begonias and geraniums

[paulthewall]

I have some begonias in pots inthe garden, which are presently in flower. The leaves have started to turn yellow at the edges and there is a white dusty covering on the surface of the leaves. The begonias seem to have stopped growing, and are looking very unhealthy. Any ideas?
In addition, my geraniums (red and white) which are in pots also, are not thriving well, and leaves are turning yellow and dying off. Is this too much or too little water?
Or any other bright ideas?