Powdery Mildew Prevention and Treatment

As a gardener, powdery mildew has likely been a problem for you before. Many species of fungi can cause powdery mildew, but a fungus species called Golovinomyces cichoracearum is the one that commonly infects the dahlia plants. If powdery mildew is present on your plants, let’s dig into why it’s there, how to treat it, and ways to prevent it from popping up in your garden again.

What is Powdery Mildew?

Powdery mildew is a common fungal disease found on many different types of plants. It is a dry, white, mealy, fungal growth commonly found on the upper surface of leaves. It may also infect buds, shoots, and even flowers and fruits. 

Powdery mildew spores cover the plant leaves with silvery white powdery spores made up of threads of mycelia that feed on living plant material. The mycelia grow inside the plant cells, producing spores on the surface. The good news is that this disease won’t kill your plants. Sometimes leaves may yellow and fall off or leaves and shoots may distort, but your plants can recover from powdery mildew.

The main cause of powdery mildew on plants

The ideal conditions for powdery mildew to grow are temperatures between 60-80°F, but it can be active in temperatures from 55-90°F. This disease is more prevalent in late summer and fall because of the fluctuation between day and night temperatures that can occur.

Primarily, it’s a cosmetic issue. But every so often – especially when there’s high relative humidity, moderate temperatures between 68-86°F, and cloudy or foggy days – it can become a serious problem. Spore formation is favored by high humidity. Water in the air (humidity) is necessary for the spores to germinate, and the dispersal of spores is accelerated when the humidity is low. So these fungi cause the most severe infections when the humidity is high at night and low during the day. Think foggy overnight conditions and clear daytime skies. This is when powdery mildew flourishes. 

Many different types of powdery mildew fungi can cause the disease and spores can be spread to new plants by wind. While the spores are often spread by the wind, aphids and other pests can also spread the infection as they feed on plants. Additionally, infection is common in crowded plantings with poor air circulation or unshaded and damp areas. 

The good news? You can prevent powdery mildew by growing healthy plants. Healthy plants are more resilient, able to ward off infection, and more likely to recover on their own. Stressed plants growing in unsuitable locations or being cared for improperly are more prone to this fungal disease. 

7 Ways to Prevent Powdery Mildew Without Pesticides

With powdery mildew, prevention is the main task. Integrated pest management (IPM) helps you with that by teaching you how to manage pests – from unwanted insects to fungal diseases – without pesticides. To keep your garden free of this fungus, focus on prevention using eco-friendly management practices. 

1. Use cultural controls

There are many techniques to use to cultivate a resilient garden and create an environment that discourages pests:

  • Place plants where their needs are met (i.e., plants that require 6-8+ hours of direct sunlight are prone to powdery mildew if they get less)

  • Don’t overcrowd plants and provide good air circulation

  • Avoid synthetic fertilizers that stimulate new growth that’s more prone to the fungus

  • Use compost and organic fertilizers to prevent excessive tender, overgrown foliage that shades the leaves and provides the right conditions for the fungus 

  • Prune out small infestations, cut off infected leaves

  • Wash off the powdery mildew with a blast of water mid-morning to kill the spores and allow leaves to dry quickly to avoid other fungal infections.

  • Clean up dead plant material and fallen leaves so spores don’t spread and overwinter in plant tissue

  • Choose plant varieties that are resistant to powdery mildew

  • Remove the plant, if needed

2. Increase soil health 

To boost the health of your soil and keep diseases at bay, amend it with compost when you plant. The benefits include improved soil structure, water retention, and microbiology in the soil. It also helps to balance pH levels, recycle nutrients, and reduce the need for synthetic fertilizers or pesticides. Plus, healthy soil fights pathogens and bad bacteria in the soil to prevent powdery mildew growth.

You can also add worm castings during the planting process to provide dynamic root growth, neutralize high pH, and introduce essential nutrients and minerals to help your garden thrive. This superfood is highly effective at deterring diseases that lead to pest infestations and plant death. 

3. Feed with organic fertilizers 

Another way to prevent powdery mildew is to use organic fertilizers to feed your plants. This feeds your soil microbes, which increases soil health and prevents growth spurts that can attract pests and diseases.

A bonus to opting for organic over synthetic is that it doesn’t run off into your local waterways or pollute the environment. Plus, because it’s not high in salts, it doesn’t build up in the soil and hinder plant health and development.

4. Sanitize pots, planters, and tools 

The best advice for preventing most bacteria and fungi from growing also applies here: Keep things clean. It’s important to sterilize your pots and containers before planting or repotting, and to sterilize your shears when pruning to prevent the spread of diseases.

5. Increase biodiversity

Beneficial bugs will eat or parasitize pests and pollinate flowers. In fact, there’s a specific species of ladybug (Psyllobora vigintimaculata) that feeds on powdery mildew. Over 90% of insects in the garden are beneficial, so don’t be afraid to attract garden allies! 

6. My secret weapon

Want to keep your plants strong and resilient? Apply a soil drench of compost tea and liquid kelp. You can also apply a foliar application to boost plant cell health, especially when you notice your plants are stressed.

Tips for Treating Powdery Mildew

Maybe you missed the early signs of powdery mildew, or you tried everything to prevent it, and it still showed up. Don’t stress – plants can recover from the disease with proper treatment.

While temperatures above 90°F kill some powdery mildew fungi and their spores, the best way to get rid of powdery mildew is to hose it off the leaves. The presence of free water reduces spore germination, so syringing the infected leaves is also an eco-friendly way to treat powdery mildew. 

Treating powdery mildew doesn’t require you to reach for fungicides. This should always be done as a last resort. If you do decide to use one, choose an organic option that’s proven safe around pets, people, and the environment. 

Should you use fungicides?

Several eco-friendly products can help you manage powdery mildew. Most of these products are best used to prevent powdery mildew, so apply them to plants susceptible to the disease before you see powdery mildew (or in the very beginning stages). 

If the fungus is already there, these products can be useful:

  • Bio-pesticides (Bacillus amyloliquefaciens) will kill the fungus

  • Horticultural oil, neem oil, sesame oil, and Organocide will suffocate the spores of the fungus

Thoroughly cover all plant parts, including under the leaves. Additional applications may be needed as the plant grows. Always apply according to the label and wear PPE to protect your skin, eyes, and lungs.

Ways to Grow Together

The best way to manage powdery mildew is to prevent it. To address the root cause of the disease and manage it sustainably, grow a garden full of healthy, resilient plants. Because while pesticides and fungicides kill the fungus, they don’t solve the problem. 

“When we try to pick out anything by itself, we find it hitched to everything else in the universe.” –John Muir

Suzanne Bontempo