JARS v46n2 - Powdery Mildew: The Unknown Garden Intruder

Powdery Mildew: The Unknown Garden Intruder
Ken Gibson
Tofino, British Columbia, Canada

The unknown intruder I am referring to is a new form of powdery mildew which began to be noticed on rhododendrons in the United Kingdom during the early 1980s. I feel the best authority is Kenneth Cox of Perth, Scotland, and my purpose here is to emphasize his warning of two years ago: "The most worrying thing in the Pacific Northwest was that some people didn't realize they had the disease in their gardens or were not aware of its likely effects." I, like many others failed to take his warning seriously. In a brief trip to northern Washington this past fall, I concluded that many other gardeners have not taken him seriously either. Warren Berg of Port Ludlow, Wash., pointed out evidence of the intruder in my garden in July 1990. My first thought was that Rhododendron Heaven couldn't have aliens. Needless to say, I've since changed my thinking.

Powdery mildew on Rhododendron leaves
The stricken 'Cherry Float' shows the fine, raised, brown
freckles on the underside of the leaves.
Photo by Ken Gibson

Cox states further: "I would hazard to guess that every rhododendron collection in the Pacific Northwest, unless very isolated, will become affected by the disease in the next few years." Perhaps I thought of Tofino as isolated. But I estimate now that 60 percent of my garden is infected. I am sure you can appreciate my concern.

'Leverett Richards' defoliation
All five plants of Mr. Gibson's 'Leverett Richards'
are, like this one, 90 percent defoliated.
Photo by Ken Gibson

To begin with, I am no writer or scientist and I certainly do not have the answers. I merely would like to put forward, or point out to my fellow gardeners, some findings and possible treatment while the 1992 blooming season is in progress. I am sure, when more independent studies are complete, much of the mystery we now face will be behind us. I must admit I am disappointed in what the "professionals" have been able to tell me, to date, other than to suggest I quit growing affected varieties. I have approximately 900 different varieties and would have been more than pleased if any one of the professionals had shown some interest to view them.

'Lionel's Triumph' shows bluish mosaic.
'Lionel's Triumph' shows bluish mosaic.
Photo by Ken Gibson
Mildew on R. 'Ripe Corn' leaves
'Ripe Corn', a very susceptible plant, shows yellow blotched and
light brown spots on undersides of leaves.
Photo by Ken Gibson