[harrygermany]

Hello Solo Verde,

I think that the roots of your Oncidium have suffered from too much water and fertilizer.

You can find out easily if you look at the roots. Not all of them should be black. And if you rub them between your fingers, they should stay unharmed.

And smell at the roots. There should not be a scent like mildew or mold.

Plants with destroyed roots because of too much water show the same signs as those with too little water.

The "rusty colored powder" very likely is a fungus. So an insecticide will not help. What you should try is a fungicide (antifungal agent, antimycotic agent).

Regards

Harry