Straw mulch, a ground cover of white clover interplanted with the crops, and temporary flooding all provide effective weed control in my fields.
Since I turned the fields back to their natural state, I can't say I've had any really difficult problems with insects or disease.
If a farmer does abandon his or her "tame" fields completely to nature, mistakes and destruction are inevitable.
As far as my planting program goes, I simply broadcast rye and barley seed on separate fields in the fall . . . while the rice in those areas is still standing. A few weeks after that I harvest the rice, and then spread its straw back over the fields as mulch.