Just like the house owner that does his own work isn't a general contractor. And like that, the distinction is gradual, and there is benefit to the home owner in knowing how to do their own repairs and construction to a point, and then hiring out for the stuff that's too much for you.

I don't think most people thought that the theoretical general farmer described is literally a crop doctor, but that they should know enough about it to know when to call in a professional, and when there are initial steps that might be taken to hamper or eliminate a disease before it takes hold.