Japan anticipates an eventual betrayal by Russia, particularly after Operation Bagration really gets underway during the summer of 1944. They order their infamous Bio-warfare team, Unit 731 to prepare biological attacks against the Russians if and when this invasion occurs. Given a year to prepare, Unit 731 creates millions of cannisters to deploy bubonic-plague infected fleas, and anthrax spores. These cannisters are loaded into planes, piloted by Kamakazees, whose mission will be to explode their planes, with their deadly cargo above enemy lines. Ceramic cannisters will also be deployed to artillery crews near the Manchurian border.
When operation August Storm commences in August, 1945 most of these plans are foiled by Soviet air superiority. However, some do get through, infecting thousands of Soviet Personnel with virtually incurable, and extremely commutable diseases. If proper quarantine is not observed, potentially hundreds of thousands of soldiers, but also civilians could die in both the Soviet Union, and those areas of Manchuria and Korea they occupied.
The Japanese could also attempt biological weapons attacks against massed US troop concentrations in the event that Operation Downfall actually commenced, though I suspect they would be less effective, as the US Army and Marines were better organized, and operating so far from home that any infections could more easily be quarantined.