In late summer, I sometimes notice discoloration of Hydrangea macrophylla leaves. The leaves near the base of the plant may turn yellow and develop 1/8 to ¼ inch circular gray or tan spots with brown or purple borders. The discoloration will move up the hydrangea, and the leaves may fall off. This is Cercospora leafspot, a fungal disease that is unsightly, but not likely to damage or kill the plant.

I recognize this fungal disease because I have past experience with it. You may recognize the effects of some plant diseases, too, but what if some new signs and symptoms appear on one of your perennials or trees? Will you know how to sift through the endless amount of gardening information in your garden books and on the internet? That is an impossible task, so the first step is to ask pertinent questions that will help to identify your plant’s problem.

Start at the beginning by learning about the normal characteristics of your plant. Perhaps it is a dwarf cultivar or is supposed to...