Latest episode (I've followed this program from the beginning) SEEMS to be saying that Americans are planning to taint/poison agricultural products that are sold to the USSR, with the intention of causing large-scale disease and/or death.
I never heard of this, either as a plan or certainly as something that was actually done. OTOH, I doubt the writers would pull something like this right out of their asses, so maybe there is a grain of truth (play on words intended) in the story line.
I am a Conserva-tarian, but have not seen this as a LW propaganda tool. They show good and bad on both sides, and in fact, make the Commies look murderous and foolish frequently.