Crape myrtle problem: Herbicide damage on windy days final determination

Jobs and WorkplaceU.S. Department of Agriculture

Below is an e-mail detailing the outcome of possible disease problems associated with dozens of crape myrtles in Elizabeth City, N.C., earlier this year. McDonald Garden Center assisted with the investigation.

"Happily I can tell you our crape myrtle trees have fading symptoms which were widespread in spring. Few tiny narrowed leaves remain on the more than 200 trees affected in Elizabeth City, NC.

With the passage of time, it appears all affected crape myrtle trees will be normal again.

All tests for infectious pathogens turned up negative for crape myrtles affected this spring.
Testing for bacteria, viruses, and phytoplasmas were done in state and Federal laboratories.

Most trees have recovered or are in recovery.  Some of those I sprayed with dormant oil
and systemic fungicide (propiconazole) recovered more quickly than unsprayed trees.
But all have recovered fully or are making progress recovering as each week passes.

It is obvious now that our widespread mysterious symptoms on crape myrtle trees were
due to herbicide injury, rather than an infectious causal agent, such as phytoplasmas.
Symptoms of both can be very similar.  The far ranging occurrence of the symptoms
and time of year it happened, made it hard to believe that herbicides were the cause.
I had received assurances that no herbicides were used broadly like that in our city.
However, I had not then found the right City department to interview.

After many weeks of interviews with homeowners, and city workers, after getting the
results of the plant clinic at NC State University and USDA Laboratories in Washington, DC,
I have concluded the entire problem was due to widespread herbicide injury by city workers'
application of glyphosate to the curbs in certain neighborhoods on hot, perhaps breezy days,
in April 2012.  We had some days in that month when the day time high temperature reached 89 F.
This was very unusual weather.  Other neighborhoods which received identical treatments showed
little or no reaction from the crape myrtle trees in their vicinity.

I have educated staff of the City of Elizabeth City on drift reduction nozzles and drift reduction
adjuvants, useful for the spraying of herbicides, to keep them on-target.   We also discussed the
relative volatility of various herbicide formulations.  We've also discussed pre-emergence chemicals
which may substitute for, or augment glyphosate use, requiring less glyphosate to be needed.

I'm glad the crape myrtle trees will be fine." -- Margaret Pooler, research leader,  USDA/ARS U.S. National Arboretum, Floral and Nursery Plants Research Unit

Meet the plant

Learn about the 29 U.S. National Arboretum crape myrtle introductions at: http://www.usna.usda.gov/PhotoGallery/CrapemyrtleGallery/index.html

Posted by Kathy Van Mullekomkvanmullekom@aol.com

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