Question
Four of my five rose bushes are completely bare.  I have one thriving rose bush.  
What could have caused all the leaves to fall off the other four bushes?  I
recognize no visible signs of disease or parasites.

Answer
When something happens suddenly to rose leaves and they fall off, then the problem is usually something wrong with the roots. Such as they didn't get enough water meaning the water didn't go down far enough or the opposite in that they got too much water and the feeder roots suffacated.Not being able to see the roses means I have to guess. Scrape the canes of the roses without leaves and if there is green under the bark then they will be okay and will put out new growth. Florida roses are usually grafted onto an understock called fortuiana sp? because some Floridas soils contain nematodes.  If the roses were not grafted onto this understock then that will be your problem. Dig down along side and look at a root and if it has little black or dark brown nodules on it then you have found the problem. Sorry I couldn't be more specific for you.