Nutrients - phosphite vs. phosphate.
 

http://eseap.org/ppiweb/bcrops.nsf/$webindex/3EF696A6E5851563852572140026EACD/$file/06-4p11.pdf

From this article I see that phosphites have been use for foliar feeding and can help the plant resist attack by microrganisms but that if the concentration of phosphite is too high it can also cause necrosis.

Has anyone used phosphite on orchids?

Can you explain phosphite? I've never heard of it

Hmm, something is wrong with the link I tried to place in the first post.I will try again:
http://eseap.org/ppiweb/bcrops.nsf/$webindex/3EF696A6E5851563852572140026EACD/$file/06-4p11.pdf

Edit: The link still does not allow you to click on it. Copy and paste the link into your browser, it explains about phosphite.

An interesting article for sure. Its strange how the first 2 (of 3) pages really downplay phosphite as a potentially toxic, inferior alternative to phosphate; but then talks about the increased flower and fruit activity in commercial food crops when used properly. Certainly a product that could prove useful in future orchid flower studies. If I was a botany grad student, I think I could make a pretty good thesis argument using phosphite (hint, hint to all you young orchid growers!).

Another phosphite link, phosphite as a fungicide:
POTASSIUM PHOSPHITE - Endeavour Turf Products Pty Ltd