HORTICULTURE
Roundupâ Injury Symptoms in Blueberries
Roundupâ (glyphosate) herbicide and generic glyphosate
products are often used for weed control in Michigan fruit plantings. These
materials interfere with amino acid synthesis disrupting protein synthesis. They are
nonselective contact herbicides that work well against both annuals and perennials.
Many Michigan blueberry growers are using glyphosate as a total weed control program with
several applications per year to kill weeds. Blueberries are one of the plants where
I see a lot of injury symptoms. The symptom I see most often is tiny leaves
crowded close together. Whenever I see this symptom when there are very clean weed
free rows, I am sure that I am seeing roundup injury. This is the symptom I see on
older canes.
Glyphosate materials cause no damage if they not do contact green
tissues such as green leaves and young stems. This is a problem since blueberries
send up new shoots each year. This means there are leaves and new green shoots at
the base of the plant all year long. Growers need to be very careful to keep the
glyphosate from contacting the plant. Glyphosate does not cause any striking
symptoms so growers think they are doing a good job. There may be some yellowing of
the leaves where the material was applied. Glyphosate does not break down rapidly in
the plant and moves with the photosynthate to the growing tip where it may cause the tip
to die. I believe that many of these shoots are weakened and may be more likely to
suffer from disease infection.
The shoot on the right shows what I think are typical spring symptoms of glyphosate injury from an application the previous year. I believe the green shoot recieved a small dose of the herbicide. The small straplike leaves and short internodes (stem between the leaves) are a common symptom. Compare those leaves to the leaves on the shoot in the background. The tip of this shoot was killed by a canker forcing new shoots to start growing below the dead tissue. Large blueberry plants survive by outgrowing or diluting the herbicide application, but individual shoots are often stunted and deformed.
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. I see dead or stunted shoots in the crown of the crown of the blueberry bush. These stunted shoots often have lots of short branches at the tip. I think this is due to repeated applications of glyphosate, which drift into the crown stunting and killing the new shoots.
