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Cacti & Succulents

My pencil cactus is dying

Theresa Mancarella

7 years ago

I had to over water my pencil cactus to treat it for fungus gnats. Now it's stalks are turning brown and it is slowly dying. What can I do?

Featured Answer

Sue

7 years ago

Fungus gnat larva like moist decomposing organic matter. The problem is that the soil has been consistently too wet, even before the pesticide drenches. Unpot it, trim off the soft or rotten parts, and repot it in a soil mix that is much better draining. Perlite is readily available, and adding at least 50% of that to a commercial cactus mix would be a good place to start. Then don't overwater. The fungus gnats will not be a problem if the soil dries out regularly, no poisons needed.

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Comments (6)

gardengal48 (PNW Z8/9)
7 years ago
I'm not sure I understand.........overwatering is what causes problems with fungus gnats to begin with. It's certainly not a method for controlling them!

If removed from any potting soil and the roots examined, you should be able to determine by color and condition how far progressed any root rot is. Cut off all dead, mushy or damaged roots (and any similar brown or mushy top growth) and repot in fresh potting soil specifically intended for cacti and succulents. And water very sparingly going forward - my pencil cactus (Firesticks) gets watered maybe once or twice a month in winter.
Theresa Mancarella
Original Author
7 years ago
I was using a pesticide because the gnats were out of control. It called for 3 soil saturation s in one week. All my plants were infested because of bad soil

Can I clip and put directly in the soil?
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The lighter colored skin in the yellow oval is typical. It doesn't look like a problem. It seems to be a reaction to the cactus skin being close to the moist soil mix. I think that's just corking or something like that and that is a common issue with cacti as they age. It doesn't look nice but doesn't really harm the plant. If the skin becomes a different color, is softer, and looks wet inside then that is rot and you should worry about it. The area in the orange circle is a bit concerning. It doesn't look rotten but it does look like maybe you let the rootstock go a bit too long without water so the rootstock has died back a bit at that top edge. Keep an eye on it. Mainly I'm concerned about it because it is close to the graft between the two plants. I'm not sure what happened to the edges of the rootstock but it looks like corking- the same as the base. The picture is out of focus but so far the top red Gymnoclycium looks OK.
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rina_Ontario,Canada 5a
7 years ago
last modified: 7 years ago
If the soil is badly infected, it should be disposed of.

It would be best to eliminate (or at least considerably decrease) the conditions favourable to fungus gnats...

Pls. post photos of your plant.
Theresa Mancarella
Original Author
7 years ago
I have read disposing of the soil does not help because the gnat larvae attach to the roots. As you can see in the pictures one stalk is already brown almost halfway up.
rina_Ontario,Canada 5a
7 years ago
last modified: 7 years ago
There are so many threads on this forum about fungus gnats...

Personally, I favour potting mix that is made of inorganics: grit, turface, perlite, pumice, lava, scoria...Very little of soil - if any at all - is needed. This will make the potting mix much more inhospitable to fungus gnats. It is larvae that is harder to eliminate since it lives in the soil, so eliminate excessive dampness. And any debris from the plants. And any standing water. And make sure there is good air circulation, especially in more humid environment.

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