Question
When cutting off part of an orchid which has rotted, I cut it back to clear tissue with a sterilized knife. The tissue will then, on occassion, turn a brownish red. If I blot it with a paper towel, it leaves a pink stain. Or, sometimes when I pour hydrogen peroxide on the exposed cut, the fizz from the peroxide turns pink. Even though the diagnostic pink ring around the stem tissue was not present, is the pink tissue indicative of Fusariim Wilt or does it occur because the tissue is being exposed to oxygen?

Answer
The diagnostic marker for fusarium wilt is the presence of pink fibrovascular bundles (xylem and phloem).  The American Orchid Society's publication on Orchid Pests and Diseases recommends the following: "Discard that part of the rhizome and pseudobulbs showing infection.  Plant only only that part of the plant showing no purple discoloration.  All diseased material should be destroyed and general sanitation practices followed.  Since osmunda can carry fusarium spores, it should be treated before use with a 4% formalin drench.  Drench or dip plants in benomyl (Benlate) at the rate of 1 tsp per gallon of water.  Repot in new potting materials with adequate drainage".

I don't know what reaction took place with the use of hydrogen peroxide. I have not experienced this but your speculation that oxidation in the presence of plant pigment or pigment precursors could produce this response.