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Downy Mildew on Ornamentals

What ornamental diseases are particularly difficult to control?

Ornamental diseases caused by downy mildews are making headlines in the ornamental industry press, and my field observations in Santa Cruz County likewise support the concern that these diseases are more common and difficult to control. Snapdragon downy mildew is still prevalent in greenhouses, occurring during winter and spring sometimes with crushing destructiveness. Rose downy mildew commonly occurs in winter, and seems to wallop susceptible cultivars with a vengeance. We are finding new downy mildews on California ornamentals too. In Santa Cruz County alone, we have found two new downy mildew pathogens- one on foxglove and one on delphinium- just in the last two years.

The same fungus causes disease on all those plant species?

First, the downy mildews are caused by a group of fungus-like micro-organisms. They are not true fungi. When it comes to causing diseases however they have similar ways of causing infection and are controlled with similar tactics.

Actually the pathogens that cause downy mildew diseases on snapdragon, rose, foxglove, and delphinium are all different species. They all have specific and, in this case, different host ranges. For example the pathogen that causes downy mildew disease on rose will not cause downy mildew on snapdragon.

What do these diseases look like?

Disease symptoms vary greatly depending on the plant. Generally there can be gray to purplish fungus-like growth on the lower sides of leaves and this is often associated with leaf yellowing. Sometimes leaves drop from the plant, as they do commonly in roses.  

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Anything else I should know? 

Be on the lookout for a new downy mildew on marguerite daisy (Argyranthemum frutescens = Chyrsanthemum frutescens). This disease was observed in Santa Cruz and Monterey Counties in the spring of 2003 and in San Mateo in the winter of 2004. We did not confirm the particular species, as the isolates did not survive during experimentation. Presently there is no documentation of downy mildews on Argyranthemum in California. Following a very rainy period, the purple-gray growth can be readily seen sporulating on the undersides of newly developing leaves. Dry weather dries the spores up quickly.

We need your help to confirm the presence of this disease and identify the pathogen. Please report any suspicious observations.