Downy Mildew

Gary W. Moorman, Professor of Plant Pathology

Downy mildew is the common name for a group of highly specialized obligate parasites of vascular plants. These organisms are distinctly different from the powdery mildews. Although they have traditionally been included taxonomically with true fungi, these organisms and their relatives in the genera Phytophthora and Pythium and others in the Oomycota are now not believed to be closely related to true fungi. Morphologically, they are similar to fungi and have absorptive nutrition. The chemicals used to control downy mildews are similar to those used for Pythium and Phytophthora and different from most of those used for true fungi.

Symptoms

Downy mildew colonies often appear first on the underside of leaves, and they sometimes have a bluish tinge (1; 3). In many cases, they can grow systemically throughout the plant. If growing abundantly on a leaf, downy mildew colonies can be confused with gray mold (Botrytis) or with powdery mildew. Microscopically, they are very easy to tell apart from powdery mildew and Botrytis. On the foliage, small yellow spots develop on the upper sides of the leaf while white to bluish-white fluffy growth forms on the underside of the leaf. As the leaf spot dies, the fluffy growth darkens to gray in color. Infected leaves and branches may be distorted and die.

Life history

Downy mildews are generally favored by cool temperatures (15-23° C = 58-72° F) and relative humidity above 85% at the leaf surface. While some downy mildews survive from year to year locally in plant debris, in the soil, or on weeds, others do not survive in cold climates and must be blown back to our region from southern climates each spring. Species of downy mildews reproduce sexually via oogonia and antheridia and asexually via sporangia. From the time of infection until new spores form can be as short as 4 days but is usually 7-10 days. In all species, the sporangia germinate directly by forming a germ tube that penetrates the plant. In some species (Sclerospora and Plasmopara), sporangia can also germinate 'indirectly' by releasing zoospores. Peronospora, Pseudoperonospora, and Bremia rarely if ever form zoospores.

Downy mildew on underside of leaf.  Note grayish-blue color.

Downy mildew on Rudbeckia. White color could be confused

powdery mildew.

White Downy mildew branch with spore attached.

Management

Plant debris should be removed from the area around the plant and buried, burned, or placed in a closed container. Plants should be spaced to ensure good air circulation around them so that when irrigated or subject to rainfall, leaf surfaces dry quickly. Overhead irrigation should not be used when the weather is generally cool. IT IS IMPORTANT, during cool damp weather in the spring, TO SCOUT HIGHLY SUSCEPTIBLE PLANTS in order to detect first infections. In particular, roses, pansy and impatiens should be monitored carefully. The following fungicides are available to control downy mildews to protect plants during cool, damp weather.

Active Ingredients and Trade Names of the Chemicals

Fungicides and Fungicide Resistance Management - Certain fungicides, usually systemic fungicides, are said to be 'at risk' to the development of resistance if they are used repeatedly. See the Risk Level in the above table (1 = low risk; 3 = high risk). The Fungicide Resistance Action Committee has developed a numbering system in which chemicals with the same FRAC Group number have the same mode of action (See http://www.frac.info/frac/index.htm ). It is recommended that chemicals at high risk be used sparingly and in rotation or mixed with chemicals with different modes of actions (different FRAC number).

Literature Cited

1. Agrios, G. N. 1997. Plant Pathology. Fourth ed. New York: Academic Press. 635 pp.

2. Farr, D. F., Bills, G. F., Chamuris, G. P., and Rossman, A. Y. 1989. Fungi on plants and plant products in the United States. St. Paul: APS Press. 1252 pp.

3. Yarwood, C. E. 1947. Snapdragon downy mildew. Hilgardia 17:241-250.