. Growers who purchase seed from reputable providers, manage pests and diseases properly, and follow a good fertility program are sometimes puzzled – this occurs when they have poor emergence, multiple sprouting underground and/or a poor crop at harvest (and know they can’t blame the weather). Research completed at the University of Idaho suggests the problem could be due to glyphosate damage.

Dr. Pam Hutchinson, a potato cropping systems weed scientist at the university’s Aberdeen Research and Extension Center, says herbicide carryover in the mother crop can affect the daughter crop. This could explain some crop failures. The problem lies with identifying how the carryover occurred, so growers need to be diligent in their spray practices and be aware of what their neighbours are doing.

While herbicide damage can occur from any product that is not registered for potatoes, Hutchinson focused her attention on glyphosate. “With the introduction of glyphosate-tolerant sugar beets, I began to wonder about drift or poor spray tank clean-out and damage to the mother crop plus carry over in the seed,” she says. “I set up a trial in potatoes with different rates of glyphosate applied at different times of plant development.” She made applications at the small plant stage, tuber formation and tuber bulking. She stored the resulting tubers and planted them the following year.

Hutchinson says the most damage to the mother crop plants occurred when the plants were sprayed when small (10 centimetres tall), and the least amount of damage was observed when the tubers were bulking in the latter stages of crop development. “We saw no indication of injury at bulking that could be noticed unless you knew that glyphosate had been applied and had a check plot for comparison,” she comments.

However, the tables turned the following year when the daughter tubers were planted. The mother crop plants that had damage at the earlier spray times developed healthy tubers. Problems emerged from the tubers harvested from plants that showed no damage at the later spray times.

“We had multiple sprouting from those daughter tubers and in some cases we had only 20 per cent emergence of the tubers that had been sprayed during the later time period,” Hutchinson explains. “Growers may not notice the damage on the mother crop, but they could still have it.”

Hutchinson says her findings will be of particular interest to seed potato growers, but growers of table or processing varieties will want to understand why their crop may have failed or feel confident that the crop they are selling is undamaged.

As a systemic product, glyphosate will work down to the tubers in the ground causing damage, especially during the bulking period. She says glyphosate damage is rarely detected in tubers, so a problem may go unidentified. The only indication of damage may be small defects in the tubers that have to be looked for, such as the bud end folding in. If the tubers are intended for consumption and are stored, the herbicide will break down to acceptable Health Canada levels and will pose no threat. However, seed potatoes may pass the problem on causing damage in the following crop.

How does this happen? Growers who are diligent in their crop production and yet learn they have glyphosate damage in their crop may be scratching their heads. But, Hutchinson says the damage can come from unconsidered sources. For example, she says a neighbour may spray for late-season weed control and to desiccate a cereal crop and drift will occur onto the potato crop in the next field. Poorly cleaned sprayers could also be the culprits.

“Growers need to be aware if glyphosate is being used near their potato crop and they need to be diligent in cleaning their sprayers from tank to nozzle,” Hutchinson advises. “Consider having a sprayer that is dedicated only for potato use and will never have product in it that is not registered for potatoes.” She also suggests talking to neighbours about cropping intentions, so they will be aware that a potato crop in close proximity to their fields may be sprayed with products damaging to potatoes.

“This isn’t just about glyphosate; I have seen damage from other herbicides,” notes Hutchinson. However, her research focused on glyphosate, and the extent or effect of damage from other products has not been proven.

Hutchinson says the next question was whether the damage could continue into the third or granddaughter potato crop. After growing out the granddaughter crop, no discernable damage was noted. The issues were with the daughter crop if it failed to emerge and produce an acceptable yield.

“The take home message in all this is to be aware of the herbicides that could come in contact with your seed potato crop,” she cautions. For example, if a seed grower goes away during the period when the crop is bulking and a neighbour sprays a cereal crop with glyphosate and there is drift, the damage may not be noticed. But, if that potato crop is sold for seed to a commercial grower who ends up with yield loss and damage due to the affected seed, the reason for the failure will not be apparent. Who would be liable for the failure? Assuming it’s possible to determine what caused it.

Hutchinson’s research shows that potatoes are susceptible to glyphosate even at the smallest amounts. She says the main point is that “a high level of damage to the mother crop doesn’t mean daughter tubers will be affected when planted the following year.” But, timing is everything, she adds, and at mid-bulking the glyphosate will move to the developing daughter tubers.

Even though potatoes are a sturdy, dependable crop, they are not without their weaknesses, and growers need to be aware of the challenges facing their crop. Therefore, growers should continue to do everything possible to ensure a healthy, high-yielding crop, and now, complete sprayer cleaning and guarding against drift need to be added to the list of things to do.