Ah, if only this coloration were desirable, if we could affectionately refer to it as a silvery flecking or a sparkling sheen.
But no, it's dull white and it's a sign of disease, one aptly called powdery mildew.
Your lilac is also probably getting powdery mildew. And if you don't grow lilac, you may have mildew developing on the leaves of your cucumber, rose, peony or phlox.
You might worry that all that powdery mildew will spread from one plant to another all over your garden. But fear not: A few different kinds of fungi cause powdery mildew, and the one that attacks lilac bushes won't attack every other plant — surely not your cucumber or rose plants. Unfortunately, that lilac powder mildew fungus can attack catalpa, dogwood and some other trees. The same goes for the fungus that turns zinnia leaves white with powdery mildew; it also preys on a slew of other garden flowers. Each of the powdery mildews attacks a restricted group of plants.
KNOW YOUR ENEMY
Powdery mildew, like other common names of plant diseases, describes a symptom. Other descriptive common names for diseases are scab, leaf spot and brown rot. "Anthracnose" describes a symptom not obvious from its name; plants afflicted with anthracnose have dead areas in leaves.
Diseases having the same descriptive name are not necessarily all caused by the same "germs," or even by germs related to each other. Blight of pear, for example, is caused by a bacterium, while blight of peony is caused by a fungus. The term "blight" is generally used for any disease that causes a plant or plant part to die suddenly.
Descriptive names for plant diseases can get more specific. Fire blight is the blight attacking pear, so-named because blackened leaves cling to the curled-over stems as if they have been singed. The peony affliction is called botrytis blight, "botrytis" meaning grapelike and referring to the grapelike clusters of fungal spores as seen under magnification.
The common name of a disease is the first step towards finding the culprit's true name. I don't fear that lilac will spread blight — whoops, I mean powdery mildew — to nearby phlox or peony because lilac's disease is caused by the fungus Microsphaera alni, while phlox's is caused by the fungus Erysiphe cichoracearum, and peony's powdery mildew is caused by Erysiphe polygoni.
In some cases, the common name may be all that's needed to find out how to help a plant. "Westcott's Plant Disease Handbook" by R. Kenneth Horst is a good reference to finding both the Latin name and the cure for just about any disease that might turn up in your garden. "The Organic Gardener's Handbook of Natural Pest and Disease Control," by Fern Marshall Bradley, Barbara W. Ellis and Deborah L. Martin, presents a more organic approach to weeds, with less emphasis on nomenclature.
Figuring out what a disease is called makes for better gardening because it forces a close look at just what's going on. Next time something seems amiss with a plant, note first what part or parts are affected. Then characterize the symptoms with such descriptive terms as rust, rot, dieback, canker or leaf spot.
ONCE NAMED, WHAT TO DO?
Closer inspection brings on more descriptive names — brown rot or black rot, for example — or scientific names. Look at those ugly spots on your tomato plants' leaves: Alternaria solani marks leaves with dark-brown, round spots a half-inch in diameter, each surrounded by concentric rings. Septoria lycopsersici causes spots that are small, round and grey, each surrounded by a single, dark margin. The cure for both is the same: crop rotation and thorough garden cleanup in the fall.
Not all diseases need attention; powdery mildew does lilac little harm. And those that require attention, such as leaf spots of tomato, can be controlled by methods other than spraying.
When spraying is needed for a particular disease, it might be something as benign as baking soda, 1 tablespoon added to a gallon of water along with 2 1/2 tablespoons of cooking oil.