I am now with the recent rains starting to get black rot on the new growth tips of a few of my orchids, which is killing me. Is this the best product to use or something else to kill it. Also will this stop the spread of the rot or must I remove the growth tips...ugh.

I use it WHENEVER I see black and it seems to stop it in its tracks. The bottle lasts a long time and I think it was about $20. Got mine from Quarter Acre Orchids, which is local for me, but she sells online.

Can anyone tell me the Physan 20 dosage per gal.?

Before you go spending $20 - $50 for a bottle of Physan 20, go to your local pool supply or Home Depot.
The main ingredient is:

n-alkyl (60% C14, 30% C16, 5%C12, 5% C18)
dimethyl benzyl ammonium chloride� 10%
n-alkyl (68% C12, 32% C14) dimethyl
ethylbenzyl ammonium chloride

Which is the same ingredient that is used in most pool algicides, but t a 5% ratio, as someone who commercially grows, we use the pool supply version which is just as good. Instead of 1 capful of the Physan 20, we use 2 of the pool algicide and the pool algicide, last we bought it, was in the range of $5 - $7. A no brainer. Also, what works well is hydrogen peroxide H2O2 at a 3% solution, what you buy at a drug store.

Thanks Orchidguy.

Tropicalfreak