Powdery mildew poses a challenge to island maples

Posted September 3, 2022 at 9:40 am by

Julia Tur­ney, San Juan Coun­ty Mas­ter Gar­den­er, sends along an update about the sta­tus of our local bigleaf maples.

Our bigleaf maple leaves have been look­ing very gray this sum­mer and many trees have leaves that are turn­ing brown and drop­ping from the trees.

The cul­prit is pow­dery mildew. There are sev­er­al types of fun­gi that attack only maples. Pow­dery mildew fun­gi thrive with cool, humid nights that stim­u­late spore pro­duc­tion and warm (70 to 80 F), dry days that allow for spore spread. Fun­gal spores are spread by the wind. The fruit­ing body of the fun­gus can over­win­ter or live in buds infect­ed in the pre­vi­ous sea­son. The com­bi­na­tion of stress from our dry sum­mer and dam­age from fun­gi is caus­ing the leaves to turn brown and drop.

There are fungi­cides that treat pow­dery mildew but they do best when used before symp­toms devel­op and few are good at erad­i­cat­ing fun­gi. Many have to be used every sev­en to 14 days. The San Juan Coun­ty Exten­sion WSU Mas­ter Gar­den­er pro­gram does not rec­om­mend that home­own­ers spray trees over ten feet tall. Giv­en the num­ber and size of bigleaf maples in our coun­ty, treat­ment is not practical.

Bigleaf maple trees should recov­er next year, and as long as the trees are not sub­ject to the same stress for a cou­ple of years in a row, they will not die from the pow­dery mildew infes­ta­tion. Addi­tion­al­ly, there are so many affect­ed trees and leaf lit­ter that treat­ment isn’t prac­ti­cal. Leaves can be col­lect­ed and com­post­ed for oth­er uses, as the fun­gus is maple spe­cif­ic. Com­post­ing is a good option for man­ag­ing falling leaves. The fun­gus feeds on live leaf tis­sue so it will not mul­ti­ply on dead leaves and the spores will break down in the com­post process.

For fur­ther ref­er­ence, Wash­ing­ton State Uni­ver­si­ty Hort­sense’s web­site has gen­er­al infor­ma­tion on pow­dery mildew. The San Juan Coun­ty Exten­sion WSU Mas­ter Gar­den­er pro­gram office in Fri­day Har­bor is avail­able to pro­vide answers to gar­den­ing and land­scape ques­tions. It can be reached at 360–370-7663 or mg.sanjuancounty@wsu.edu.

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