HORTICULTURE

Roundup^â Injury Symptoms in Apples

Mark Longstroth District Extension Fruit Educator

Leaf Symptoms

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 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.  See my article on the interaction of glyphosate and disease on apple trunks

 

 

 

At right are two pictures of the trunk of an apple tree.  These pictures show that there was damage to the trunk causing sproats to appear on the trunk.  This grower applied glyphosate products four times a year to control weeds and over time had killed the bark close to the ground on either side of the tree. 

 

The grower had hit this tree with a mower and noticed that the large chunk of bark that he had ripped off the tree was dead and there were large cankers on either side of the tree.  After he noticed that many of his weaker trees had basal cankers he called me.  This was the second time in a month that I had been called by a grower about weak trees with basal cankers and both growers used only roundup for weed control applied several times a year.