There are small brown blobs on the underside of my snap dragon (I can't spell the other name) leaves. The tops of the leaves are turning yellow.
I noticed this last year towards the end of the summer. It's much earlier this year.
They have self-seeded, so are in exactly the same space as last year, and the year before.
Is there anything organic I can do to stop it, other than not having them in the same ground again?
What causes it? Will it spread to other plants?