[Farael]

I agree with much of what has been said here, particularly on the fact that Sauron had no desire of being king over an empty desert. After all, even Mordor had arable lands!

But I think there's one more thing to consider: Plagues don't hold territories. Yes, they can weaken your foe and make it easier for you to conquer them, but at some point you have to get out and conquer them. So I would say that, as has been said before, Sauron felt the time was ripe for conequest and so a plague was not what he needed, he needed armies for control and occupation.

Furthermore, we don't really know how plagues were spread. It seems that the writers of the annals where such information was collected to make the appendixes of LoTR (after all, it's not Tolkien who speaks to us as a narrator) attributed the plagues to "ill winds" but then their societies probably knew nothing about disease vectors and microorganisms.

Is it possible that these plagues sperad by conventional methods? which include human-to-human (elf-to human?) contact, vector organisms like mosquitoes, aerosols and the like

Perhaps to spread a plague Sauron first had to infect a significant amount of individuals and send them to coexist with his targets. Then he created a "foul wind" which probably encouraged people to stay inside... and voila! Good situation for a rapid spread of a virulent and contagious disease! However, during times of high military tension, peoples would tend to be more xenophobic (See Rohan) and thus the disease would not have been easily introduced to their target lands!

Therefore (and finishing up a long, rambling post) Sauron's need for armed conquest of territories decreased his ability to spread disease among his enemies so that he would have reaped little or no benefit from trying.

Or perhaps, he did try and, for the reasons explained before, failed and thus we never found out!