Did you Know?Fungi make up a diverse kingdom. There is an estimated 1.5 million species of fungi, though only 10% have been identified. That's second only to insects (which estimations for insect species is upwards of 10 million)! Fungi do have a record in one category: Largest living organism. In Oregon, there is a honey fungus that spans 2.4 MILES across the Blue Mountains! Interestingly, another fungus in the same genus, Armillaria, actually infects trees and causes Armillaria root rot.
Powdery Mildew – White Film on Leaves
Powdery Mildew refers to many disease causing fungi that cause a white, powder-like substance to grow on leaves and branches. This is one of the most common diseases in the landscape.
A white, powdery film is easily recognizable on leaves and stems in mid to late summer. In severe infections, mycelia growth can become so extensive that it looks almost like cobwebs coming off the leaves and stems. Typically, powdery mildew occurs on the tops of younger leaves, but can also occur on other parts throughout the plant.
Spores overwinter in leaf litter on the ground. As weather conditions become favorable, spores are released from their overwintering structures and spread by either wind or splashed by rain. The spores germinate on leaves and begin growing mycelium. Unlike many fungal pathogens of plants, powdery mildew does not require high moisture to grow. More spores are created and sent out to infect new plants and plant parts until spore-containing overwintering structures are formed when weather conditions become unfavorable.
Typically, powdery mildew is not a huge concern because it usually occurs later in the season. However, repeated infections or early appearance of powdery mildew can be problematic as landscape plants become aesthetically displeasing and overall health is compromised. These types of infections usually cause leaves to become twisted and drop early.
Good cultural practices are best for managing powdery mildew. When planting new trees, try to find resistant cultivars to avoid the problem altogether. Pruning out the canopy can help as it increases air flow and reduces the surface area for spores to take hold. Also, cleaning up leaf debris in the fall reduces the likelihood of reinfection.
There are some fungicides that are available for powdery mildew and will reduce spreading or infection, but will not cure existing damage.