Sophronitis new pb rotted at base
 

This new growth turned yellow overnight. I check this plant daily, and it was growing strong. Then one of the new leads turned yellow. I cut it off (clean razor) and see a dark brown center. Should I use physan or something else?

I would use hydrogen peroxide on the cut, avoiding any roots.
More importantly you should think about mounting the plant. All specimen plants I've seen are mounted. See photo of mine. Reason for mounting essentially is to ensure plant dries out quickly as excessive and continual moisture is fatal.

You had a heat wave recently? They really don't like heat.

Thanks for the tips on hydrogen peroxide, water and heat. This is S. coccinea, that I got last spring. I bought 3 of them potted in fine bark and pumice in 2" pots, and all have done really well. I keep them moist for the most part, and they have produced 4, 4, and 3 new leads this year (they're 4n). One bloomed last year. Losing this lead knocks the plant down to 3 new growths, but that's still really good in my book. I'm just uneasy because I don't know if I caused the disease. (I just added pics of all three plants.)

I've read that S. cernua tolerates higher temps than coccinea. I have some cernuas that are mounted, but are not thriving. I think they are too dry. My growing area gets a steady ocean breeze (no salt, but cooling and drying at this time of year).