I have several healty sego palms, but one in a pothas turned yellow and has a
Scale looking substance on it. I've tried everything Know of and several things
That have been recommended.

I don't know what this pest is, but I was told to remove all of the seriously spotted fronds and spray with deluted NEEM Concentrate. Do not cut all of the fronts off the pant will die. Do not allow the infected plant or cuttings to come into contact with other plants. Mulch if possible. If his advise doesn't seem to help in a few months, check with a nursery which specializes in these palms.They grow very slowly, but are worth it. :eek:

Could be some sort of white flies, or something called scales, there are a couple of different varieties of scales. The best way to get rid of them, at the nursery that I work at, we use a systemic fungicide... some places have stopped selling systemic. The best that I found is a 3 in 1, for fungicide, miticide, and insecticide. Also, before you do that, you should take off the infected libs/branches before and follow the instructions on the label. Hope it helps.

Since you submitted your Sego Palm infestation problem question I have been checking around and was advised that to improve the plants general health is to distribute a soil amendment high in minerals such as manganese, potassium, iron, magnesium and, of course, nitrogen. Good Luck!