You’ve seemingly done everything right in your garden. You’ve chosen good soil, fertilized and provided plenty of water – and yet you notice yellow splotches or gray speckles on some plants. It could be downy mildew. We’ll tell you how to spot downy mildew and how to get rid of it.
What is downy mildew?
Downy mildew is a microscopic fungus that attacks flowers, fruit, stems, and the leaves of plants. It generally emerges in rainy seasons when leaves stay wet, or from too much irrigation.
There are thousands of different kinds of water molds that are referred to as downy mildew, but almost all have this in common: Left unchecked, they destroy plants.
Some of the different kinds are:
Cucurbit downy mildew: Infects melons, gourds, cucumbers, squash, and pumpkin
Brassicas downy mildew: Infects crops like cabbage, broccoli, Brussels sprouts, kale, kohlrabi, turnip, radish, cauliflower, Chinese cabbage, and mustard
Grapes downy mildew: Common in viney plants; attacks the flowers, fruit, leaves, and stems, killing the plant.
Impatiens downy mildew: Affects roses, blackberry, and boysenberry.
Causes of downy mildew
The key cause of downy mildew is moisture. Major outbreaks occur when leaves stay wet for extended periods of time, particularly in humid conditions during spring and fall when the sun may not be intense enough to dry out plants.
Downy mildew is spread by spores that latch onto the undersides of leaves. These spores often live in the soil but then are splashed onto leaves by rain or blown onto plants by wind.
Some of the factors that contribute to the development of downy mildew are:
- Humid conditions
- Shade
- Prolonged periods of rain
- Poorly drained soils
- Overcrowding of plants
Symptoms of downy mildew
Downy mildew disease can easily be confused with other diseases such as powdery mildew and gray mold. Close inspection is key because, left untreated, the plant may die.
Look for these signs:
- The underside of the leaf develops white, purple or bluish-gray speckling.
- Next, the top of the leaf develops yellow spots.
- Eventually, the yellow spots die off and the mildew turns gray and powdery.
- Branches of the plant appear distorted, wilt, and die.
Getting rid of downy mildew
It’s important to know how severe your case of downy mildew is. A mild case may not require as much intervention and will probably go away on its own when the warmer weather sets in because downy mildew hates heat. You will certainly need to take action if the case is more severe.
A word of caution: While some fungicides may help the fungus, some strains of downy mildew have proven resistant to fungicides.
Having said that, here are some things you can do to get rid of downy mildew:
- Remove severely infected plants from the garden to keep them from passing the fungus to healthier plants.
- Identify plants that are salvageable, and carefully prune them to improve air circulation.
- Thin out the garden to improve circulation.
- Apply a copper or sulfur fungicide to the plants, making sure to completely wet both the tops and bottoms of leaves because spores cling to both sides.
- Spray a broad spectrum fungicide and drench the base of the plants that are severely affected by downy mildew.
- Apply neem oil to the plants to stop further damage.
- Eliminate weeds to increase airflow.
Preventing downy mildew
Preventing downy mildew is much easier than eradicating it once it gets a foothold. Here are some steps you can take to prevent downy mildew:
- Maintain some space between plants to help them breathe.
- Choose disease-resistant cultivars of plants. Many plants have been bred to resist downy mildew.
- Water at the base of the plant, not on top. Minimizing moisture on the leaf surfaces is key.
- Mulch the garden. Mulch, as this Lawn Love article explains, not only keeps moisture in the ground, but it keeps soil infected with downy mildew microbes from splattering on the leaves in the rain.
- Make sure soil drains well.
- Don’t plant in heavily shaded areas where sunlight can’t evaporate moisture.
- Avoid watering your garden in the late evenings.
- Apply fungicides during the growing season when the humidity is high to prevent the disease from getting a foothold.
FAQ
Yes, downy mildew more commonly harms plants but it can affect lawns as well. Inspect your grass blades for signs of the disease.
Downy mildew spores live in the soil but can be splattered on leaves by heavy rain or carried to plants by wind.
Conclusion
The best offense against downy mildew is a good defense. Take steps to ensure your plants get adequate air, that they live in well-drained soil and that the leaves of the plants don’t get any more water than is necessary.
Need help with your garden? Contact a Lawn Love lawn care professional.
Main photo credit: NY State IPM Program | Flickr
