What the Heck is Wrong with My Dogwood?!

 This beautiful small tree is both the state flower and state tree of Virginia, and for good reason.  The tree flourishes in the woodland understory throughout our state, with showy spring flowers, a deep green tiered canopy, and a knock-out deep purple fall color.  It’s no wonder that these trees have been a staple of Richmond landscapes and are usually put on a pedestal at key locations around the home.  Any stress or decline on a mature Flowering Dogwood can cause great distress, as the tree symbolizes almost more than a simple tree.

With all of that said, Flowering Dogwoods have had a doozy of a year, and they are looking quite terrible throughout the region.  Fear not though, this is not a repeat of tthe mid 90’s die-off caused by Dogwood Anthracnose.  It’s really a confluence of several unfavorable events that have Dogwoods looking so shabby.

In an unusual but wholly pleasant surprise, we had a true spring in Richmond this year, and the trees and shrubs could not have been more pleased.  With cooler temperatures and steady rain, plants were able to grow lushly without much pressure from fungal attacks.  It looked like it was going to be a banner year for Richmond landscapes.  Then a warm but usually wet June turned into a hot and dry month followed by an even hotter and drier July that quickly put a stop to happy and healthy plants.  Plants were forced to slam on the brakes, and many were not ready for the stress so early in the year (as this type of weather is more typical in August and early September).

The combination of the early heat and dry weather was particularly cruel to Dogwoods as it fosters the development of Dogwood Powdery Mildew, which is one of the uncommon fungal diseases caused by two different fungi that prefer and thrive in humid but dry conditions.  The early timing of the dry spell was particularly damaging, as the trees were stressed by the lack of water early in the season, when they would normally be thriving.  This meant that the trees still had very lush leaves that were not hardened off and ready for the onslaught of dry weather.  When typical dry spells occur in late summer, the trees are well acclimated to the heat and have developed thicker cuticles to the leaves, which can help fend off Powdery Mildew.  With the early dry spell, the leaves were perfect hosts for the fungi to colonize and multiply.

Heavily damaged trees look as if they have been scorched by a flame, with leaves drooping and turning black.  Typically the worst damage will be on the lower interior leaves of the tree where the humidity can remain high, even during extremely hot days.  New growth may be emerging past the damage, which will likely become infected as well, due to its extremely vegetative and vulnerable state.  Understandingly, many homeowners are dismayed and assume that the tree is dying – but luckily that is not the case.

powdery mildew on dogwood – photo by Bob Mulrooney, U of DE

Powdery Mildew are caused by fungi that are categorized as Obligate Parasites, which are organisms that require another organism to complete its life cycle.  This is good news as it means that the fungus does not actually kill the tissue, but instead needs to keep it alive.  Heavily infected trees appear to have dieback along the margins of the leaves, as the foliage is being so heavily parasitized.  While this is very stressful to the trees, it is much less damaging than the dreaded Dogwood Anthracnose (Discula destructiva) that wiped out numerous Virginia Dogwoods in the late 90s and early 2000s.  With the damage from the two being very similar and easily confused, it is understandable that many homeowners view such dieback as a serious issue.

In most years, Powdery Mildew appears in late summer and the trees are prepared so damage is limited to some twisting and cupping of affected leaves. This limited damage does little to the overall health of the trees and any treatments or remediation are usually unwarranted.  Early season infections like this year can be a bit of a different story.  With significant damage to affected foliage, the tree is essentially put on an involuntary diet that will limit its overall vigor for the rest of the year.  Early damage is unlikely to kill the tree but will cause decline and reduced flowering the following spring.  Consecutive years of infection can force a tree into an irrecoverable spiral of decline, leading to the death of the tree.

When Powdery Mildew is damaging early in the season, it is best to do little to the tree, as it is likely to recover.  Deep watering to help the tree through the dry period will help reduce stress, but do not fertilize the trees as that will likely push new succulent growth (which will likely be damaged by further infection).  If two consecutive years of damage and defoliation occur, it is then time to start planning for remedial treatments.  The fungi that cause Powdery Mildew are weak pathogens, so nearly any fungicide will work to kill the fungi, or it can be simply washed off with a coarse spray of water that resembles a heavy rain.  By hosing down the foliage of the tree, it acts as a mechanical control that simply washes away the fungi and provides water to the tree.