Hi everyone, I am repotting a bunch of cuttings I started earlier this year, but the last two I have unpotted have had thin, white strings almost like dry threads of paper on the roots, some spines, and bits of soil and bark below the soil level. Is this fungal growth of some kind or is this a symptom of root mealybugs? There is no white residue left stuck to the pots themselves, they are completely spotless except for dust and a few dusty perlite streaks that brushed right off. Looking through the roots, I don't see anything that looks like mealybugs and I have crumbled some between my fingers to be sure it's perlite and not a bug. Some bark pieces were completely covered in these threads. I have ceased repotting and left the two I have found so far sitting out until I know if this needs to be treated or if it's nothing.

Many cacti will make a really fine network of thin threadlike roots in addition to bigger roots. To me it looks like it could be the start of those, but it is hard to see/say with complete certainty.

Other than the white threads, there isn't anything else wrong with them. They are either still plump from being watered about a month and a half ago or so, or growing. They all look healthy and they all have nice little root systems. Some of the other ones I unpotted had a few of these threads on the insides of the pots after I emptied them, but nothing sticky or anything. There wasn't any white residue or anything else on the bottom of their containers or poking out of the pots either, just some small cobwebs from little spiders underneath.

To me. it looks like a plant grown from a cutting (offset) that was planted too deep.

Yes, I have some other ones that I planted too deeply as well, some even deeper than this one I am embarrassed to say. Would that cause it to grow these? The light was fading, but I couldn't find anything resembling a mealybug on any of them. Unless they can be smaller than is visible with the naked eye

Honesty the more I look at them the more they seem to be integral parts of the root system. That you find them on the inside of pots from other plants seems to confirm that. Those fine root systems are usually an indication that the plant is settled in and growing well. When a plant goes dormant it will let these roots die off since they are relatively easy to re-establish.

I see no reason to worry at this point in time. Stick them back in soil before these roots die.

That's a relief, I'll repot them straight away. I saw all the white threads and wondered if perhaps I had had root mealybugs sneak in. I've read about them but never seen any before so I wasn't sure.

Whew. Thanks all!

Root mealies should be relatively easy to identify based on the size of the actual insects inside all the cottony thread. You can squeeze one between your fingers and feel it pretty easily. In fact I do that whenever I suspect mealy bug activity, because then I can confirm that I removed a bug, not just a wad of the cottony stuff (with a toothpick usually). And that it's history.

I have seen the white hairs on several of my plants too.
It is known as mycorrhizal fungi.
they are good for plants as they help get nutrients to the roots.
it is a symbiotic relationship where they both benefit.

I'll read more about that, that sounds fascinating. I remember reading that Pachycereus pringlei has some form of fungi or bacteria on its roots which can actually break down rock, but I didn't think that any form of beneficial fungi would find a dry pot sitting out in the baking hot sun to be a very good place to be. Pretty cool!

Baja, I plucked off probably ten bits to see if any were anything mealybuggy and crushed them between my fingers and they all crumbled to perlite dust - I've never seen root mealybugs before and only remembered reading that they left white stuff everywhere. Having to constantly deal with spider mites, gophers, and ground squirrels is starting to make me jump at shadows and see possible pests everywhere

It's good to be alert, regardless. You'll miss fewer things that way.

Root mealies tend to like it when the soil is dry for a lot of the watering cycle. Here they tend to be most problematic in late summer and early fall, and greatly reduced or eliminated when it rains. I've seen them on agaves and a few other plants, usually accompanied by ants. You can pretty effectively drown them to mimic the natural effect of rainfall. They're among the easiest bugs to eliminate once you identify them. Way easier than gophers.

We have gophers and ground squirrels but no spider mites, owing to the coastal humidity. The mammals start to get real hungry/thirsty this time of the year, and they'll be snacking on the garden more and more until it rains again.