Perennial geranium leaf spots
by Joan Higgs
(Seven Valleys, PA)
Doug says there are a large number of leaf spot type problems that bother perennial geranium leaves. There are both bacterial leaf spots and these can be a major pain in the garden - leading to major problems with the plant (and really difficult to get rid of - never mind treat). In annual geraniums, these wipe out entire crops but in perennial geraniums, they persist for years doing exactly what you describe. The problem here is that there's no treatment other than to remove the affected leaves (don't compost them) and hope the plant can outgrow the problem. Alternately - dig and destroy the plant so it doesn't spread the infection to other geraniums.
There are also leaf spot fungus problems (about nine different kinds of fungus) that do almost the same thing. The good news is that these can be controlled by organic fungicides such as lime-sulfur. But again, removing infected leaves is part of the control recommendations when you have severe infections. Fall cleanup is critical so the problems don't overwinter as is the health of the plant (feed and water) to fight off the problem.
So - which do you have?
One solution here is to take some leaves to the local extension folks and tell them what I pointed out - and that you need an expert to tell you what you have. It might actually take a lab to identify the Bacterial Leaf Spots.
Another option is to spray with the fungicides in a major way this year and next spring. Clean up this fall and remove all foliage. (Read the label on the fungicide) If the fungicide makes things better - you know what you have. If it doesn't make things better - you also have a better sense of what you have.
But step one is the remove and destroy all infected leaves. The plant will regrow new leaves this year so don't worry about doing it right now (mid-July)
Good luck
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