Cut a gall open and search for signs of an insect inside- if you find none, the gall is probably caused by a disease. The leaves pucker and curl as they expand. Certain insects cause similar symptoms, but on closer inspection, if insects are the cause the insects themselves or other signs of their presence can be seen. Downy mildew is usually a white to purple, fuzzy growth, usually on the undersides of leaves and along stems. It turns black with age.
Powdery mildew is a white to grayish powdery growth on the upper surfaces of leaves.
They can be soft and squishy or hard and dry, and color can be either light or dark. Most symptoms you find in your yard or garden will be caused by insects or environmental problems such as water stress or nutrient deficiencies, so try to rule out these possibilities first . If you have yard and garden reference books, you can often identify the disease based on the categories of symptoms listed above. The species of plant on which the symptoms occur will also help identify the problem as most diseases are quite specific in the kinds of plants they will attack. If you need additional assistance with identification, your county Extension agent can help.