Seeking help from everyone, what kind of bacteria is this and how to deal with it?

[keith]

Organic material in the substrate invites mold, so a purely mineral substrate with a layer of potting sand doesn't give the mold anything to feast on"

I agree I don't use raw organic soil for cactus seeds and when I did its often with bad results. Be careful mold can grow on the seed coat itself so best to clean the seeds first I use Hydrogen peroxide then rinse with water.

Some kind of loam or top soil and you don't need much like 10-20% with small pumice or course sand or both. If the loam or topsoil floats in water I wouldn't use it. You can microwave this mix if you want probably a good Idea for very small cactus seeds that need a constant humidity in a sealed baggie.

[MrXeric]

keith is right. You can sterilize the substrate and pot all you want, but the pathogens can come from the seeds themselves.

I use the baggie method and I heat my mineral based seedling substrate in the microwave (which is moistened) until it steams, wait a few minutes, then heat again. I soak pots in a bleach solution and rinse well before sowing. I sow the seeds on a thin layer of fine pumice sand (similarly heat treated). I spray the inside of the bags with hydrogen peroxide before placing the pots inside.

I still get some fungus growing every now and then, despite rinsing the seeds with hydrogen peroxide (an then rinsing again with water before sowing). The fungus always starts growing on the seeds and spreads from the infected seeds with thin filaments (mycelia?). I check on pots frequently so I can catch infections early. When spotted, I scoop out the infected seeds and the substrate around them before spraying the surface with hydrogen peroxide and reclose the bags. This usually works, but sometimes the fungus comes back, so I open the bags and let the surface of the substrate dry out. This slows the seedlings' growth, but at least they survive!

I haven't tried commercial fungicides because I've read they can inhibit germination? Probably not an issue when you're sowing dozens of seed of single species, but lately I've been sowing seed that comes in packets of 3 or 5 seeds per species, so I rather not risk losing out on germination!

[Jangaudi]

I put the substrate in the glass jars, including a top layer of course sand, add some water, and put in the pressure cooker. It comes out sterile as a whole, no need to handle it later. I give my seeds a bleach bath, diluted, for about 10 minutes, rinse with tap water, then sow them in the jars, and cover it with a plastic sheet. Rarely have mold issues this way, no need to use any anti fungal products either. Only issue that turns up sometimes, is that some seeds have a cavity, like astro seeds, and the bleach doesn't always penetrate it, so for astros, I will take out the seed pods after germination, and close again.

[Aztekium123]

I put the substrate in the glass jars, including a top layer of course sand, add some water, and put in the pressure cooker. It comes out sterile as a whole, no need to handle it later. I give my seeds a bleach bath, diluted, for about 10 minutes, rinse with tap water, then sow them in the jars, and cover it with a plastic sheet. Rarely have mold issues this way, no need to use any anti fungal products either. Only issue that turns up sometimes, is that some seeds have a cavity, like astro seeds, and the bleach doesn't always penetrate it, so for astros, I will take out the seed pods after germination, and close again.

If the diameter of the seed is relatively large, this method should be effective. Because after sowing, those seeds that do not sprout or have already died can be removed. But for very small seeds like Aztekuim, it's basically impossible to take them out again if you sprinkle them inside, unless you use a microscope? Haha, thank you all for your help. I took three faulty sowing pots, and now my method is to take out those moldy small plants from each one to two days. Taking out those moldy plants from inside, this method cannot completely eradicate this mold. I just hope that the plants in these pots that have not yet been infected can live smoothly until summer.