My cactus turned yellow!

[cactusnewbie]

I have just started growing cacti/succulents and one of them has turned yellow. It is an angled cactus (Pitahaya maybe?) with a red cactus grafted onto the top. The red cactus is fine, but the green one below has turned yellow.

In the same pot, I also have a molded wax agave (Echeveria agavoides) that is starting to turn yellow.

Also in the pot are an elephant bush, pincushion, yellow tower, little crocodile, and aloe. These all seem to be fine.

I am afraid there may be multiple problems and I don't know where to start. Some things I have thought of are too little light, overwatering, or pests. I have seen some small, black, gnat-looking bugs hanging around that pot. I don't know what they are or how threatening they can be.

Please help!!!

[cactusnewbie]

Hopefully this will work...here are two pictures:





You can see the grafted one there in the middle with the whole base turned yellowish brown and the agave in the forefront with one part just starting to turn yellowish brown.

[phil_SK]

That's fatal rot. The previously green triangular bit will pull apart as mush if you twist it a bit.
The Echeveria seems to only have a rotting leaf which shouldn't be a problem if it's removed

Your plants need much more light and a fair bit less water.

[cactusnewbie]

Thank you phil_SK. You have been very helpful. Should I remove the entire grafted cactus and throw it away? Will any of the others be affected and if so is there anything I can do to prevent the same thing from happening to them?

[daiv]

Yes the grafted one is doomed.

The others look like they'll recover if taken care of. Is there a drainage hole in the bottom of that pot? Also, don't give strong light too fast! Be sure to acclimate them to the light a little at a time.