As you approach weed control this spring, remember the word SUPPLY.
That’s the acronym Dr. Hannah Mathers developed to help orchardists and nurserymen prevent injury to their trees as they go about the process of controlling weeds with herbicides, especially with glyphosate.
The Ohio State University nursery and landscape extension horticulturist, while investigating the cause of what looks like cold injury, has linked bark cracking on fruit and ornamental trees to improper use of the herbicide glyphosate. In the research, she found that many growers were taking a very casual approach to herbicides in general and glyphosate in particular. Glyphosate, which has dropped dramatically in price since it went off patent in 2000, has been used more intensively in recent years and used in ways that can cause injury.
Hence the acronym SUPPLY.
“S” stands for shield. Never use Roundup or any generic glyphosate product without shielding the tree, she said. Even paraquat, when sprayed on tree trunks, can cause injury, usually looking like a weakness at the graft. With glyphosate, drift can cause injury, even from 30 to 60 feet away.
“U” stands for use proper sucker removal products. Glyphosate cannot be used to control suckers, either to kill them directly or in ways that put glyphosate on the stubs where they were removed. The only product that is labeled for use for sucker removal is Scythe, she said, and it is expensive. An alternative is to remove suckers by hand and then, perhaps, to use NAA to suppress regrowth.
“P” stands for preemergence. “Preemergence products should always be the backbone of your weed control program,” she said, “not the postemergence products.”
“P” also stands for prune. Prune late in winter or early in spring before trees come out of dormancy. That is the best time for wound healing, and wounds provide a source of direct entry of herbicides into trees and also an injury point from which cracks can develop.
“L” stands for limit. Never use more than two applications of glyphosate per year, and make no applications after mid-July, Mathers advised. Be careful what formulation you use. Growers should never use glyphosate formulated with a full adjuvant load, and don’t add adjuvant yourself. “The glyphosate formulations that work really fast are the ones that are really bad to use around trees,” she said.
“Y” stands for you. The orchardist needs to take steps to prevent bark injuries. Bark cracks always develop from a point of injury, she said, and something as simple as removing a sucker can provide that injury point.
The role of glyphosate
“Before 2002, if you’d asked me what causes bark cracking, I’d have said it was one of nine kinds of cold injury,” she said. “Now, my answer is, bark cracking is caused by an injury point. You have to have an injury point, a point of weakness, for a crack to grow from. All the stress accumulates at that one point.”
In 2003, responding to complaints from the landscape nursery industry in Ohio, she began to investigate cracks that did not occur on the southwest side of trees, the place where cold injury cracks normally occur. She found cracks all around the trees and even on scaffold limbs and branches. They occur mostly on thin-barked trees, especially maples.
That led to her closer look at the way they were using herbicides and ultimately to the link to glyphosate.
At first, she looked at obvious problems. Some growers were using glyphosate after sucker removal, or even for sucker removal. “If they took off suckers first, it was like a stump treatment,” she said. “You kill stumps by making a clean cut and putting glyphosate on it.”
Some were assuming that glyphosate would not be taken up from tree trunks. “The key is active growth of photosynthetic tissue,” she said. “That includes green bark, especially on thin-bark trees. Any time you can peel up the bark and see green below, that is green bark. You can find that on 15-year-old apple trees.”
Growers also didn’t consider the effects of multiple applications, and some were using glyphosate repeatedly as a total replacement for preemergence herbicides. “Once inside plants, glyphosate is not broken down,” she said. “Each successive application is building up in the plant like a sublethal dose until it can start causing problems.” Even small applications, or drift, contribute to the overall dose.
What has been called glyphosate carryover injury is merely the way glyphosate expresses itself, she said. “It doesn’t come out until the next spring. It acts upon where it is carried to and comes out as sporadic injury, like auxin injury, on one branch, not another. It can be caused by even one application, and injury can show up one, two, three years later.”
How glyphosate works
People have known for many years the way glyphosate works, she said. It blocks the shikimic acid pathway in the plant. As shikimic acid builds up, secondary metabolism and the production of phenolics, tannins, lignins, and flavonoids are suppressed. Even at sublethal doses, it interferes with healing of injuries and increases sensitivity to cold injury. It affects how well apples will store.
In New York, she said, David Rosenberger found that a one-thirtieth rate of glyphosate on trees, equivalent to what might be caused by drift, caused internal browning on Empire apples in storage. Empire and Macoun apples are very sensitive to glyphosate.
One single drift event can cause a tenfold increase in shikimic acid levels two years later, Mathers said.