Jams and jellies have a very high sugar content. The sugar binds with the liquid present making it difficult for microorganisms to grow. The purpose of water bath processing jams and jellies is to prevent surface contamination. If you processed the plum jam in the water bath canner within 24 hours of preparing the jam, it should be safe, however, keep a close eye when you open the jars to use them for any signs of mold or fermentation. If there are signs of mold or fermentation, throw the jam away and do not eat it. You could also freeze the jam to prolong the shelf life.