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Downy Mildew

Q: My hickory tree leaves are turning brown and the undersides have white spots.

Q:  The hickory trees in my yard and those around the neighborhood seem to be doing poorly. Some of the tops of the leaves are turning brown and the undersides have white spots.

A:  I am so glad you brought me some clippings of the tree because it was hard to do a diagnosis over the phone.  After looking at the leaves under a microscope I was able to identify the culprit as downy mildew, which is a common disease on walnut and hickory.  The white mold occurs on the underside of the leaves in early spring when temperatures are low but moisture is high.  This year we have had an abnormally cool spring which contributed to the large downy mildew outbreak on the hickory trees around your neighborhood.  You can expect the disease to reappear next year if weather conditions are repeated.  Downy mildew will not kill the tree and is really more of a nuisance than a true problem.  During the summer the leaves will shrivel and drop and this would be the best time to clean up the leaf litter.  This simple sanitation method is important in disease control but there is no chemical we would recommend to manage the disease.  This is one of those grin and bare it disease situations.