HORTICULTURE
Roundupâ Injury Symptoms in Apples
Roundupâ (glyphosate) herbicide, and its generic knockoffs
are often used for weed control in Michigan apple plantings. These materials disrupt
protein synthesis. They are nonselective contact materials that work well against
both annuals and perennials. Glyphosate generally causes no damage if it does not
contact green tissues such as leaves and young stems. Many growers and homeowners
are using glyphosate as their weed control program with several applications per year to
kill weeds.
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 also a good time to kill any perennial plant. 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.
Early season applications are not as risky and some growers use
Roundupâ to kill or burn back root suckers at
the base of their trees. Roundupâ 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 in the
growing shoot tips where it poisons them. This is a good way to kill annuals or burn back
the tops of perennials. I tell growers that they should be careful after July 1. This is a
good cutoff date for using glyphosate around green tissue on plants you want to save.
Glyphosate does not break down rapidly in the plant and often causes dramatic
effects the next year. Large plants survive by outgrowing or diluting the
application. Where the injury is apparent on only a small part of the plant, new
tissues will show little damage and you will be hard pressed to find the affected limb
again.
Over
the last few years, I have seen what I believe is glyphosate injury to the trunks of older
trees. I have seen 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. The active ingredient gets deep enough into the bark to kill the cambium layer that makes new wood and bark in the woody parts of the tree. This causes a canker to form on the trunk. These cankers are usually located on the sides of the tree where unshielded sprayers would deposit the spray. There is not much that can be done to other than avoid repeated application of glyphosate materials with penetrants to the trunks of trees.
