Download March 6th, 2015 Re: Downy Mildew on Viburnum suspensum An

MEMO
March 6th, 2015
Re: Downy Mildew on Viburnum suspensum
An unexpected outbreak of a disease called Downy Mildew was found last week
infecting about 95% of the Viburnum suspensum hedges throughout Kings Point. The
pathogen, Plasmopara viburni, is a water mold closely related to fungus. The pathogen
favors wet, foggy or humid and cool conditions, which can cause the disease to worsen
and spread. Whereas Downy Mildew used to be only a minor and occasional problem in
the landscape, it is becoming increasingly problematic and increasing its host range. As
an example, it has been so devastating to impatiens over the past couple years that
common impatiens are difficult to grow in some areas. It has also been a challenge to
manage over the past few years on “Awabuki” viburnum (Mirror Leaf Viburnum).
However, we have not observed it infecting Viburnum suspensum on such a large scale
and causing significant defoliation of hedges until now.
Downy mildew can cause severe defoliation and re-infect new leaves if measures aren’t
taken to correct the problem. Repeated loss and regrowth of leaves results in excessive
plant stress, which could lead to the loss of aging plants. Therefore it is important that
treatment begin immediately. A widespread fungicide application began this morning
on all infected viburnum hedges at Kings Point. We will also be removing leaves that
have dropped to reduce the chances of re-infection. As favorable conditions for the
disease alleviate, our hope is that the disease pressure will also subside, however a
follow-up treatment may be needed.
As a future consideration, water streaming from irrigation heads that directly contacts
plant foliage can spread an infection throughout a hedge by splashing spores. If
fungicide applications alone prove not to be effective, an evaluation of irrigation heads
near Viburnum suspensum may be needed.
We will continue to monitor this disease and communicate our progress in getting it
under control.
Dr. Stephanie Bledsoe
Regional Director of Agronomy