Have you ever flipped through the pages of a glossy gardening magazine and marveled at the perfection of every blossom, every leaf in every photo? Not one torn petal, not a single yellow or spotty leaf. How do they do it? I don’t know, but just looking at the pictures frustrates me because I know that no matter what I do, a fungal disease can turn up.

In last week’s “Gardening Corner,” I wrote about powdery and downy mildews, two fungal diseases that affect garden perennials, but many other fungi also cause diseases. About 85 percent of plant diseases are caused by fungi.

Three fungal leaf spot diseases of specific garden perennials and shrubs aggravate gardeners every summer, and the sad fact is that once your plants fall prey to one of these diseases, it becomes difficult to eradicate because the spores overwinter on old leaves and plant debris and re-infect the plants in the spring.

Iris leaf spot is caused by Cladosporium iridis. Bearded iris (Iris germanica) is most comm...