Growers often express concern that a herbicide program using glyphosate (e.g., Roundup) may not be totally safe for their trees.
Some won’t use the herbicide when trees are young. Most use directed sprays, shields, and tree guards to protect tree trunks and low limbs when killing weeds in strips under the trees. Others paint tree trunks to provide protection.
Now, it appears, these precautions—and even more—should be used when applying glyphosate in fruit orchards.
Besides being perhaps the most effective and nonselective plant killer ever devised—and thus worthy of respect when it is applied near any desirable plant—glyphosate has been linked to many other indirect or subtle effects. In recently published scientific reports, use of glyphosate has been linked to:
—An increase in plant diseases, including cankers in fruit, caused by reduced resistance to diseases and pests
—An increase in bark cracking and brittleness in tree trunks
—A tying-up of nutritionally important minerals, resulting in less thrifty plants and a decrease in the nutritional quality of the crops produced by these plants. This property, chelation, is the mode by which glyphosate kills plants.
—The destruction of important soil flora, plants that are important in nitrogen fixation, mineralization, and other soil fertility processes
Moreover, glyphosate is cumulative in both plant tissue and in the soil. Repeated applications add to previous doses, so even minor drift can add up to problems in future years. Glyphosate injury is often attributed to other causes such as drought, high temperatures, water fluctuations, or cool soils.
Glyphosate gets into the soil by direct application to the ground, from decomposing residues of plants that are killed by glyphosate, and from root exudates from glyphosate-resistant plants that grow after glyphosate is applied. In orchard situations, glyphosate could be introduced into the soil in cover crops or vegetation sprayed during orchard preplanting operations as well as by sprays aimed at weeds in the tree rows.
In addition, the effects of glyphosate occur at much lower doses than growers typically apply for weed control.
Much of this story is detailed in recent reports, two of them published by Dr. Don Huber, a plant pathologist and professor emeritus at Purdue University. Some appears in work done by Dr. Hannah Mathers, an Ohio State University Extension specialist working in the nursery industry.
Huber’s main work is with field crops and citrus fruit. Mathers works with perennial woody plants and has experience in apples and cherries, where she studied cold injury during her doctoral work at Cornell University.
Last fall, Mark Longstroth, the Michigan State University District Extension Fruit Educator in Van Buren County in southwest Michigan, published an article with pictures on his Web site describing what he thinks is glyphosate injury to apple trees. These can be seen at www.canr.msu.edu/vanburen/rndtrunk.htm.
“I had applied Roundup on young trees many times and usually saw little if any damage if the bark of the trunk was a least a year old,” he said. “This was a very common practice from the 1980s on. From time to time, I would see symptoms on the branches in the spring from what I assumed was a fall application that was absorbed by basal root suckers. In 2005, I saw what I believe was glyphosate injury to the trunks of older trees. I saw it is several orchards where the grower used only glyphosate herbicides several times a year.”
Since then, Longstroth has associated other injury symptoms with repeated use of the herbicide—basal cankers, dead bark at the base of the trunk, black rotlike cankers higher up the tree, and branches in the tree showing discolored and misshapen leaves.
In a paper published last year in the European Journal of Agronomy and in another published in February in the proceedings of the Fluid Fertilizer Forum, Huber reviewed the mounting evidence that glyphosate (N-phosphonomethyl glycine), the most extensively used herbicide in the history of agriculture, has many side effects.
Glyphosate was originally developed by Stauffer Chemical in the 1960s, because it was recognized as a highly effective chelating agent, Huber said, meaning it could be used as an agent to tie up minerals like iron, manganese, zinc, calcium, nickel, and copper. That is what makes it an effective herbicide, which is where Monsanto entered the picture with Roundup. Many of the pesticides used in agriculture are metal chelators, Huber said; glyphosate is just much more effective at it.
It kills plants by blocking a critical enzyme pathway known as the shikimic acid pathway. The blocked enzyme is essential for respiration in plants, so plants that receive a full dose of glyphosate cannot survive unless they are engineered or evolve to be resistant. It also weakens plants’ defenses to infectious organisms.
Besides weakening plant defenses and increasing pathogen populations and virulence, glyphosate can have indirect effects on predisposition to diseases resulting from: immobilization of specific micronutrients involved in disease resistance; reduced growth and vigor of the plant from accumulation of glyphosate in meristematic root, shoot, and reproductive tissues; altered physiological efficiency; and modification of the soil microflora affecting the availability of nutrients involved in physiological disease resistance.
Diseases Huber associates with glyphosate include canker in apples and various other crops, black goo in grapes, and a host of plant diseases caused by Fusarium, Phytophthora, Sclerotinia, and Pythium.
In 2003, Mathers was contacted by Ohio nursery stock growers who were seeing bark cracking and scald injury. Cracks were often multiple, weren’t on the southwest side of trees, and sometimes occurred in scaffold branches. In her experience, this wasn’t cold injury. She began a literature search and experiments to determine the cause.
She found increasing reports of bark splitting in pears, cherries, and crab apples used in landscaping.
“Apple growers in Pennsylvania have observed decline and death of different cultivars grafted on M.26, M.9, and B.9 rootstocks,” she wrote in one report. “The problem is widespread across Pennsylvania. Pomologists in New York have observed the severity of damage to be variety-dependent, with especially severe injury noted on Cortland and Macoun trees.”
Cracks need a point of injury from which to grow, she said. A necrotic spot on the tree, injury from a machine, hoe, or string trimmer, pruning scars, graft unions, or places where suckers are removed can be starting points. While these points can also be places of easier entry for glyphosate into the tree, she believes the accumulation of glyphosate in the tree can cause stress and injury.
The use of more effective surfactants with glyphosate means it penetrates better and faster. Thin-barked trees like apples and stone fruit are particularly susceptible to glyphosate, even to small amounts of drift. “It drifts very readily from 30 feet or more away. It shows up as injury in odd places years later,” she said.
When the patent went off glyphosate and the price began to come down, use increased greatly, Mathers said. “People got lazy over the use of glyphosate,” she said. “Some even were using it to control suckers. This is a very serious plant killer,” she said.