http://www.emfnews.org/

No, they don't, a Dutch study finds. A rumor that Wi-Fi signals sicken trees has been circulating the Internet. It's not true, says a Dutch study.

It's an Internet rumor that is spreading, appropriately enough, like wildfire: Wi-Fi signals can make trees and other plants sick, causing cracks in their bark and killing off portions of their leaves. The outlandish claim, supposedly based on a Dutch study, cropped up late last week and has since been repeated in countless blog posts.

In response, the Dutch government's Antenna Agency, which provides information on the health effects of electromagnetic fields, has issued a statement urging caution on the unpublished, unverified and otherwise very preliminary findings.

As rendered via Google Translate, the Antenna Agency wrote (with a few [sics]):
"Based on the information now available can not be concluded that the WiFi radio signals...

http://www.emfnews.org/store/
http://www.emfnews.org/products.html