Issue 5, May 21, 2012

Bacterial Leaf Spot on Oakleaf Hydrangea

This weekend, while working in my yard, I noticed my Little Honey wasn't looking too good. Of course, I am referring to my Little Honey Oakleaf Hydrangea. The normal gold to chartreuse leaves had developed numerous dark reddish-purple angular spots. Hydrangeas are known to be hosts to several leaf spots, both fungal and bacterial. Oakleaf Hydrangeas, in particular, are known to develop leaf spots caused by the bacterial pathogen Xanthomonas campestris. A few minutes in our plant clinic's diagnostic lab confirmed my suspicions, as I observed bacteria oozing from the leaf spot tissue.

Bacterial Leaf Spot on Little Honey Oakleaf Hydrangea

Bacteria oozing from hydrangea leaf spot.

Bacterial leaf spot on hydrangea (Xanthomonas campestris) likely overwinters in diseased plant debris from one year to the next. It is transferred to new plant tissues by splashing rain and irrigation water. The pathogen likely enters the plant through stomata, other natural openings, and/or plant wounds. Symptoms of infection first appear as water-soak spots. The spots then darken and develop an angular shape. Several spots may enlarge, coalesce and cause death to mature leaves. Disease development is favored by warm, wet conditions which allow for increased bacterial production and dissemination.

Cultural disease control options should be your first course of action.

  1. Remove diseased debris from the site. Debris harbors the bacteria and provides inoculum for future infections. Infected debris should be burned, buried or discarded. On-site composting is not advised.
  2. Monitor the plants closely during the growing season. Leaves displaying leaf spot symptoms should be removed from the plant and site.
  3. Avoid pruning or working near problematic plants when they are wet. These activities will likely spread the pathogen as well as create wounds and entry points. Disinfecting your pruners between cuts will further help reduce the spread of the pathogen.
  4. Avoid overhead irrigation and wetting the foliage. If un-avoidable, irrigate at a time of day that minimizes the duration of leaf wetness. Adequate plant spacing will also help limit the duration of leaf wetness.

Pesticides containing copper sulfate and copper octanoate (copper soap) are broadly labeled for control of leaf spots on ornamentals, but are only marginally effective. They should be applied preventatively or at first sign of disease. Use products containing copper with caution as they may cause phytotoxicity. (Travis Cleveland)

Travis Cleveland

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