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| Welcome to our tree column, “Ask your local arborist.” Each month, arborists from the Davey Tree Expert Company in Menlo Park discuss a tree-related topic from general care and planting tips to how trees can impact your property. If you have a tree-related question or an idea for a topic to discuss, email lifestyle@embarcaderopublishing.com or post in the comments section at the end of this column. |
Right about now, you may start to notice a powdery substance covering the leaves on your trees and shrubs – most likely, this is mildew. Our region has endured long-term drought, followed by wetter winters and variable humidity. The extremes of this roller-coaster weather ride can stress trees and make them less able to fend off diseases, including fungal threats. While powdery mildew can appear throughout the year, it is most common during the late summer and early fall when the days are warm and the nights are cool, creating favorable conditions for its development and spread.
Leaves with brown or tan spots are another sign of a common fungal infection called anthracnose.
These spots are typically more noticeable in late spring and early summer.
Powdery mildew and anthracnose are found on a wide variety of trees and shrubs: Peninsula maples and dogwoods are among the trees on the Peninsula that often struggle with these common fungal diseases.
These fungal infections won’t usually kill mature trees, but it can weaken them. It is more serious for young trees and shrubs and annuals, like garden vegetables, and can cause severe leaf and branch loss.
Understanding why these diseases thrive locally, and how to manage them, can save your trees.
Powdery mildew
Powdery mildew starts with a fine, white dusting on leaf surfaces and tender shoots, hence the name. Unlike many pathogens, it doesn’t invade leaf tissue, only growing on the surface. Young leaves often become distorted or dry out.
In the greater Bay Area, we commonly see it develop in summer and fall along the coast. Nighttime humidity from marine layers, combined with warm daytime temperatures, creates ideal conditions. Inland, it often flares up in poorly ventilated, irrigated areas.
What homeowners can do: Rake and remove fallen infected leaves and thin canopies to improve airflow.
When to bring in the pros: If you see repeated outbreaks year after year, if young trees are affected, if the above treatment actions are not working.
Anthracnose
This fungal disease thrives in cool, wet springs, with symptoms ranging from leaf spots to twig dieback, and in the case of dogwood anthracnose, even branch and trunk cankers. Coastal and foothill climates are particularly prone.
What homeowners can do: Prune out any dead or infected branches and dispose of them properly to reduce the risk of spread. Healthy trees benefit from a fresh layer of mulch and slow-release nutrients from compost or fertilizer to help them recover.
When to bring in the pros: If the tree has reoccurring anthracnose for more than two years; if you require a ladder to remove diseased branches; or if the tree has extensive leaf loss, especially in the spring season.
Next steps before fall
- Assess susceptible trees.
- Thin canopies, clear up debris. This will help reduce the spread of spores.
- Treat proactively: Prune, deep water, mulch and apply fungicides, when recommended. (For best results, start fungicide treatments in mid-summer or when the disease is first detected and repeat at intervals throughout the fall.)
George Reno is a local arborist at The Davey Tree Expert Company in Menlo Park, which serves communities on the Peninsula, including Menlo Park, Atherton, Palo Alto and Redwood City. Reno can be reached at 866-923-5658.
