Rhododendron Problems: How To Get Rid Of Sooty Mold On Rhododendrons

Sooty Mold On Rhododendron Plants
(Image credit: Artur Henryk Bialosiewicz)

Rhododendrons are at their best in spring when they produce large clusters of showy flowers against a backdrop of glossy green foliage. Rhododendron problems such as sooty mold on leaves ruin the display with unsightly black splotches on the foliage. Although the sooty mold fungus grows on the surface of the leaves and rarely causes permanent damage, it can seriously impact the appearance of the rhododendrons.

How to Get Rid of Sooty Mold on Rhododendrons

Sooty mold on rhododendron foliage is easily rubbed off with your fingers. You may be able to remove part of it with a strong spray of water from a hose. These measures are only temporary, however, and the only way to keep the mold from returning is to treat the cause of the problem. Small, sucking insects such as scale, whiteflies, and aphids secrete a sweet sticky substance called honeydew as they feed. Within a few days, honeydew becomes infested with sooty mold. The best way of controlling sooty mold is to control the insects that produce honeydew.

Pests Causing Sooty Mold Leaves

As soon as you notice the black fungus on rhododendron shrubs, check the leaves carefully to determine which insect is responsible and treat it appropriately.

  • Scale - Scale insects commonly cause black fungus on rhododendron. These insects are flat, tan-colored discs on foliage and stems that at first glance appear to be growths on the leaves rather than insects. You can sometimes pry them off of the leaves with your fingernail or a sharp knife. Use insecticidal soaps, horticultural oils, or a product that contains both soaps and oils against scale. Follow the label carefully, especially regarding timing. Oils sprayed at the wrong time can damage the plant and will not kill the insect. Several repeat applications of the sprays may be necessary.
  • Whiteflies - Whiteflies are very tiny flying insects that rise above the shrub in a cloud when it is shaken. You can vacuum up these insects with a hand-held vacuum cleaner. Kill the vacuumed insects by freezing the bag overnight and disposing of it the following morning. Aluminum foil or other reflective mulch is very effective against whiteflies, but it is unsightly in the garden. Insecticidal soap is effective if it comes into direct contact with the insect. Pay particular attention to the undersides of leaves when using insecticidal soap when controlling sooty mold caused by these pests.
  • Aphids - Aphids are tiny, pear-shaped insects that can be almost any color. Treatment for sooty mold on leaves caused from aphids is much the same as you would for scale insects.

Rhododendron problems like sooty mold don't have to be an issue. Learning how to get rid of sooty mold on rhododendrons means getting rid of the pests that contribute to the fungal disease.

Jackie Carroll

Jackie Carroll has written over 500 articles for Gardening Know How on a wide range of topics.