IMO it would depend on the varieties of apples....

Many of the apples we eat are inbred (see konrads link on apple inbreeding). We all know the downside to doing so....

Old orchards don't necessarily have more viruses. If the orchard consisted of trees all bought from one conscientious nursery at one time then the odds are low. But, if the trees had a lot of grafts put on them and were bought from many nurseries over the years the odds are higher.

Scott

I manage lots of old apple trees and don't give viruses a thought. They have yet to become an observable issue although I don't sterilize my pruning tools. I've never read of it being recommended, but it is an interesting question.

For me there are so many other things that you do have to worry about- I will wait to see (or at least hear of) local virus issues before I worry about them as well.