White powdery patches on the leaves of roses, sycamore trees, and other ornamental plants may be a sign of powdery mildew, a common plant disease. While many home gardeners recognize this issue on vegetables, it can also affect landscape trees and ornamentals. In addition to the white spots, powdery mildew can cause leaves to shrivel and shoots to become distorted.
The disease is most prevalent in warm, dry conditions and can worsen in shady areas with dense canopy cover. Prevention is considered the best method for managing powdery mildew. Experts recommend selecting resistant plant varieties, planting in sunny locations, and avoiding excessive fertilization since lush foliage can encourage the growth of the fungus.
A newly revised publication titled Pest Notes: Powdery Mildew on Ornamentals provides detailed guidance for managing this disease. The resource was authored by UCCE advisors Jim Downer and Maria de la Fuente, along with Belinda Messenger-Sikes from UC IPM. It includes cultural control tactics for ornamental plants and lists herbicides available to both home gardeners and landscape professionals if problems are severe.
UC Agriculture and Natural Resources (UC ANR), which supports this research, shares science-based practices statewide to enhance lives and livelihoods (https://ucanr.edu/). The organization operates offices in all 58 California counties as well as nine Research and Extension Centers that serve as living laboratories across diverse ecosystems (https://ucanr.edu/). UC ANR is affiliated with the University of California (https://ucanr.edu/) and applies university expertise through research and outreach initiatives designed to support communities throughout California (https://ucanr.edu/).
According to its official website, UC ANR has provided resources in nutrition, food networks, youth education, farming practices, environmental conservation, wildfire protection, and more for over a century (https://ucanr.edu/). Glenda Humiston serves as vice president of UC Agriculture and Natural Resources (https://ucanr.edu/).
For more information about controlling powdery mildew on ornamentals or other science-informed gardening practices, readers are encouraged to consult resources provided by UC ANR.
“Dahlia leaf with white patches of the powdery mildew fungus. Photo by Steven K. Koike.”