Field Guide     Crop Diseases     Downy Mildew

Downy Mildew

CROPS IMPACTED: Grapevines, lettuce, peas, onions, tobacco, cabbage, carrots

Downy Mildew

Family: Peronosporaceae

Downy Mildew Downy Mildew

About Downy Mildew

Life Cycle

Downy mildew is a plant disease that mainly affects host plant’s foliage. It reproduces both asexually through the production of spores, or sexually via its female and male sex organs (the oogonium and the antheridium). Once an infection is present, spore formation can occur as soon as 4 days after initial infection. However, the typical time period for these spores to germinate is 7 to 10 days. There are many different downy mildew species, but they are all capable of spore germination by creating a germ tube that enters the host plant. Some species also germinate through the formation of zoospores. This disease can overwinter with plant debris in the soil. However, some downy mildew species cannot handle the cool weather and as a result, must be reintroduced to the area in order for another infection to break out. The spores may be carried by wind, water, insects, and even contaminated equipment.

Downy Mildew Identification and Habitat


This disease is caused by oomycete organisms, which are similar to fungi and are considered a water mold. This mold will appear in colonies and usually develops first on leaves undersides. In severe cases, downy mildew may be misdiagnosed as powdery mildew or botrytis. However, under a microscope they are easy to distinguish. On the undersides of leaves, downy mildew will appear as a fluffy growth that ranges from white to blue in color. Yellow spots will begin to form on leaves upper sides, and may also appear speckled. Eventually the mold will kill the spots it has infected, which will cause the growth to turn from the white or blue to more of a gray color. Leaves will often curl and may look wilted. A plants blossoms fall to the ground first, typically followed by the leaves and then stems. Branches can also become infected and may appear distorted in shape. Early infection will result in stunted growth if the plant survives.


Downy mildew is generally prevalent in cool and damp weather conditions. This often takes place early in the spring or late in the fall. These cool conditions refer to temperatures ranging from 58 to 72 degrees Fahrenheit. Spore formation is particularly favored by temperatures below 65 degrees Fahrenheit, especially when humidity levels are close to 100 percent near the plant’s foliage.

Downy Mildew Management and Control Methods

Cultural Control

Infected plant debris should be removed from any fields or gardens. This reduces overwintering sites for the spores and helps to lessen the chance of infection in the next year. Plant debris can be burned or buried. Ensure possible downy mildew host plants are spaced out enough to promote air circulation, allowing leaves to dry quickly from rainfall or irrigation. It is important to note that when the weather is cool, the use of overhead irrigation should be avoided. Additionally, when conditions are optimal for downy mildew, scout susceptible plants to catch possible infections early to treat as soon as possible and avoid further damage. Whenever possible, select plant varieties that are resistant to downy mildew. Practice crop rotation with plants that are less susceptible to reduce the chance of soil-borne resting spore infection.

Chemical Control

There are a number of fungicides that have proven to be effective in fighting downy mildew infection. However, no fungicide can completely eradicate the disease; it only helps to suppress the infection. Copper spray can be applied if the infection is caught in its earlier stages. This spray can be applied every 7 to 10 days. Also, fungicides with the following active ingredients have also had effective results: mancozeb, azoxystrobin, cyazofamid, fluopicolide, mafenoxam, and phosphouous acid. Often several applications are needed for visible results. That being said, always be sure to carefully read the label for cautions and proper application before use.

Alternative Downy Mildew Names

  • • Water mold