I would like some information on how to stop my sago palm tree from growing.
While you can't really prevent it from growing, you can prune it back to keep its growth under control. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/trees/sago-palm/prune-sago-palm.htm
My Sago Palm has white powder-like spittle on the underside of its leaves. What can I do to get rid of this problem?
Is it powdery like flour or actually sudsy and reminiscent of soap bubbles? If it's powdery in nature, then it is probably powdery mildew. If it's foamy looking or like spit, you could have spittle bugs. These articles will help determine which is the likely cause: https://www.gardeningknowhow.com/plant-problems/disease/get-the-cure-for-powdery-mildew.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/spittlebug-control.htm
Two of my sago palm plants seem to have a red spider disease. The fronds are brown, as if they were sunburned. I was told by the fellow at the nursery to cut off all the fronds (which I did) and apply fungicide. Did I do the right thing? Would appreciate your suggestions or comments.
I'm not sure why you would be treating with a fungicide, as the culprit here is red spider mites, not a fungus. Cutting the foliage is a start but you could also treat the plant with Malathion or Neem oil. Here is an article that you may find helpful: https://www.gardeningknowhow.com/plant-problems/pests/insects/spider-mite-treatment.htm
What is the lowest temp. a sago palm can live in? I have one that is so big I can't move it.
These plants are generally quite hardy, surviving to 20 degrees F.
I have a sago palm in the house that grows long 18 inch stalks, but the leaves don't open. I see tiny furled leaves starting, but they are only 1/4 long. I've had the plant about 4 months and it's in a brightly lit window with late afternoon sun. I never had a sago palm before, so is this normal slow growth or is something wrong?
Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/trees/sago-palm/how-to-care-for-sago-palms.htm
I have a white powder on the center and some leaves of my sago. What is it, and if necessary, how do I remove it? It is mostly center towards the bottom of the stems.
It sounds like powdery mildew, this article will help:
https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm
I have also found neem oil to be effective against powdery mildew:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
My sago is about 6 feet tall. It is starting to grow new fronds in two places. How can I keep it from splitting into two trunks?
It sounds like it may be growing a pup. This article will help you remove it (instructions near the bottom of the article):
https://www.gardeningknowhow.com/ornamental/trees/sago-palm/prune-sago-palm.htm