FACT SHEETS>PEST MANAGEMENT>DOWNY MILDEWS OF ORNAMENTAL PLANTS

Downy Mildews of Ornamental Plants

Downy mildews have become an increasing problem in the horticultural industry and are currently causing serious losses in many floricultural crops. Headlines in the ornamental industry press support the concern that these diseases are becoming more common and more difficult to control. The pathogens are very different from Powdery Mildews-they attack different plants under very different environmental conditions, and are controlled by different classes of fungicides. Downy Mildew diseases are caused by a group of fungus-like organisms: they are not true fungi, and are similar to Pythium and Phytophthora species. Most of the Downy mildew fungi are host specific and infect only one plant family. Pathogens include species of Peronospora, Bremia, Plasmopara, and Basidiophora. Some of the Downy mildews are more aggressive than others.

Downy mildews infect almost all ornamental plants, as well as, some indoor plants. Greenhouse crops reported to have Downy mildew diseases include snapdragon, Salvia , alyssum, pansy, rose, rosemary, Primula, Osteospermum, Impatiens (not wallerana ), coleus, statice Verbena, ornamental cabbage, and cineraria. Perennial hosts include Aster, Buddleia, Coreopsis, Geranium (not Pelargonium ), Geum, Gerbera, Lamium, Delphinium, Veronica and Viola.

Photos: Top left -Downy mildew on snapdragon, Lower right - Close-up of spores,
Bottom, left - Microscopic photo
Symptoms of Downy mildew start as yellow to light green, irregular leaf lesions which can become purple to dark brown and be delimited by leaf veins. Under humid conditions, the fungus sporulates on leaf undersides, producing white, tan, gray or purple, downy growth. Symptoms will vary along with the host plant. As the disease progresses, lower leaves can wither and fall off. The fungus grows both locally and systemically and it can escape notice until conditions are right for sporulation. Systemic symptoms can include stunting, leaf distortion and epinasty, shortened internodes, and a decrease in the quantity and quality of flower production. Some Downy mildews start from contaminated seed or from weed hosts around the production area. These diseases thrive under the cool, wet conditions of spring and fall. Downy mildews need wet leaves and high relative humidity (greater than 85 %) to cause disease; under these conditions, disease can occur very rapidly and is difficult to control.

The first tactic in disease management is to provide the required cultural conditions for plant growth and development. Management of Downy mildew diseases consists of managing relative humidity in the greenhouse, strict sanitation, and preventive fungicide applications. It is critical to keep relative humidity below 85 % to decrease sporulation on infected plants and stop germination of spores on healthy plants. This can be done by venting and heating, which will fill the greenhousewith warm, drier air. The use of fans in greenhouses can keep air moving and prevent cold spots where condensation develops from occurring. Space plants to allow for maximum air circulation and sunlight availability. Remove all infected plant parts and discard well away from production area. Phosphorous acid products (Aliette, Alude, Vital), cinnamic acid derivatives (Stature), and strobilurins (Heritage, Compass) provide excellent control when used preventively. Mancozeb chemicals (Dithane Rainshield, Protect T & O) give good to very good control. Copper (Phyton 27) has also been reported to give control although some phytotoxicity ( Do not use on alyssum ) can occur. Aliette and Heritage are both systemic chemicals and are able to penetrate within infected leaf tissue and allow better control. Many products which give excellent control preventively result in less or no control when used as curatives; once sporulation occurs control is difficult to impossible. The use of fungicides to control has made more difficult because of the development of resistant strains of downy mildew pathogens and loss of effectiveness. Most products with systemic activity are subject to an anti-resistance strategy. These measures include: beginning a regular spray program with a protectant fungicide from a different activity group, limiting the number of applications of a particular fungicide applied per season, applying systemic chemicals in combination with a protectant like mancozeb, and applying only in periods of greatest disease pressure.

 References

 A.R. Chase. Downy Mildew on Ornamental Crops. Chase Research Gardens, Inc.

Downy Mildew of Roses. http://www2.dpi.qld.gov.au/horticulture .

Disease of the Month: Powdery Mildew vs. Downy Mildew. http://www.county.ces.uga.edu/cobb/Horticulture/Factsheets .

UC Pest Management Guidelines. UC IPM Online. http://www.ipm.ucdavis.edu .

University of Florida IFAS Extension. Professional Disease Management Guide for Ornamental Plants. http://www.edis.ifas.ufl.edu .

Acknowledgements: Pictures by Dr. Robert Wick

Prepared by M. Bess Dicklow, UMass Plant Extension Diagnostic Lab, 108 Holdsworth Hall, University of Massachusetts , Amherst , MA 01003 -9285. (312)545-3209. mbdicklo@umext.umass.edu .

Updated 9/06