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

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

By: Jackie Carroll

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 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.

This article was last updated on

Read more about Rhododendrons

Sooty Mold: An Unsightly Problem with A Simple Solution

    hillshort1 , Neighbor

In my backyard I have a small overlooked corner that has been untended for years. In this corner there is a hedge that up until very recently was covered in a dark black soot. It came as quite a shock to me to realize that the hedge was actually a plant (a podocarpus to be specific) and it was supposed to be GREEN! What can turn a perfectly healthy shrub into an unrecognizable cluster of twigs? Sooty Mold!

Sooty mold, as its name implies, is a dark soot like covering on the leaves and stems of a plant. This "mold" can be scraped off with a fingernail to reveal a healthy green leaf below. Sooty mold is actually a fungus that grows on plants with heavy aphid, scale or whitefly damage. When these pests munch on your plants they excrete a residue euphemistically called "honeydew". Adorable, right? It is on this honeydew that the sooty mold takes up residence. If the infected plant is under a tree, then the pests may be in the tree above causing the honeydew to fall to the shrub below.

Controlling sooty mold is simple. First you need to address the root cause of the sooty mold: aphids, scale, and whiteflies. These critters are rather small and might not be easily visible. But if you have sooty mold it is a sure bet they are around. If the problem plant is under a tree, pest management may be out of reach. In these instances you can still treat the cosmetic problem by skipping to step 2.

To treat your plant against these pest, spray the leaves with a solution of either horticultural oil or neem oil. Horticultural oil and neem oil are less toxic pesticides that will rid your plant of heavy infestations. Spray in the early evening after the plant is no longer in direct sunlight. Just like tanning oil, neem and horticultural oil can exaggerate the effects of the sun and you run the risk of sunburning the leaves if you apply in full sun. Depending on the severity of the infestation, repeat applications may need to be made every two weeks or so. Always follow the directions when using any pesticide.

You remove the soot in the same way you would remove greasy caked on goo in the kitchen, by letting it soak in dishwashing liquid. Use your sprayer and hose down the plant with a solution of dishwashing soap. This step does double-duty by cleaning up the sooty mold and killing some of the pests that cause sooty mold. Avoid using a degreaser or soap/detergent for an automatic washer. Let the soap stay on the plant for a while as it loosens up the soot. Next time it rains (or if you are impatient you can spray it down yourself) you will see much of the black coating simply washing away revealing healthy green leaves.

Chances are that the problem did not come up overnight so be patient with the treatment. You can repeat these steps until you are satisfied with the appearance of your plant but don't rush out to hit it with a pressure washer your plant won't thank you for that! By staying on top of the pest problem the sooty mold will not occur on new growth. Eventually you will have a beautiful, healthy and appropriately green plant once again!

What You’ll Learn

  • What Is Sooty Mold?
  • Damage to Plants
  • Production of Honeydew by Insects
  • Conditions that Favor Sooty Mold
  • Plants That Are at Risk
  • How to Control Sooty Mold
    1. Control Insects That Produce Honeydew
    2. Wash Sooty Mold Off the Plants
  • Insects and Fungi Produce a Perfect Storm

Effective treatments against sooty mold

The only effective treatment against sooty mold is to first treat the insects that are the cause of it.

Here is our advice to avoid or fight them off:

Once the plant is treated and these invasive pests have been removed, clean the leaves so that they may again perform their life-giving function.

Cleaning is easy with a damp cloth or rag dipped in lightly soapy water, which you can then rinse with clear water.

Using organic treatments to treat sooty mold protects the ecosystem that surrounds the plant, most especially bees that come pollinate the flowers.

There is nothing more disappointing than seeing your Rhododendron covered in buds and then not getting any flowers!

These are the 2 common causes of this that I’ve seen.

Bud Blast

Bud Blast is a fungal infection that causes the buds on the Rhododendron to die and turn black before they bloom.

It isn’t a life-threatening condition for the plant, but it is really disappointing for the gardener!

It is thought to be spread by the Rhododendron Leaf Hopper, an insect that doesn’t do much damage to the plant other than spreading this fungus around.

You can get insecticides to kill the bugs, but I prefer to just pick off the infected blooms as soon as I see them (less detrimental to the environment). That decreases the number of fungus spores that are available to be transported and limits the spread of the issue.

Frost Bite

If you have a late frost, it’s possible that the buds on your Rhododendrons were affected.

In this case, they usually turn a light brown color and get a little mushy (for lack of a better term). In other words, they look like they’re been frozen and thawed.

Other than covering the plant during the frost, there isn’t much you can do about this condition. But fortunately, it usually doesn’t cause any lasting damage either. Unless you are trying to grow Rhododendrons that aren’t zoned for your area…then you might want to look for some other varieties that won’t be affected so much by the cold.


Stay ahead of this aggravating situation by catching any sight of sooty mold EARLY as possible. It’s relatively easy to treat but can be a huge hassle if left to spread. It can also lead to the loss of a prized tree or shrub in your landscape!

You Might Also Like

How to Propagate Coleus from Cuttings

Watch the video: Rhododendron Pests u0026 Diseases