Cause Pseudomonas syringae pv. viburni, a bacterium that can be a problem in cool, wet springs. Pseudomonas syringae pv. syringae may also occur on viburnum in the PNW. Two common genetic traits increase the bacteria's ability to cause disease. Most produce a powerful plant toxin, syringomycin, which destroys plant tissues as bacteria multiply in a wound. Bacteria also produce a protein that acts as an ice nucleus, increasing frost wounds that bacteria easily colonize and expand.
Symptoms Spots are at first water soaked, then turn brown and irregular in shape. Oregon observations are that spots may be angular. Margins frequently develop a bacterial exudate. Numerous spots during early shoot growth can deform leaves. Stem lesions are elongate and generally not as obvious. Severe infections can result in a shoot dieback.
- Remove and destroy infected leaves or plant debris.
- Space plants for good air circulation.
- Avoid high nitrogen fertility that promotes excessive growth.
- Protect plants from extremes of weather early in the season. Plastic shelters have been as good as or better than chemical methods against the same disease on other crops.
- Cultivars reportedly resistant include V. x burkwoodii 'Mohawk,' V. x carlcephalum 'Cayuga,' V. lantana 'Mohican,' and V. rhytidophyllum 'Alleghany.'
Chemical control Bacteria resistant to copper products have been detected in many nurseries.
- Copper-Count-N at 1 quart/100 gal water. 12-hr reentry.
- CuPRO 2005 T/N/O at 0.75 to 3 lb/A (or 1 to 3 Tbsp/1000 sq ft) dormant or at 0.75 to 2 lb/A when new growth is present. 24-hr reentry.
- Junction at 1.5 lb/100 gal water. 24-hr reentry.
- Monterey Liqui-Cop at 3 Tbsp/gal water. H
References Bradbury, J.F. 1986. Guide to Plant Pathogenic Bacteria. CAB International Mycological Institute.
Scheck, H. J., Canfield, M. L., Pscheidt, J. W., and Moore, L. W. 1997. Rapid evaluation of pathogenicity in Pseudomonas syringae pv. syringae with a lilac tissue culture bioassay and syringomycin DNA probes. Plant Dis. 81:905-910.