Downy Mildew: Prevention and Control

Milosh Potikj | 11 octobre 2023 | 8 MIN READ

If you are a horticulturist, agricultural professional, or garden enthusiast, chances are you’ve had to deal with downy mildew at least once. This fungal infection affects plants, from vegetables and fruits to ornamental flowers and vines.

While it's easy to spot the obvious signs of downy mildew on leaves - like yellowing or splotchy spots - there is much more beneath the surface that can have devastating consequences for your plant’s health if left unchecked.

To avoid these problems, read on as we provide insight into effective prevention strategies and discuss options for controlling this widespread problem once it has taken hold.

Development Stage Key Contributing Factors

Initial Infection

  • Presence of pathogen (usually via wind-blown
  • Susceptible host plants
  • Wet leaves (rain, dew, or overhead irrigation)
  • Moderate temperatures
Incubation Period
  • Continued humidity or wet conditions
  • Warm temperatures (usually 10-20°C but can vary depending on specific pathogen)
  • Nighttime humidity near 100%
  • Cool to moderate nighttime temperatures
  • Presence of water on leaf surfaces
Progression and Damage
  • Lack of fungicide applications or resistant varieties
  • Continued favorable environmental conditions
  • High pathogen pressure
Overwintering/ Survival
  • In soil as oospores or in plant debris
  • On alternative hosts
  • Inside seeds in some cases

Understanding Downy Mildew

Understanding Downy Mildew

Downy Mildew is a common disease caused by numerous species of fungus-like organisms. The impact of this disease can be detrimental to both commercial growers and hobby gardeners. Recognizing the symptoms and understanding the disease development process is crucial for effective control measures. Let's delve deeper into what causes downy mildew and how it affects your plants.

Disease Development

Disease development in plants affected by downy mildew is a complex process involving interaction between the pathogen, the plant, and environmental conditions. The process begins when fungal spores (Powdery Mildew Spores or Downy Mildew Pathogens) land on the leaf surfaces of susceptible plants, including various fruits, stems, and foliage. Favorable conditions, such as high humidity and cool temperatures, allow these spores to germinate and penetrate the plant tissue, leading to powdery or downy growth on the upper leaf surfaces or leaf veins.

Once inside, the fungi spread their mycelium throughout the infected leaves and stems, extracting nutrients and disrupting the plant's normal growth patterns. Infected leaves may exhibit various symptoms, including powdery or downy growth, discoloration, and distortion. If left unchecked, the disease can spread to fruit clusters, reducing the marketable yield and causing significant yield loss. In severe cases, the entire plant may wilt and die.

The Downy Mildew Pathogens

The Downy Mildew pathogens are various species of oomycetes, fungus-like organisms that thrive in cool, moist conditions. Two of the most common species include Erysiphe cichoracearum and Sphaerotheca fuliginea, responsible for many mildew outbreaks on crops across the United States. These pathogens produce spores easily spread by wind, rain, and even insects, making disease control particularly challenging.

Infected plant debris also plays a significant role in pathogen transmission, as it can harbor both Powdery Mildew Fungi and Downy Mildew Pathogens, which can overwinter and re-infect plants in the following growing season. Therefore, good cultural practices, such as regularly removing plant debris and using resistant cultivars or varieties, are vital in controlling these pathogens and preventing disease development.

Read More: Cercospora Leaf Spot

Identifying Susceptible Plants

Susceptible plants to downy mildew include fruits, ornamental plants, and squash varieties such as butternut squash and yellow crookneck squash. The disease particularly impacts susceptible cultivars of these plants. Identifying these susceptible plants and implementing preventive measures early is the key to disease control. These include using resistant cultivars or varieties and maintaining good cultural practices to prevent disease development.

Control Measures

Control Measures

With the understanding of Downy and Powdery Mildew diseases, their development, and susceptible plants, we now dive into the control measures. From cultural practices to the application of fungicides, we'll explore various methods available to combat these common adversaries of growers and plant enthusiasts alike. Let's uncover the best strategies to keep your plants healthy and productive.

Cultural Practices for Disease Control

Cultural practices involve the day-to-day activities carried out by growers in the garden or on the farm. These practices significantly influence the prevention and control of downy and powdery mildew diseases. The first step involves proper site selection. A well-drained site with good air circulation reduces humidity levels, limiting conditions that favor mildew growth. Regularly removing plant debris is another crucial practice, as it eliminates potential overwintering sites for powdery mildew fungi and downy mildew pathogens.

Crop rotation is also recommended, especially if you previously experienced mildew issues in your garden. Swapping out susceptible plants with resistant varieties or cultivars can break the disease cycle. Moreover, overhead watering should be avoided as it creates a conducive environment for germinating and spreading fungal spores on leaf surfaces. Instead, consider drip irrigation or watering at the base of the plants.

Lastly, monitor your plants regularly to detect the onset of disease early. Look for signs such as powdery growth on the upper leaf surface or infected leaves. Early detection allows for the timely application of control measures, reducing the likelihood of disease severity and widespread losses. Adopting these cultural practices can go a long way in preventing and controlling the development of mildew diseases in your garden or farm.

Learn More: Brown Spot Plant Mysteries

Fungicide Treatments

Fungicide treatments are essential in preventive and curative strategies against downy and powdery mildew diseases.

  • Preventative Treatments

Preventative treatments involve the application of fungicides before the onset of disease. These are typically based on weather forecasts and disease models that predict high-risk periods for infection. The fungicides used in preventive treatments are often protectant fungicides that create a barrier on the leaf surface, preventing fungal spores from germinating and infecting the plant. Examples of these include sulfur products and some biological fungicides. Regular applications at 7-10 day intervals, or even shorter intervals during favorable disease conditions, are crucial for maintaining the effectiveness of preventative treatments.

