Sooty Mold Solutions

Familiar with black sooty mold on plants? I am. This familiarity is nothing I wanted. It simply came my way… by way of something as sweet-sounding as “honeydew”.

I first noticed sooty mold on my holly, which had developed a black interior and one blackened side. Most of the new branch tips showed the expected green foliage, but deeper within the shrub, the foliage had become a mess.

I panicked, envisioning all my plants being overtaken by the sticky black dust. I called my local nursery for advice. Upon seeing a photo I had sent by cell phone, the voice on the other end exclaimed, “That’s sooty mold. It’s okay, you can fix this!”

So What Is Sooty Mold?

Sooty mold, which transformed my holly into something that looked as though it had come out of a fire sale, clings to foliage, stems, and trunks. The term “sooty” comes into play because, as with fireplace soot, this stuff does not wash off easily. Under the normal spray of a hose, it remains. You can scratch it off with your fingernail enough to know that this is a covering and not something that has blackened the underlying plant material. But the clinging nature of this “soot” lets you know you’ve got a problem on your hands (and under your fingernail).

The “mold” is actually fungi that grow on excretions (sounds better to say “honeydew”, doesn’t it?) left behind by tiny insects (scale, aphids, whiteflies). These and other tiny sap-sucking insects take in protein and sugar by way of sap; they must expel excess sugar from their bodies. Honeydew.

While “black mold” makes us worry about human health, there is widespread agreement that black sooty “mold” will not harm us gardeners. (It’s not even mold, after all.) Further, it will not necessarily harm plants. But if left to get out-of-hand, it can get in the way of photosynthesis for obvious reasons. This is what causes damage over time.

What To Do About Black Sooty Mold

With black sooty mold, you must start at the start: Focus on the tiny creatures that are excreting the honeydew. If you check the leaves, you might notice white spots, waxy patches, cottony growths, or yellowish eggs/bugs. (I had such a bad problem in one area that white spots appeared atop the sooty mold... a mess!) These sights will signal a pest infestation. But truthfully, you don’t need to check, as you can pretty much rest assured your plant is being bothered by tiny insects.

I hate to spray with anything other than water, so the methods I use to eradicate insects are time-consuming and sometimes have limited impact. But here is what I have done to lessen the problem of black sooty mold on my plants.

Manual Soap-and-Water Scrub

With my holly, my first method of attack was to try a soap-and-water scrub of the blackened foliage. I filled a bucket with a 10-90 mix of dish detergent and warm water and went to work. With wet cloth in hand, I rubbed each leaf. My cloth turned dark very quickly, so I knew my efforts were having some impact. In addition to removing the “soot”, I knew I was doing a thing or two to remove the scale. I followed up with a high-powered spray from my watering hose, rinsing off any remaining soap residue. (IMPACT: very effective; summer-long impact; admittedly awkward and messy)

Improving Light and Air Circulation

Sooty mold thrives in places with limited sunlight and poor air circulation. While insect excretions of honeydew exist, to some extent, throughout the garden (charming, right?), sooty mold develops in areas that are dark and cramped. So I got to work on trimming back nearby bushes, opening up the area around my holly as much as possible. (IMPACT: very good; helpful in many ways)

Green Liquid Cleaner

Some gardeners use a green liquid cleaner/degreaser (think: “basic vert” [French for “green”] and you’ll know the one I mean). This cleaner is widely available at hardware and big box stores. Some gardeners spray a diluted solution of this cleaner onto sooty mold-susceptible plants. They do this weekly, letting it dry on the foliage and eradicate scale. I would use this only for ornamental plants. I tried this on a Japanese andromeda that got covered in black sooty mold. (Holly hasn’t been the only plant with this issue in my yard.) This method seemed to curb an outbreak, but it must be used regularly. (IMPACT: good, but requires frequent treatments)

Hard Freeze

Stubborn “soot” can flake off shrubs and plants after a hard freeze. I always look forward to this. While a hard freeze will not help you to address the insect/honeydew root cause, it can help make things look better over time, once your insect problem is solved. (IMPACT: cosmetic, but nice to know the weather will help remove the mess)

Summing Up

Gardeners who are comfortable using insecticidal soaps, neem oil, copper fungicides… they’d share other methods of eradicating pests and fungi. But I find the methods just noted effective enough for me. They help my plants, their effects last a while, and they give me peace of mind.

One thing I will say is that although the problem of sooty mold on plants may cause gardeners to panic, as I did, my nursery friends were right: It can be fixed (see last photo) with effort, at least for a time. You might find a remedy that helps you for a summer, and then the effect wears off and prompts the need for additional treatments. Still, that’s something. I suspect that even the folks who use some of the harsher sprays have to repeat the treatments. And you can rest assured that your plants will not perish while you are finding the best solution to your black sooty mold problem. This is a case where things look worse than they are. Still, the problem is not entirely cosmetic, and so this is something you should address. 

Ah, the life of a gardener. It's not all roses and honey... sometimes we get honeydew. But it's fascinating nonetheless.