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Plasmopara viticola, also known as downy mildew, is a problem for many different types of plants.
Many gardeners think of downy mildew as a fungal infection plant disease, but it is a highly specialized parasite.
Downy mildew and powdery mildew are not the same things; even though, they have often been listed taxonomically alongside true fungi.
Downy mildew is related to Phytophthora diseases and also to Pythium members of the oomycota family.
Water mold organisms from the Plasmopara or Peronospora genus cause the infection.
This is a small distinction because all of these organisms are similar morphologically, and the chemicals used to control one can effectively be used to control another.
What Are The Damage And Symptoms Of Downy Mildew?
Infected plants of this fungus-like parasite display leaf spots of white or yellow spots on the upper leaf surface of mature leaves.
When you turn the leaf over and look at the underside of the leaf, you will find grayish white, cottony spores up to the leaf veins.
It is this downy mass that gives this pest its common name, downy mildew.
You’ll most often see this phenomenon during damp weather.
For example, you might see the plant disease spores after a rainstorm or during times when dew accumulation is especially heavy.
When the weather dries up and the sun returns, the masses of spores may disappear, but the infestation is still present on the leaf surface.
As time passes and the downy mildew matures, the condition of the leaves worsens.
Eventually, the leaves will turn brown and crisp and fall off, no matter how much water you give the plant.
Downy mildew does the most damage in the early springtime and late in the autumn when the weather is moist and cool.
Spore production is higher when the temperatures are below 65° degrees Fahrenheit and relative humidity is close to 100%.
The fungal spores of the downy mildew pathogen can overwinter in the soil and on plant debris such as leaf litter.
These spores can also be carried in on the wind, by the rain, by insects or on contaminated gardening tools.
What Can You Do About Downy Mildew COntrol and Pest Management?
Prevention is the best cure for downy mildew. Strive to avoid creating the environmental conditions during the growing season that allow the disease to thrive.
Here are 7 tips to follow to help prevent downy mildew in your garden:
- Keep your yard and garden well weeded so that there’s plenty of room for good air circulation.
- Prune your plants regularly to encourage better air circulation and to remove any foliage that may be affected by downy mildew.
- Stake up plants that tend to fall over. Preventing leaf contact with the soil can help prevent downy mildew.
- Avoid overhead irrigation. Use a soaker hose to prevent having spores splashed up from the soil.
- Water early in the morning so that your plants will have plenty of time to dry during the daylight hours.
- Do a thorough cleanup in the late autumn, and keep debris off the ground throughout the winter. Keeping the ground clear prevents spores from overwintering.
- If you’ve experienced a problem with downy mildew in your yard and garden, look for disease-resistant varieties of plants.
Use Appropriate Fungicidal Products Promptly
Downy mildew reproduces very rapidly. Once infection sets in, new spores can form within 4 days. Generally speaking, though, disease development takes a week to 10 days.
To head downy mildew off at the pass, keep a close eye on your plants during the cool, damp, high humidity days of early spring and late autumn.
If you can detect the infection early, try removing affected foliage and treating the rest of the plant with appropriate fungicides.
For example, use copper fungicide spray to keep cucurbit downy mildew spores under control.
Start treating plants a couple of weeks before cool, damp weather is expected.
If your plants are already infected with downy mildew, treat with copper spray as soon as you can and repeat the treatment about once a week until all symptoms disappear.
When spraying make sure to spray the undersides of leaves and rotate the fungicide treatments to reduce the possibility of fungicide resistance to the disease.
Use systemic treatments such as Organocide Plant Doctor to help boost your plants’ resistance to fungal and infections.
All plants are at some risk of downy mildew infestation. Some that are especially susceptible to downy mildew infestation include:
- Impatiens (New Guinea Impatiens are resistant varieties)
- Pansy flowers
- Roses (Knockout Rose care tips)
For specific guidance on treating your plants in your setting, contact your county cooperative extension agent for recommendations and mode of action on the right fungicides to use.