Downy Mildew of Viburnum Diagnostic Guide
Downy mildew (Plasmopara viburni) disease causes leaf spots and defoliation on viburnum varieties. Some literature indicates that downy mildew is favored by nighttime temperatures of about 50 to 72°F, and the disease has been reported on Viburnum from November through late March in Florida. However, the disease may be present as early as October and as late as June in central parts of Florida (Spencer-Phillips, Gisi, and Lebeda 2002; Spencer-Phillips and Jeger 2004).
Trees at Risk
Downy mildew has been reported on several species of Viburnum including V. acerifolium, V. dentatum, V. opulus, V. pubescens, V. odoratissimum 'Awabuki', V. suspensum, V. tinus, and V. trilobum. The two predominant species in Florida are Viburnum odoratissimum ‘Awabuki’ and V. suspensum.
Signs of Damage
- New growth is most susceptible and may show symptoms first. Initially, leaves may look yellowish or speckled.
- The upper leaf surface often appears faded and blotchy with reddish brown spots.
- Downy mildew symptoms on ‘Awabuki’ have been described as a bronzing of the leaf surface.
- Dark brown to angular black spots on the upper leaf of V. suspensum are typically less blotchy and more distinct.
- This disease can be seen colonizing the underside of diseased leaves and appears whitish gray and downy-like on Viburnum ‘Awabuki’ and V. suspensum leaves.
- As the disease progresses leaves begin to drop off, and large portions of the canopy can become defoliated.
Downy mildew is prominent in high humidity, cool temperatures, and overcrowding. Literature indicates that downy mildew is favored by nighttime temperatures of about 50 to 72°F. The pathogen that causes downy mildew is a type of water mold and is more closely related to algae than to fungi. Downy mildew can spread by two different types of spores. One type, zoospores, moves through water. The sporangia that contain and release zoospores are easily windborne. The other type of spore, the oospore, forms inside plant tissues where the pathogen can survive for years. Downy mildew is very aggressive and can spread rapidly, so action should be taken quickly if it is found.
Field diagnosis of the disease and the host plant are usually sufficient. Take leaf samples and submit them to local extension agencies for definitive diagnosis.
Provide adequate fertilizer and water. Avoid overhead irrigation, and/or water early in the day. Avoid excessive pruning of hedges. Rake and dispose of fallen leaves offsite.
Other Treatment Practices
- Watering - Irrigate once a week during extended drought periods. Do not use overhead irrigation: use drip or trickle irrigation instead to keep the foliage from getting wet.
- Mulching - Mulch is very beneficial for all trees because it reduces competition with turf and moderates soil temperature and moisture levels. Maintain a 2-3 inch deep layer of composted mulch over the root zone.
- Sanitation - Rake and dispose of fallen leaves offsite.
Treatment Strategy Expectations
Downy mildew will always be present in the landscape, and treatment must focus on suppression, not eradication.
A Diagnostic Guide is designed to help you identify a pest issue and management solutions. Always refer to product label for all rates and approved uses. Some images courtesy forestryimages.org. Use of the images does not imply endorsement of treatments by forestryimages.org