Trees and herbicide drift
by Bob Hartzler

May 24, 1999 -  Delays in field work have put farmers and commercial applicators under pressure to apply herbicides when winds exceed appropriate levels.  Applications made during these periods are likely to have significant movement of herbicide from the target area, regardless of nozzle type, spray pressure or drift retardants used.  Unfortunately, many applicators are willing to spray during high winds when using herbicides with a relatively low risk of damaging off-target plants (most preemergence products).  Applicators need to recognize that herbicide drift is unacceptable, regardless of the potential for off-target injury.   The Pesticide Applicator Training program at the University of Wisconsin has produced a new bulletin discussing drift issues.  It is available on the web in an Adobe Acrobat file ( http://ipcm.wisc.edu/pat/pdfs/drift_pdf.pdf .  This bulletin not only discusses the mechanics of drift, but also the responsibilities of pesticide applicators.

While herbicide drift often is responsible for injury to non-target plants, plant diseases or environmental stress can cause abnormalities on plants that are confused for herbicide injury.  This spring's weather has been as tough on ornamentals and shade trees as it has on the corn crop.  Many trees across the state, particularly ashes, are experiencing foliar problems this spring.  A common disease of ashes during wet springs is anthracnose, and this disease is widespread this year  (anthracnose) . According to Paula Flynn, Department of Plant Pathology, symptoms of anthracnose include brown to black blotches on leaflets that commonly start at the margin and move towards the midvein. The leaflets tend to curl toward the dead areas, resulting in a twisted leaf which may be confused with epinastic responses caused by growth regulator herbicides.  The lower branches are often infected more severely, and premature defoliation often occurs.   Anthracnose is also a problem found on other trees, including oak, maple and walnut.  Jeff Iles, Department of Horticulture, has reported that ash trees are losing leaves without the typical anthracnose symptoms.  He speculates that low temperatures (mid 20's) in late April and early May might have damaged leaves as they emerged.  These damaged leaves often don't fall until several weeks after the event responsible for the damage.   Plant Pathology's Plant Disease Clinic maintains a website has descriptions of other diseases that sometimes are mistaken for herbicide injury http://www.exnet.iastate.edu/Pages/plantpath/diseases.html.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University