I think the main response would be to try and scale up alternative food supplies such as those being explored by https://allfed.info/, as well as trying to scale up cultivation of whatever plants are immune to the virus (it would presumably be difficult to make a single disease which could wipe out all plants, so if perhaps wheat crops start failing worldwide, maybe we start re-seeding those farms with potatoes/corn/rice or whatever is climactically appropriate).

In general, I think out-of-the-box GCBR ideas are not discussed much in public due to the infohazard risk of inspiring evildoers with outlines of clever potential exploits that could bring down civilization.  (eg, you are definitely not the first pandemic researcher to wonder about the potential for engineered crop disesases)  For that reason, IMO we should refrain from speculating too much about what such a disease might look like or how plausible the threat is.

But this justified secrecy does put somewhat of a damper on attempts to brainstorm potential societal responses, like you say.

Along similar lines, from a few days ago: Animal Agrocrime: An Overlooked Biological Threat