Seedlings disappearing. White mold instead.

[From0to10in2weeks]

Hi,

Last week I noticed that in a pot of seedlings (Melocactus Matanzanus, sown In February) a few of them had vanished and only the skeletal spines remained. Today I noticed that there is now a very fine whitish mold. And that a few more had gone missing. Not sure if you can see it in the attached photo well.

What do you think this might be? And how do deal with it? Throw them out? Spray with diluted hydrogen peroxide? Burn it?

I have them now out in air. Other pots with seedlings appear to do well. Besides that some have Alge growing which is the reason that I removed the lid about 2 weeks ago. I water now about weekly with half strength succulent fertilizer.

Thanks. Best.

[Jangaudi]

Best to take the molded seedlings out asap, they are lost. If you give them more air now the others will survive easily I think.

[Aiko]

This is just something that can happen. I suggest to listen to Jangaudi. Take the mold out, and allow fresh air to dry out the soil a bit. Don't put them back in bags again. These seedlings don't mind to be in dry soil for a while just as long as they are not in very intense sun light. The mold does not like things dry.

Also check for any remaining ungerminated seeds that might start to mold for the next few weeks. It's not just the seedlings that could start to mold.

[Download]

I spray on some fungicide when I spot mould. If it's possible to do so, I'll keep them exposed to open air as well.

[keith]

Take out the dead plants its time to dry it out some more. fungicide if you have it I mix rootone with water and spray it on the seedlings for the fungicide . The hormone probably doesn't do anything.

[From0to10in2weeks]

Thank you all for the suggestions. Sorry for not updating but life got in the way…

I scooped up the molded part and adjacent areas. I was surprised how wet the substrate was.

I wasn’t very clear in my post but I have them in a room grow house. The vents are open already for a long time. And about 2 - 3 weeks ago, I started removing the lid - first only during day times but then entirely.
So, I thought I compensate by giving them a weekly deep soak. But I appear to have overdone it.

A question I have is what kills the seedlings? Is it the mold? Or is the mold only growing on the remains of the dead seedlings?

Thanks. Best.

[Jangaudi]

A question I have is what kills the seedlings? Is it the mold? Or is the mold only growing on the remains of the dead seedlings?

Probably both, mold attack which kills them. In theory it could also be damping off first and molding later. Either way, get the soil dry asap. In my experience other seedlings don't seem to bother much with an attack like this, if the soil gets dry enough, they just continue their business

[MikeInOz]

A question I have is what kills the seedlings? Is it the mold?

Thanks. Best.

Unlikely. What usually attacks seedlings (especially cacti) are oomycetes not fungi. They tend to live in the soil water. Examples are Pythium and Phytophthora. Therefore you need a spray formulated specifically for those. Most normal fungicides will do nothing against them. Banrot works but it's always better to have a low population in the mix in the first place.


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[greenknight]

A weekly deep soaking is the wrong approach with cactus seedlings - the substrate should be kept evenly damp, never saturated. Commercial nurseries achieve this with capillary matting - the pots sit on a mat of wet, spongy material, pulling water up by capillary action They will never draw up more than just enough to moisten the media.

You can achieve the same thing by bottom watering with or without an absorbent mat under the pots. As long as the media isn't underwater it won't get saturated, so a thin film of water in the tray under the pots works well enough, though you have to water daily - I use a sprayer bottle for this.

Once the seedlings reach the point where they've completely filled the pot with roots, then you can transition to top watering.

[From0to10in2weeks]

A weekly deep soaking is the wrong approach with cactus seedlings - the substrate should be kept evenly damp, never saturated.

I guess I learned that the hard way. Though all other pots in this grow house appear to be fine still. But I will change of course.

Commercial nurseries achieve this with capillary matting - the pots sit on a mat of wet, spongy material, pulling water up by capillary action They will never draw up more than just enough to moisten the media.

That's exactly what I did! In short, since this was my first attempt I very closely followed the instructions by Kakteen Uhlig. They recommend an "irrigation mat" with a microperforated foil on top. And it worked just fine. Until I started to observe algae growth in some (not all) pots. Then, I started removing the lid of the grow house to dry out the substrate. The mistake I apparently made was to transition too early to treating them like regularly cacti with a thorough bottom soak and letting them dry in between. The latter likely didn't work beyond the top layer because I kept them still out of the sun (but under a grow light and some filtered morning light). Lesson learned.

Thanks! Best.

[From0to10in2weeks]

Unlikely. What usually attacks seedlings (especially cacti) are oomycetes not fungi. They tend to live in the soil water.

Thanks. Where do they come from? I used distilled water for watering the pots. But of course conditions are not sterile...

Best.

[MikeInOz]

Unlikely. What usually attacks seedlings (especially cacti) are oomycetes not fungi. They tend to live in the soil water.

Thanks. Where do they come from? I used distilled water for watering the pots. But of course conditions are not sterile...

Best.

Assume they are everywhere including the screen you're looking at, your clothes and in every pot you touch. They don't always cause problems unless the environment is right for them. They are called water molds so that gives a clue to the conditions they prefer.
Don't forget too that sometimes a seedling will have some kind of physiological disorder not related to disease at all and just give up the ghost for no apparent reason! Low temps are also bad especially with tropical species like Melo which should be kept at about 25 - 32C!

[greenknight]

You shouldn't worry about algae, it's harmless to your cactus seedlings.

Those seedlings are far enough along that it was okay to remove the lid and give them fresh air. Changing the watering method too soon was your only mistake.

As for the oomycetes - their spores are everywhere, just like algae and fungi.

[From0to10in2weeks]

Low temps are also bad especially with tropical species like Melo which should be kept at about 25 - 32C!

And that's another good hint! When I removed the lid I of course turned off the heating pad. So, the temperature the seedlings then were exposed to were around 20 C rather than 25 C as before.

Big thanks.

[From0to10in2weeks]

Those seedlings are far enough along that it was okay to remove the lid and give them fresh air. Changing the watering method too soon was your only mistake.

Yep. Lesson learned. Thankfully most of them are still alive. So far.

Thanks for the coaching all along!