Taxonomy

Domain	Eukarya
Kingdom	Chromista
Phylum	Oomycota
Class	Oomycetes
Order	Peronosporales
Family	Peronosporaceae
Genus	Plasmopara

Scientific Name

Plasmopara viburni

Common Name

downy mildew

Author: Stephanie Suarez, University of Florida

Reviewed by: Jeffrey Rollins, University of Florida

Pathogen

Plasmopara viburni is an oomycete in the order Peronosporales that causes downy mildew host-specifically on the ornamental crop viburnum. Host range includes many species in the genus Viburnum (Caprifoliaceae), some including Viburnum suspensum, V. acerifolium, and V. odoratissimum (3).

Symptoms and Signs

Symptoms include large bronze to black-colored spots on the upper side of the leaf that eventually merge together until the entire leaf curls and ultimately drops. Signs of the pathogen include white or gray fuzzy downy growth on the underside of spotted leaves. Sparser white growth is the youngest and the gray patchy growth is older.

Ecology and Spread

P. viburni flourishes in high relative humidity and cool, wet conditions of the late nights or early mornings (1). The disease is usually prevalent on viburnum hedges in the landscape in late winter. It lasts until early spring, ending around March or April when humidity decreases and temperature increases (4). Oospores can persist for long periods of time in infested soil and infect healthy viburnum plants in the next season when conditions are favorable. Upon oospore germinatation, sporangiophores are formed. Sporangiophores then give rise to sporangia, each one filled with zoospores that spread to susceptible plants by way of free water or wind (1).

Geographic Distribution

It is spread throughout the central and eastern regions of the U.S, including states like Florida, Georgia, Alabama, and New York (4).

Management

The most critical points in controlling this disease are removing fallen infected leaf debris (i.e., discard and do NOT compost), and making sure that irrigation is set up in a way that water does not directly hit the foliage. This will reduce the chance of sporangia dispersal by water splashing. If chemical control is desired, fungicides labeled for ornamentals may be used in nursery or greenhouse settings. Repeated applications of fosetyl-Al, azoxystrobin, pyraclostrobin, and fluopicolide are all effective chemistries (2). Consult your local extension specialist for legal and efficacious fungicide products available in your state. Remember, the label is the law and the product applicator is responsible for reading and following all chemical labeling.

Diagnostic Procedures

As this pathogen is an obligate biotroph, it cannot be cultured. The most common diagnostic method of this disease is looking for signs of the pathogen microscopically (i.e., sporangia or sporangiophores). Another disease affecting Viburnum spp. is powdery mildew. Symptoms and signs are similar, but powdery mildew usually appears on the upper leaf surface of the plant and during the warmer summer months. Downy mildew appears on the lower leaf surface during spring months when there are cooler temperatures (5).

Resources and References

1. Beckerman, J. 2009. Diseases of Landscape Plants: Downy Mildew. Purdue Extension.

2. Caldwell, D. 2012. Viburnum Downy Mildew Blight of Awabuki (mirror-leaf) Viburnum. Naples Daily News.

3. Dick, M.W. 2001. Straminipilous fungi: systematics of the Peronosporomycetes including accounts of the marine straminipilous protists, the plasmodiophorids and similar organisms. Kluwer Academic Publishers, 670 pg.

4. Farr, D.F., & Rossman, A.Y. Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA, from http://nt.ars-grin.gov/fungaldatabases/

5. Williamson, J. 2013. Viburnum Diseases and Insect Pests. Clemson University: Cooperative Extension.

Acknowledgments

National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)