Powdery mildew on trees and shrubs: what the extension services record
What UC IPM, the University of Connecticut and Colorado State University Extension say about powdery mildew on trees and woody ornamentals: the many host-specific fungi, the white growth, the dry-leaf infection that needs no water on the surface, the overwintering bodies and the harm the sources describe.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page describes what the named sources say. It is not a diagnosis of any tree and does not say what should be done with one. None of the three pages read lists powdery mildew as regulated or notifiable. Each section names its source, all read on October 9, 2026. The sources cover garden plants as well as trees, so only what they say about trees and woody shrubs is reported. The full list of diseases is on the tree diseases page.
A group of host-specific fungi
Colorado State University Extension calls powdery mildews one of the most widespread and easily recognized plant diseases, affecting virtually all kinds of plants including fruit trees and broad-leaved shade and forest trees. UC IPM, the University of California's pest management program, says there are many species of powdery mildew fungi and each infects a range of different plants. The University of Connecticut gives the example that powdery mildew on oak will not infect zinnia. Colorado says the fungi are host specific and cannot survive without the proper host, and gives the examples that the species causing mildew on grape and linden does not attack lilac, and that the species affecting elm, catalpa, lilac and oak does not attack turfgrass.
Trees and shrubs the sources name
| Source | Woody hosts named |
|---|---|
| UC IPM | Rose, crape myrtle, sycamore and oak, where it hits new growth (buds, shoots, flowers and leaves), and says it can be serious on these woody species across California. It lists London plane tree among the ornamentals for which resistant or less susceptible varieties can be found. |
| University of Connecticut | More commonly infected woody ornamentals include lilac, dogwood, azalea, sycamore and rose; it also describes powdery mildew on oak. |
| Colorado State University Extension | In Colorado, ash, lilac, euonymus, cherry, apple, crabapple, pear and Virginia creeper, among others; elm, catalpa, lilac and oak for one named fungus. |
What it looks like
Colorado describes spots or patches of white to grayish, talcum-powder-like growth, mostly on the upper side of leaves but also the underside, young stems, buds, flowers and young fruit. It says infected leaves may distort, turn yellow with small green patches and fall early, and infected buds may fail to open. It also describes tiny pinhead-sized spherical bodies, first white, later yellow-brown and finally black, which are the cleistothecia or overwintering bodies. UC IPM describes white powdery spots on both leaf surfaces that expand as the infection grows, leaves that yellow or brown and drop, leaving the plant or fruit open to sunburn, and weblike russet scars or corky areas on apples, grapes and stone fruits. Connecticut says home gardeners can recognize it by white fungus patches on the upper leaf surface.
How it lives and what weather it likes
The three sources agree on a feature unusual among leaf fungi: the spores do not need water on the leaf to germinate. UC IPM says powdery mildew does not require moist conditions to grow, Connecticut says the spores do not require water on the leaf surface to germinate and infect, and Colorado says the fungus does not need water on the leaf for infection and that infection does not occur while leaf surfaces are wet, as in a rain shower. Connecticut says the disease thrives from mid-summer to fall, because warm days followed by cool nights favour it, and also develops in late spring, with spores blown to new hosts on warm dry days and high night humidity helping germination. Colorado says mildews are severe in spring and fall at 70 to 80 degrees Fahrenheit, need both high and low humidity to finish their life cycle, are common in crowded plantings with poor air circulation and in damp shade, and that young succulent growth is more susceptible than older tissue.
On overwintering, Connecticut says the fungus may survive the winter as small black pepper-like spores on infected leaves in autumn, so new infections may begin from the leaf litter, and that some species overwinter in the buds. It says the fungus needs live tissue to grow and reproduce, sending sucker-like structures into the cells of the leaf surface.
Harm to the tree
Connecticut says the disease arrives late in the season, so it may not lower the lasting health of trees and shrubs, and that it is mainly a cosmetic matter. UC IPM says it can be serious on woody species where it attacks new growth. Colorado says severity depends on the host variety, the age and condition of the plant and the weather during the growing season.
Status and control wording
None of the pages names a quarantine or a report line. The sources point to resistant varieties and plant spacing for airflow as the first considerations; UC IPM adds that sunny sites help. This page gives no treatment steps, product names, rates or timings, and does not repeat the resistant-variety lists. For another leaf disease of plane trees see anthracnose of plane. For who may apply pesticides see pesticide licences.