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What is that Black Soot on My Plants?

Got soot?

Black soot left / Aphids Right

Are your plants looking like you have a coal mine right next to you? If so, read on. This summer I came across something I hadn’t seen before. I first spotted it on some 1st year small trees, then on a crape myrtle and then on some photinias. In addition to this black soot, there were some white/light green little egg looking things that covered some of the new growth. Puzzled, I did some quick research and discovered the source of this mystery. The culprit, aphids. I had read about aphids and new they existed but had no clue they caused this black sooty stuff on plants. As it turns out, aphids suck on the new growth to extract the plant juices. In turn, they secrete this stuff called “honeydew” which then molds (black stuff that you see). In addition to the black stuff, you may also notice that a lot of the new growth is deformed.

The incredible thing is that though the aphids are pretty small, the black stuff covers the entire plant! Also, if you look at the ground, you will find that it completely covers that as well including any understory shade plantings you may have. So it would seem that aphids are quite the voracious eaters!

The good news is that aphids are pretty easy to get rid of with organic non invasive products. Below are some suggestions depending on severity and/or style of gardening:

Severe Infestation

A severe infestation will probably require spraying.  Since aphids are quite delicate, no need to break out the big guns. A good parafinic oil based product will work.  I use Bonide All season spray which works great on aphids, best of all it’s organic so you can spray without the guilt. I like this option because it provides all around protection and one application is enough to do the trick for a while. You can also apply any time of the year so if you spray early, you can get the eggs before your plant looks like the inside of a chimney.   Another alternative is organic  insecticidal soap. It’s also an organic option so just as safe as spraying the parfinic (Neem) oil.  Use this one for spot spraying.

Medium Infestations or Organic Gardening

If you are an organic gardener or your infestation is medium, then you can simply spot spray the plant with organic insecticidal soap. The soap strips the wax covering the aphid’s body so a few sprayings may be needed to do the trick. Works great and very safe. Home remedies can also do the trick. Cooking oil along with your dish soap may work but  I would try that as a last resort  or if you are dead set on not buying a commercial organic option.  It should not be used on hot sunny days or on plants that get direct sun as the oil will cause sun damage on the foliage and could kill your plant right along with the aphids.

Mild Infestations

For really mild infestations or at the first signs of aphids,  simply knock them off with a stream of water from a garden hose nozzle.  You can also use a manual approach and simply crush the little suckers with your finders (they pop very easily but watch out for PETA! =-). The downside to these methods is that you may miss some and they will quickly come back. So constant vigilance is required  if this is your preferred method. Even for mild infestations, consider using either the insecticidal soap or parafinic (Neem) oil simply because they are faster and way more effective. Remember, if you miss a few, they will be back and you’d be surprised at how fast these suckers spread and turn your beloved plant into a sootie mess.

The bad news is that regardless of what you use, the black stuff/mold will remain on the foliage. You will have to clean that off manually leaf by leaf which is impossible unless you have a small plant. Of course you can leave it since it will not really hurt the plant. Just be vigilant the following year and eliminate the aphids as soon as you see them or  use a preventive spray of the above mentioned products early in the year.  I am not sure on what causes aphids, but they seem to target the same plants/bushes every year so just because you get them early one year it does not mean they wont come back the next.

4 comments to What is that Black Soot on My Plants?

  • Angela

    Thanks for the information. My big Oak tree is inundated with this black soot and it is so tall I can’t possibly spray it all. Any ideas?

  • Hi there, on a big oak tree I don’t suppose there is much you can do about it. Fortunately, the soot does not seem to harm the three it is more of a cosmetic thing. Even if you could spray, it still is not going get rid of the soot just kill the bugs causing the soot. I wouldn’t worry about it too much and since it is not necessarily a yearly recurring thing, your tree should be ok. I have only had the aphid problem once and have not seen them since.

  • betsy

    I have sixty rhodos on banks in the mountains. Nearly every leaf is affected by soot. I am saddened as the leaves were so glossy and green until August…then the soot occurred. I have used sprays of insecticides and fungicides; see no aphids underneath but guess they are there. Should I now use soapy water spray and clean out under bushes?

  • Hi there, unfortunately, once the soot occurs it must be removed manually. In deciduous shrubs, you just leave it until leaves fall off. Since these are evergreens, I would suggest trying an insecticidal soap early in the morning before the sun is strong. Even if you try soap, i do not believe the soot will simply come off right away without some sort of hand cleaning. I have found it to be quite stubborn. Since you have 60 of them, that will be quite a chore. I have not had this experience with evergreens so I honestly do not know what would happen if you do not clean the soot off somehow. I imagine that as long as you get rid of the aphids, the elements will probably do the cleaning for you but it may take some time.