[Scribbler]

Mrs Scribbler has a bed with some rather special geraniums. Some are showing signs of  what looks like a fungal disease, with spotty blister-like growths on the undersides of the tips of the leaves. Any thoughts what this might be please?

[Goosegirl]

Mmmm - see something like this before. I think it is geranium rust but I would Google RHS and see what they say.

[Scribbler]

I've just been advised to clarify the type of plant here. These are cranesbill type geraniums, rather than pelargoniums. Does this make any sense? 

[Goosegirl]

Yes - cranesbill geraniums are the perennial herbaceous plants ranging from quite tall ones to flat, spreading rockery types. Pelargoniums are the greenhouse/bedding ones you see in pots, troughs, park bedding schemes etc and include upright, and ivy-leaf trailing types and bloom all summer. I had a look at my RHS book on pests and diseases and, though it didn't help too much, I think it is more of a fungal disease than a bacterial one as the blisters could be fruiting bodies where the spores develop. Personally, I would cut off all affected leaves, give it a spray with a fungicide then a good feed with blood, fish and bone, growmore, or the like but with not too much nitrogen in it so you don't get sappy growth that may be susceptible to more disease.