I'm sorry profplum. It sounds as though you are in an impossible situation.

I wonder if there's not a way to communicate to your wife that you care deeply about her and love her, but that the concern/preoccupation with the farmer is her issue, not yours. That you don't have a problem with the farmer; that you can accept that she does, but that it is hers to contend with, not yours. I.e., you state that you are not going to confront the farmer or spend time or energy discussing the situation. That your refusal to take action around this concern of hers does not mean that you love or care about her any less (although I'm sure she'll accuse you of such). Alternatively put, this is a boundary that you can draw in a compassionate and loving way. If she storms off and threatens or even files for divorce because you won't confront the farmer, well, then that's quite absurd and tragic, but that's the piece you can't control...