Beginner Gardening: What is this powdery brown stuff??, 1 by SirSedum

SirSedum wrote:
Can anyone help me identify what this is on my snapdragon? It is a spotty, brown substance on the undersides of the leaves of my snapdragon. When you touch it, it is a pwdery substance, I suspect they are spores of some sort. I sent a picture and hopefully that helps. I have other stuff out there and I really do not want this to spread-whatever it is. It seems that the problem is getting worse and now the top sides of the leaves are starting to yellow slightly. Thanks for any input!