Erysiphe liriodendri, a powdery mildew specific to tulip poplars (Liriodendron tulipifera) by Scott LaGreca.
Botanical name: Erysiphe spp. and related genera, e.g. Golovinomyces, Podosphaera, Phyllactinia, etc.
Common name: Powdery mildews
Family name: Erysiphaceae (Erysiphale family)
Native range: Cosmopolitan
The bane of rose gardens everywhere, powdery mildew is one of our most persistent and hated plant diseases. In casual speech, “mildew” is often used interchangeably with “mold” to describe a wide variety of fungal forms and species; but in a horticultural context, powdery mildew is caused by fungi in the family Erysiphaceae, comprising over 900 known species. Powdery mildew afflicts not only roses, but over 10,000 other plant species, causing decreased growth, wilting and unattractive plants. It is also responsible for billions of dollars in annual losses worldwide of economically important crops such as grains, grapes, tomatoes, cucumbers, squash and other vegetables.
By fungal standards, powdery mildew fungi have a fairly simple life cycle, with most species alternating between an asexual state in the spring, followed by a sexual state in late summer and fall. The whitish, dusty mildew visible on leaves is actually a miniature “lawn” of mycelium (fungal filaments) that produce chains of asexual spores called conidia. Enormous numbers of conidia can be produced during a growing season, causing exponential growth of mildew, resulting in garden-wide epidemics.
Unlike most fungi, powdery mildew spores do not require free water for germination, so mildew spread isn’t necessarily correlated with rain. Powdery mildew rarely kills plants outright—rather, the fungus usually weakens plants and makes them unsightly to the garden observer. It is thought that in some cases, powdery mildews can weaken a plant to the point that more serious plant pathogens gain a foothold, resulting in a succession of disease leading to death.
In late summer and fall, if you look carefully, you might find dark-colored, spherical sporangia called “chasmothecia” that contain sacs of sexual spores. Considered objectively, chasmothecia have a symmetric, aesthetic quality that has caught the eye of many naturalists ever since the invention of the microscope. The size and cells of these tiny dark spheres, as well as the size and shape of their appendages, are important characters for distinguishing the approximately 19 genera that make up this fascinating family of fungi. The chasmothecial appendages are also important in the epidemiology of powdery mildews: they can overwinter and “catch” on other vegetation, perpetuating the disease over many years. For this reason, it is critically important to destroy any diseased plants and plant debris, and not include them in your mulching (as is true for all plant diseases).
Chasmothecium of Erysiphe quercus-virginianae, a powdery mildew specific to live oaks (Quercus virginiana), and Takamatsuella grandii, a powdery mildew that afflicts sugar maples (Acer saccharum), 100x magnification. Photos by Scott LaGreca.
Being obligate parasites, powdery mildew species only grow on their host plants—not on petri dishes—making them difficult to study in a laboratory setting. Luckily, agricultural researchers like the Bradshaw Lab at NC State University are making great strides in documenting the diversity, ecology, and epidemiology of these important pathogens. Powdery mildew species have traditionally been described in a broad sense for their morphology, but the Bradshaw lab has shown they are much more host-specific than traditionally thought. Last year, before I began working at Duke Gardens, I had the privilege of working in the Bradshaw Lab on a project describing five powdery mildews new to science, all from North Carolina. Thus far, Dr. Bradshaw has described over 30 species new to science. He has also documented the recent, rapid spread of the powdery mildew Erysiphe vaccinii—an economically devastating pathogen of blueberries once confined to North America—to other parts of the world, including Mexico, Morocco, and China.
If you have a powdery mildew (or any other plant disease) that is troubling you, NC State’s Plant Disease and Insect Clinic is your local, state-funded, go-to resource for diagnosis and advice—for a small fee, you can learn exactly what’s growing on your plants (and what to do about it)!