  • Curative Treatments

In contrast, curative treatments are applied post-infection to control disease growth and stop further infection. Systemic fungicides, or penetrant fungicides, are typically used for this purpose. The plant absorbs these fungicides and can move within its tissues, reaching the fungi within infected leaves and stems. However, to be effective, curative treatments must be applied soon after infection occurs, ideally before visible symptoms appear.

The choice between preventive and curative treatments depends on several factors, including the disease history in the area, the susceptibility of the plants being grown, and the weather conditions. Regardless of the approach taken, always follow label directions for all fungicides, including rates, timing of applications, and pre-harvest intervals.

  • Biological Fungicides

Biological fungicides use naturally occurring organisms or their by-products to control plant diseases. They can be very effective against powdery mildew diseases and offer an alternative to chemical fungicides. Some examples include Serenade, which contains a strain of Bacillus subtilis, and Sonata, which contains a strain of Bacillus pumilus. These products can be used in both preventative and curative treatment strategies.

  • Jojoba Oil and Other Plant-Based Oils

Plant-based oils, such as jojoba oil and neem oil, offer a more natural solution for controlling powdery mildew diseases. These products disrupt the fungi's cell membranes, causing them to dry out and die. They can be used as a standalone treatment or mixed in a tank with other fungicides to enhance their effectiveness. Applying plant-based oil sprays in the morning hours, when temperatures are lower, is important to prevent leaf burn.

Spray Applications

Spraying is a popular method for applying fungicides to plants, helping to manage diseases such as powdery mildew effectively. The aim is to ensure even coverage of the plant's leaf surfaces, stems, and other susceptible parts. For optimal results, spray applications should be done in the morning when temperatures are lower, and wind speeds are minimal. This helps maximize the product's reach and adhesion and prevents rapid evaporation or drift that can occur in hotter midday conditions.

Wetting the plants thoroughly but not to the point of runoff is essential. Remember, the goal is to cover the plant with a fine layer of fungicide to protect against fungal spores or treat already infected areas. Most fungicides, whether sulfur products, biological fungicides, or systemic fungicides, recommend spraying at intervals of 7-10 days. However, the frequency may need to be increased during favorable disease conditions.

Tank-mixing fungicides can also be beneficial for disease control, improving the spectrum of activity and potentially delaying the development of resistance. For instance, plant-based oils like jojoba oil can be mixed with other fungicides to enhance their effectiveness. Always remember to follow label instructions when mixing and applying fungicides. Improper application can lead to phytotoxicity, ineffective disease control, and residues on the harvested product.

Spray applications are an integral part of disease management in commercial and home settings, helping maintain healthy, productive plants. With careful planning and execution, spraying can be a highly effective tool in the fight against powdery and downy mildew diseases.

More: Top 8 Mistakes Made by Hydroponic Growers

Impact of Mildew on Plants

Impact of Mildew on Plants

Hydroponic systems, while offering many advantages, such as efficient use of water and essential nutrients, can also present unique challenges when managing diseases like powdery mildew. Plants in these systems are often grown nearby, which can facilitate the rapid spread of fungal spores. Additionally, the high humidity levels that are beneficial for hydroponic growth can also create an ideal environment for developing and proliferating mildew diseases.

Varieties of plants like squash, melons, and cucumbers that are commonly grown in hydroponic systems are particularly susceptible to these mildew diseases. Infected plants may exhibit powdery white spots on the leaves, stems, and fruit, drastically affecting plant health and yield. If left unchecked, the disease can cause significant leaf damage, reducing the photosynthetic ability of the plant and leading to subpar fruit production or even plant death.

Therefore, growers employing hydroponic systems must be proactive in their disease management strategies. This includes using resistant cultivars, carefully monitoring for early signs of disease, and regularly applying suitable fungicides. It is also recommended to maintain good air circulation and avoid overwatering, as these can help minimize the conditions that favor disease development. With the right knowledge and garden tools, it's possible to manage mildew diseases in hydroponic systems successfully.

Frequently Asked Questions

How do you prevent downy mildew?

Preventing downy mildew involves a multi-pronged approach. It's crucial to choose resistant varieties of plants, maintain proper air circulation, avoid overhead watering, and apply appropriate fungicides. Regular scouting and early detection are also key in stopping the disease before it becomes established.

Can downy mildew be prevented?

Downy mildew can be prevented with the right approach and measures. This involves implementing good cultural practices, selecting disease-resistant cultivars, and using effective fungicides as part of an integrated disease management program.

What is the best control of downy mildew?

The best control of downy mildew involves using preventative and curative treatments, proper cultural practices like adequate spacing for air circulation, and deploying resistant varieties when available. Consistent monitoring of the plants to identify early signs of the disease and timely application of appropriate fungicides also play a significant role in controlling downy mildew.

How do you control downy mildew naturally?

One natural way to control downy mildew is by applying homemade sprays, such as a mixture of milk and water, which has proven effective in some instances. Another approach involves using biological fungicides containing beneficial microorganisms that help control the disease by out-competing the downy mildew pathogen for resources.


In conclusion, effective disease management, particularly for common threats like powdery and downy mildew, is essential to successful gardening, whether you're a commercial grower or a backyard enthusiast. It involves the strategic application of fungicides, implementing good cultural practices, and using disease-resistant varieties.

With knowledge, vigilance, and the right tools, you can protect your plants from these pervasive fungal diseases and ensure healthy, fruitful harvests. Prevention is always better than cure, so start your disease management strategies early to keep your garden thriving.


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