Powdery Mildews
Causal Agent(s) & Hosts
Powdery mildew is a common disease of many woody and herbaceous ornamental plants. This disease is caused by fungi in several genera—including Erisyphe, Golovinomyces, Microsphaera, Phyllactinia, Podosphaera, and Sphaerotheca. These pathogens have a very limited host range. In most cases, a powdery mildew fungus only infects plants within a single genus or family.
Signs & Symptoms
All powdery mildew fungi produce easily recognizable white mycelium mats and powdery spores on the upper surfaces of plant leaves. Both old and new foliage are susceptible to powdery mildew infection, but younger, succulent foliage usually is more susceptible and more severely affected. In most cases, powdery mildew fungi do not kill their host. However, infected leaves may sometimes become distorted or die depending on host susceptibility, plant age, and environmental factors.
When Does it Happen?
On susceptible hosts, powdery mildew spores will germinate and infect the host in a process that can take up to a week, depending on the pathogen. Free water is not necessary for germination and infection; instead, infection is favored by high humidity and moderate temperatures. Therefore, shaded areas, dense canopies, close plant spacing, and poor air circulation are highly favorable for powdery mildew development.
Cultural Management
The best line of defense against powdery mildew is to plant resistant cultivars when available. Reducing shade, decreasing planting density, increasing airflow and avoiding over fertilization can help reduce disease severity. If infection occurs early in the growing season, consider removing infected plants/branches immediately to reduce inoculum availability; place diseased material in bags and dispose of away from susceptible plants.
Chemical Management
Powdery mildew does not kill its host in most cases and often gets started later in the growing season; therefore, fungicide sprays usually are not needed or recommended. However, when high-value plants are infected, particularly early in the season, a spray may be needed, and there are many effective fungicides available.
Fungicide efficacy may vary across plant species due to the large number of fungal species that cause powdery mildew. Development of fungicide resistance is highly likely among powdery mildew fungi. Always rotate fungicide chemistries between each application.
