What is soil mold and how to kill it

Question

I used neem oil spray on a succulent that had mealybugs. 5 days later, This fuzzy, stringy white mold appeared all over the soil. It stunk, and I hated it. I sprayed the mold with neem oil, and the next day there was more. The succulent died because of the mold 3 days later so I got rid of it. I thought, "end of the mold, right? Whew." Well, I was wrong. 8 days later, tiny bits of the same mold started appearing on another succulent that was healthy and never had mealybugs. The succulent was about 5 feet away from the one that first had mold, and it was on a different surface, which was 3 feet lower than the one that first had the mold. The succulent was healthy, and it was not over-watered. When the mold first appeared on this second succulent, the soil was completely dry, so I'm not sure why the mold is there. 3 days later, I saw the mold again on another succulent that was right next to (but in a different pot) the succulent with mold right now (the second one to be spotted with mold). I was about to go nuts. That succulent also was healthy and had not even bee sprayed with neem oil, so that must not have been the cause. it was also dry soil when the mold appeared, same as the other one. I have not watered either succulents in 14 days, which is before I saw the mold. In other words, I have not watered the succulents at all since even before the mold appeared. These succulents have not had any previous issues. There was 6 other healthy succulents each in separate pots next to those two, so I immediately separated the two infected succulents. Why is it there, will is spread to my other healthy succulents, and most importantly: How do I get rid of them? All of these succulents are not over-watered and are houseplants, in my house. I tried neem oil but that made it worse. I need a solution that doesn't involve destroying and/or getting rid of the plants, and that does not involve re-potting the soil, since that hasn't shown to do anything. I need a way to kill it permanently without harming the plant and that will work near-instantly, because this mold has shown to kill my succulents in 2-5 days.

Warning!: The following images are extremely gross and may leave viewers discomforted or grossed-out. Please refrain from viewing the following if you are sensitive to these gross things:

The white dots that are slightly ovular in those images are parts of the mold. I don't know what part or what it is for, but I know it is the mold because they weren't there before.. ...and what in the world or the white dots? It really yucks-up the whole scene. It also makes the mold look more hazardous to the plant. Are these germination bulbs or something that generate spores? Or is this the mold anamorphing?

Answer 1

So, first off: I have no clue what kind of mold or fungus this one is. So the advice below is generic, but you told me in chat some of it worked (or seemed to be working), so I'll write down what I know about treating mold from my time working the garden store.

Why is it there?

If we had known that, we'd never had to throw away plants again. One way or another, some mold spores made their way into the soil or onto your plants, and started growing when they found the water to do so.

Will it spread to my other healthy succulents?

It can. In some cases, mold is much like COVID. Wash your hands between handling the infected plants and the non-infected ones, and quarantine the infected plants. If the climate is suitable, you can put infected plants outside (but not near any outside plants!) to avoid the mold spreading to any indoor plants. Otherwise, separate them as much as possible.

You can follow the steps on this wikihow to prevent mold growth on your unaffected succulents. Especially, clean away dead leaves/bits, and don't overwater. Make sure there's sunlight and air circulation. (though, don't aim a fan at your infected succulents trying to fix it: you might blow spores into the air and exacerbate the problem).

How do I get rid of them?

Our first step was generally to try and get rid of the mold by cleaning the plant and as much of the soil as possible. A good description of how we cleaned plants is on wikihow: Take the plant to the parking lot, wipe the mold from the leaves with damp paper towels, prune away any bad-looking leaves, and spray with a fungicide.

Being a garden store, we usually also just repotted the plants by giving the roots a thorough rinse so all soil was gone from them, then put them in clean soil and a new plastic pot.

You said repotting the plants is not an option, but dry out the soil, take the plant outside, scoop out the mold, clean the stone pot the plant is in (if it's in one), and wipe down the shelf/windowsill the plant was on. This is just to get rid of as many spores as possible. Again, wikihow lists most of these and also adds some advice on natural things you can add to the cleaned/new soil to prevent mold from returning.

I need a way to kill it permanently without harming the plant and that will work near-instantly, because this mold has shown to kill my succulents in 2-5 days.

I can't guarantee the above steps will make sure no mold will ever return, but it will greatly reduce the chances at least. Some of our plants ended up on the compost heap because a first cleanse wasn't enough, the mold returned, and the plant ended up looking too ugly to sell and was a danger to other plants.

And most importantly: All of these succulents are not over-watered.

I'll take your word for it, though I find the soil in your pictures rather dark and wet looking. Since too much water can really cause mold growth, and proper watering can prevent it, I'll add this section for other people's benefit then.

To check if your succulent or cactus needs water, you need to look at more than just the top layer of soil. Ideally, you can gently lift the plant out of it's pot to see if the soil is dry all around. Otherwise, it's good to leave a bit of room around the plant so you can gently dig a finger into the soil and see if the ground it truly dry, or if the top layer just dried out to the air. Only water if the ground really is dry, which can be as little as once a month.

If you can take your succulents out of their pots (and they have those plastic 'inner pots' with holes in the bottom), a good way to water them is to put them in a layer of water for half and hour/an hour, so the soil around the plant can soak up the water. After that, make sure the plant isn't dripping heavily, and put it back into it's pot. This makes sure the roots won't start rotting because there's always water in the bottom of the pot.