People can survive quite well without the care of physicians. Going without food is more difficult.
This. But we're talking about a deep understanding of agriculture and plant biology, not modern farming with GPS-guided combines and Monsanto round-up seeds.One would need to know how farming was done in ages past.
And also since we're assuming a post-apocalyptic world in which computer programmers are useless due to a lack of electricity, I'd say even more important than farming knowledge is fighting knowledge. Having guns and ammo (LOTS of ammo) and knowing how to use them. Shooting a gun accurately may seem simple to the uninitiated, but it takes considerable training and practice. Also knowing how to fix guns (gunsmithing) will be an important skill.
The holy grail in this world would be having the chemistry knowledge and experience to make your own gunpowder and ammo. If you could do that, you'd become THE most important person a local warlord could have in his court.