Blog

Commonly-affected plants:        Viburnum ‘Awabuki’,
Viburnum suspensum

Downy Mildew of Viburnum, caused by the fungus Plasmopara viburni, is a common foliar disease of viburnum. This fungus requires cooler temperatures and high humidity and/or rainfall or overhead irrigation for infection. Therefore, Viburnum plants often become diseased in the cooler months of October through March, especially when watered through overhead irrigation.

Diseased Viburnums exhibit yellow to red blotches on the top of the leaves followed by white to gray “downy” growth on the bottom of the leaves. Infected leaves then quickly turn brown and drop off the plant. Eventually the entire plant may defoliate. Fungus from fallen diseased leaves re-infects the plants and the cycle continues.

Other viburnum species and varieties are usually not significantly-affected by this disease.

Over the past several years, Downy Mildew of Viburnum has become a challenging disease to manage. Application of fungicides labeled for management of this pathogen may not provide satisfactory control. Therefore, the following is recommended to increase the efficacy of treatments:

• Rake-out fallen leaves and dispose in the trash – do not move to another area in the landscape. This should be done often to remove the source of future infections.
• Water diseased Viburnums, or Viburnums that were infected in previous years, only as needed. Overwatering will exacerbate fungal spread and infection potential.
• Prune overcrowded shrubs to maximize air flow and encourage faster drying of leaves.

Following these recommendations may alleviate the spread of this disease, but this cannot be guaranteed. Should the disease continue to spread, please contact our office so we may discuss additional options.

Downy Mildew of Viburnum may cause significant to complete defoliation of entire Viburnum hedges. In the majority of cases,defoliated Viburnums will slowly recover in spring and summer. However if plants succumb to this disease, then replacement with a  plant other than Viburnum ‘Awabuki’ or Viburnum suspensum is advisable.