Ask The Expert: What’s Happening to My Rhododendrons?

Warm weather is finally here in the Hamptons, which means that you are most likely thinking about the beautiful trees that you love so much in your back yard, and what you should be doing to get them ready for spring’s official arrival. Fox Tree Service president Bart L. Fuscoa Certified Arborist and Registered Consulting Arborist—is here to share his insights.

The Question: The branches on my Rhododendrons are turning black. Are these shrubs dying?

The Answer from Fox Tree Service president Bart L. Fusco: The black flaky like material on the branch structure of many plants is called Sooty Mold.  This grows on the high sugar concentrated waste (known as honey dew) of many of the piercing-sucking insects and arachnids, such as spider mites.

A group of Rhododendrons adjacent & protected by a house or other warm structure will be showing this problem now.  Even when these plants are treated with a pesticide, the umbrella like shape of the waxy evergreen leaves protect the scale insects from direct contact with the chemical.  Especially when horticultural or dormant oil is used on the plants, as these surfactants must contact the Scale insect to be effective.

A quick inspection of plants showing black branches of other parts of the plants colonized by Sooty Mold will lead to the discovery of an insect or Arachnid infestation.

The white silk like material is created by the Scale insects and easily seen without the use of any type of magnification.  This Scale population is active and their waste, after they feed, is deposited on the branches and leaves below the feeding sites.  The mold quickly colonizes the deposits and turns black.  This has the effect of interfering with Photosynthesis and can lead to the decline of any plant whose leaves are covered.

Also this waste matter, after falling on lawn furniture or other outside equipment will also be colonized by Sooty Mold.  This problem on non-plant material can be cleaned off with soapy water.  Many people have asked me what is causing the trees to drop “sap” onto the windshields of their cars.  Again this is the honey dew from the insects infesting the trees above the car and is not sap.

As long as the piercing-sucking insects or arachnids producing the honey dew is eliminated, the sooty mold will wear off the plant material in time.  If the mold is interfering in a plants food production, then a plant wash can be used to quickly remove the problem.

For more information on the Rhododendrons in your yard or any other tree-related matters, you can contact Fox Tree Service in Suffolk at 631-283-6700, and in Nassau at 516-921-7111, or visit online at  

Read more tree tips from Bart L. Fusco right here!

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