If road & rail transportation is shut down, they won't be able to move grain anyhow. The grain will just back up in the ports and can't be unloaded. Even if it's only a few weeks or a couple of months it will cut the consumption of grain simply because the interior will be forced to do without.
And worse, if the Chinese economy slows appreciably they won't have the money to buy as much food as well.
As far as that $200 billion in additional purchases mandated under Phase One is concerned, it would seem to me that the Chinese could declare "Force Majeure" or "casus fortuitus" and get out of it completely now.
Phase One text includes this article under dispute resolution:
Article 7.6: Miscellaneous
1. The Parties affirm their existing rights and obligations with respect to each other under the WTO Agreement and other agreements to which the Parties are party.
2. In the event that a natural disaster or other unforeseeable event outside the control of the Parties delays a Party from timely complying with its obligations under this Agreement, the Parties shall consult with each other.
I think what epidemic means is: don't plan on sell beans to the Chinese this year, maybe not even next.
