Downy Mildew

Introduction
Downy mildew causes foliage blights and distortion. Although its name sounds similar to powdery mildew, downy mildew occurs during different environmental conditions and it is managed differently.

Downy mildew is not as common as foliar blights caused by Botrytis. During cool wet weather, downy mildew can occur very rapidly and is difficult to control. Some of the greenhouse crops prone to infection include: snapdragon, salvia, alyssum, pansy, rose, rosemary, and ornamental cabbage. Perennials susceptible to downy mildew include aster, coreopsis, geranium, geum, lamium, potentilla, veronica and viola.

Symptoms
Symptoms can be confused with other plant problems. Leaves may become mottled and yellowed, resembling nutritional deficiencies. On some plants, downy mildew infection may look similar to injury from foliar nematodes. In both cases, angular lesions are bounded by leaf veins. However, downy mildew infection results in a soft, fluffy gray, brown or purple fungal sporulation developing on the underside of leaves. On coreopsis, the fungal sporulation is white.

Symptoms can also vary depending upon the host plant. On snapdragons, infected plants are yellow and stunted. You may see downward leaf curling on the young seedlings. On salvia, angular yellow blotches can be seen between the leaf veins. On pansy, leaves turn mottled and off-color with purple blotches. Pale green or yellow patches develop on geum. Purplish patches occur on lamium and veronica. On roses, leaves develop angular dark purple to black areas and may turn yellow and drop.

Causal Organisms and Disease Development
Downy mildew fungi are obligate parasites that can only grow on live plant tissue. Infection occurs inside the leaf. In ornamental crops, fungi in the genus Peronospora, and sometimes Plasmopara cause downy mildews. Different species of Peronospora affect closely related plants. For example, P. lamiii affects members of the mint family including salvia and lamium.

The fungi that cause downy mildew are closely related to the Pythium and Phytophthora water molds. Downy mildews usually develop during cool (50^o - 75^o F), wet conditions with high relative humidity. A film of water is needed for spore germination and infection. Prolonged periods of leaf wetness promote this disease. Fungal spores are easily spread by wind and splashing water. Infection is sometimes carried in the seed or bulbs.

Prevention
In the greenhouse, keep relative humidity below 85 percent to decrease fungal sporulation and disease development. The use of computer control systems for environmental modification allows reduction of humidity levels to less than 85%. Reduce condensation by using horizontal air flow (HAF). HAF fans help to keep air moving in the greenhouse and to prevent cold spots developing where condensation occurs. Condensation can also be reduced by heating and venting the warm, moist greenhouse air. Heat and vent 2 to 3 times per hour in the evening after the sun sets and early in the morning at sunrise. Always water early in the day, so leaves can dry before nightfall. Use drip irrigation whenever feasible. Maintain proper plant spacing to reduce humidity levels within the plant canopy.

Monitoring
Become familiar with the symptoms on susceptible crops and regularly scout to detect downy mildew early when fungicides are more effective. With a 10 to 20x handlens, look on the underside of leaves for the gray, purple, brown or white blooms of sporangia (microscopic stalks bearing spores). The fungal stalks may branch in a pattern that looks like "deer antlers".

Sanitation
Remove all infected plants. Spores can be easily moved from plant to plant. Downy mildew may be carried over in infected plant debris in the soil and in weed hosts.

Chemical Controls
Protect susceptible crops with preventative fungicide applications during cool, wet weather. Repeated applications may be needed. Rotate among fungicide classes to delay the development of resistance. See the most recent edition of New England Greenhouse Floriculture Guide: A Management Guide for Insects, Diseases, Weeds and Growth Regulators for more specific up-to-date recommendations.

Leanne Pundt, Extension Educator, University of Connecticut
Photos by Leanne Pundt

References:
Chase, A. R. 2002. Cool, wet conditions are catalysts for downy mildew disease. Greenhouse Business. January 2002. 23-24.

Daughtrey, M. 2002. Downy Mildews on Flower Crops. GrowerTalks. Feb 2002. 64-66.

Daughtrey, M.L., R. Wick and J. Peterson. 1995. Compendium of Flowering Potted Plant Diseases. The American Phytopathological Society, St. Paul. Minnesota. 90 pp.

Hausbeck, M.K. 2002. Controlling Downy Mildews. Michigan State University Greenhouse Alert Newsletter. May 9, 2002.

Holcomb, G. E. 2000. First Report of Downy Mildew Caused by Peronospora lamii on Salvia splendens and Salvia coccinea. Plant Disease. 84:1154.