The first step in diagnosing plant problems is identifying the plant and recognizing normal growth. Often, what looks "sick" may actually be the normal growth habit of a plant. When the Plant Disease Clinic receives calls about sick-looking plants, it is always a joy to be able to tell the client that the plant in question is perfectly healthy.
Ferns are primitive plants that do not reproduce by seeds. Instead, ferns produce "spores" in orange or brown, dusty "sori" (singular sorus) on the undersides of their leaves. Sori may appear as small or large raised dots, or as a diffuse rusty coating under the leaf. These reproductive structures are often mistaken for rust diseases or scale insects. Rather than being a problem, they are a normal, healthy part of the life cycle of the plant.
While on the subject of reproductive structures, occasionally the normal male pollen structures on healthy pines are confused with a disease or insect problem.
Evergreens "dying" in the fall
Every year, panicked homeowners call the Plant Disease Clinic regarding their "dying" evergreens. It is normal for the inner (oldest) needles of a pine or arborvitae to turn yellow or brown and fall off in the autumn. This is most commonly noticed on white pines, Austrian pines, and arborvitae, but it may also affect spruces in some years. This seasonal needle loss is more noticeable in some years than others, and is more apparent on some individual trees than others. Bald cypress trees are an extreme case; they lose all their needles in the fall, not just the inner ones, as they are deciduous conifers.
Occasionally we receive samples from clients who are concerned that their maple trees have leaves that are partially white. Without fail, the leaves in question are from a variegated maple, sometimes called a Harlequin maple. These attractive leaves are a characteristic of this cultivar. To further add to the confusion, these variegated trees sometimes "revert" to their normal green color, so sometimes a branch or two on a variegated tree will develop plain green leaves. These reverted branches should be pruned out to maintain the variegated coloration.
Some leaves are exceptionally hairy (or "pubescent"). Sycamore leaves are especially noteworthy for the dense down they have on their undersides. Clients sometimes wonder if the dense white coating is powdery mildew, but it is perfectly normal and healthy.
Russian Olive "galls"
Russian olive trees normally develop large "galls" on their roots. Rather than being an indication of disease, these growths are called actinorhizae and are specialized structures that contain bacteria. These helpful bacteria fix nitrogen, putting it into a form that the plant can use. Actinorhizae are similar to the nitrogen-fixing nodules found on the roots of legumes such as soybeans.