We grew these Bird of Paradise from seeds and they are 6 months old growing inside. Some of the leaves have black spots on them. Do you know why and how to treat it? I used plain Japan dry soil from by back yard.
In your third pic I can see some white stuff on the stems and undersides of the leaves--I think that could be your real culprit, could be some scale or maybe mealy bugs (or maybe something else, I can't tell without seeing a closeup!). If it's scale or something like that, they make honeydew which often leads to sooty mold which could be the black spots on the leaves. I would try taking a q-tip dipped in alcohol and use it to wipe off the white stuff, then give the plant a good shower/hose off to get rid of the black stuff. The black spots could also be some other sort of fungus, soil from outside is going to have all sorts of fungal spores in it which will start to grow when you give it warm moist conditions, when you start seedlings you're better off using a sterile seed starting mix. I would also recommend using potting mix instead of garden soil for anything that you're growing in a container--garden soil is often too heavy and can lead to root rot when you use it in containers, but potting mix is designed specially for containers to give them good drainage. Changing the soil won't fix the problem you have now, but it could help save you from more problems later on.
Yes, it looks to me also, that you may have mealy bugs or something else going on on the under side of the leaves. Be sure to wash the plant well before putting it into new soil or you might just bring them with the plant. Bj
I have a BOP that got leaf scale (LS) last summer. I used a product called NEEM, judiciously on a regular spraying schedule of once a week. I had it on the run, too, until the rains came and kept washing it away. I love NEEM on the plants, cause it really makes them shine, too!
Trouble with the LS is that they can burrough down into the soil and come back again, so it's important to treat the soil too. And wash your pruning equip and tools with bleach, as you can transfer it back and forth, especially to any Sago Palms you may have. Good luck!