Glyphosate Injury to Nursery, Landscape, and Orchard Trees
Glyphosate is a systemic herbicide that travels through the phloem and accumulates in meristematic tissues (primarily in roots). It kills plants by inhibiting the synthesis of a critical enzyme used in the shikimate pathway, which is needed for plant growth and survival. It is a strong chelator that binds to calcium, manganese, copper, iron, nickle, and zinc. Although this chelating action may inhibit the growth of soil microbes that depend on these essential nutrients, it also serves to immobilize the pesticide in the soil, preventing leaching. As a result, glyphosate can accumulate in soils and perennial plants for several years.
Glyphosate-killed weeds or tree stumps exude small amounts of the chemical into the soil through the roots. Glyphosate released into the soil has been shown to affect growth of microorganisms in the vicinity of the roots and in the zone of application. Woody plants may absorb the chemical via root to root contact or when root hairs contact the exudates from treated plants. Once inside the non-target plant, it may persist for several years while disrupting the shikimate pathway, reducing plants’ defenses and leading to a variety of symptoms (Figs. 1 and 2). Several studies have shown that plants exposed to glyphosate are more susceptible to soil borne pathogens such as Phytophthora and Fusarium.
Dr. Hannah Mathers at Ohio State University has led research on the high incidence of bark split of landscape and nursery trees. She found that bark splitting occurs when drift from late season applications is absorbed into thin or pigment-barked trees. Glyphosate deteriorates the bark structure and reduces the winter hardiness of the plant. Mathers found that glyphosate products with added surfactants (a chemical that helps the glyphosate bind to the target plant) caused the greatest reduction in winter hardiness due to the increased uptake. She estimates that losses from bark cracking via glyphosate approaches $6.6 million a year in the nursery and landscape industry.
Plant pathologists, fruit specialists, and apple growers in New York, Michigan, and other locations have seen an increase in bark damage, dieback, and basal cankers of several varieties. Based on Mathers’ research, they are now hypothesizing that glyphosate-induced cold injury may be a culprit because affected orchards all received glyphosate applications at least once/year.
Many of these apple trees are being attacked by opportunistic pathogens at the point of winter injury, which occur as basal or stem cankers (Figs. 3 and 4). Michigan pathologists found at least six different species of pathogenic fungi in stem cankers. In New York, they are primarily seeing one pathogen, Botryosphaeria dothidea. Normally these pathogens are not a problem in healthy apple orchards.
The severity of damage seems to be variety-dependent, mostly affecting newer McIntosh cultivars, Macoun, Cortland, and Honeycrisp. The relationships between trunk cankers and herbicide injury or infection by opportunistic pathogens remains to be proven in Michigan and New York, but the widespread death of these particular varieties is still occurring, and is a concern.
• Avoid glyphosate applications in Macoun, Cortland, and Honeycrisp apple orchards.