There has much been much dialogue about cleaning shears and such to prevent FMV from spreading to other plants. I assume the same applies to the hydrometers and ph meters. The probes of these instruments are rounded but I probe and feel roots occasionally and move on to the next pot. If I nick a root and the plant is infected, would simply probing the next pot pass the virus on or would I have to nick the roots in the second pot?

Good question. IMO you could spread it that way, especially as pots get more and more roots. I would think wiping the probe between pots with a rag soaked in stout bleach water solution should prevent any distribution.