Much ink has been spilled by EAs on the threat posed by a pathogen intentionally engineered to infect and kill humans, which could cause societal collapse or extinction. While prevention is important, mechanisms to prepare for and respond to such a pathogen include things like PPE, refuges, vaccines, antivirals and diagnostics.
But what about a pathogen (or multiple pathogens) engineered to kill plants and disrupt global food supplies? This seems like it would pose a societal collapse risk via mass starvation, and although the approach to prevention is probably similar to prevention for human pandemics, the approaches to response will presumably need to differ, and differ greatly. (I think plant pandemics wouldn't feasibly pose an extinction risk because plants are so genetically diverse that it seems infeasible for a virus to cause that degree of destruction across different plant species)
Some previous writing on plant pandemics (not focused on collapse risks):
https://www.pnas.org/doi/10.1073/pnas.2022239118
I think one obvious direction for work on response would be to fund more research into plant vaccines.
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