HORTICULTURE
Roundupâ Injury Symptoms in Perennial Plants
This page is a
general description of how glyphosate works and the symptoms with links to specific
examples for apples, blueberries, brambles, grapes and strawberries.
Roundupâ (glyphosate) herbicide, and its generic cousins are often used for weed control in Michigan fruit plantings. These materials interfere with amino acid synthesis and disrupt protein synthesis. They are nonselective postemergent materials that work well against both annuals and perennials. Glyphosate is absorbed by the leaves and poisons the biochemical machinery in the leaf. It is then translocated with the sugars from photosynthesis to actively growing tissues where it poisons them. Glyphosate materials cause little damage if they not do drift on to green tissues such as green leaves and young stems. Many farmers and homeowners are using glyphosate as a total weed control program with several applications per year to kill weeds. This eliminates the use of soil active materials that could harm the plant.
I often recommend Roundupâ or other materials in the fall to kill invasive perennials in fruit plantings because the fall is the best time to kill perennial weeds with glyphosate materials. Unfortunately, it is an excellent time to kill any plant with this broad-spectrum plant killer. The herbicide is taken up by the plant and stored in the bark and wood of the stems as well as the root system. When the plant begins growth in the spring the herbicide stunts new growth and if the dose was high enough it will kill the plant.
Over
the years, I have seen more and more glyphosate injury symptoms in the spring. These
symptoms are the result of glyphosate use the previous year. Glyphosate does not
cause striking injury symptoms especially if it is not applied to actively growing
plants. There may be some yellowing of the leaves where the material was
applied. The glyphosate is then moved from the leaves to actively growing tissues or
to storage tissues for use next year.
The symptom I see most often is small leaves crowded close together. Whenever I see this symptom especially if it is on new growth in the spring I am sure that I am seeing roundup injury. This is the symptom that the homeowner is most likely to encounter. Zinc deficiency causes a similar symptom but I am more likely to see zinc later in the year and over most of the plant.
Glyphosate often kills actively growing shoot tips causing stunted growth. This can cause new buds below the tip to begin growth. This results in an increase in branching.
In grapes, I see a lot of branching and misshapen leaves. This looks a little like 2,4-D injury but the shape of the misshapen leaves is different. With 2,4-D the leaves are narrow at the base and widen out to the end with lots of finger like extension on the edge of the leaf, and the leaf surface is relatively smooth. With glyphosate, the leaves look more normal and are crinkled as if they were crushed. I see this symptom; the year of application when the glyphosate was before stops for the season.
Newer
formulations have very effective penetrates can even cause damage to the bark of
established trees. I saw this symptom for several years where growers were using
just glyphosate as a weed control and that seemed to be the only thing that could explain
it.
I had applied Roundupâ to young trees many times and usually saw little if any damage if the bark of the trunk was at least a year old. I was stumped as to what was the cause. I was sure the glyphosate was doing damage but I did not know why applications that caused little injury in the past now caused dramatic damage.
I have since learned that some generic glyphosate herbicides can move into the bark because of the penetrant used in that formulation.
I have posted several other glyphosate injury files with pictures of the different types of injury
