Nearly every gardener has dealt with garden diseases from time to time. The most common of these are fungal in nature, and fortunately, we can fix most of them. However, it can be disheartening to see the decline these diseases cause in your plants, seemingly overnight. 

If you’re not in the garden every day, fungal diseases can set in and spread quickly. One such disease that can seem to come out of nowhere is powdery mildew. In order to fix this problem, you’ll want to act quickly. 

Our team has experimented with many solutions to fix powdery mildew. Some work better than others, and some are more effective when used in tandem. Let’s take a look at this common fungal disease, how it affects the garden, and how to fix it.

Unlike many fungal diseases, this mildew thrives in warm, dry conditions with high humidity. This is different from other fungal diseases that proliferate in consistently wet environments. 

While it rarely kills plants outright, powdery mildew weakens them by interfering with photosynthesis. This leads to stunted growth, reduced flowering, and lower yields. It spreads quickly, but fortunately, it’s not impossible to fix. 

How Does Powdery Mildew Occur?

Mildew spreads quickly throughout the garden.

Powdery mildew happens as a result of fungal spores. When they land on a susceptible plant, under the right conditions, this is where it all begins. The fungi that cause this issue are airborne. They spread predominantly on the wind, but can also spread by insects and water splashes. 

Most fungal spores require moisture to proliferate. It’s the opposite for this one. Mildew prefers warm, dry weather and high humidity. This combination is common to late summer and early fall, as well as inside greenhouses. 

Once the spores settle on a host, they germinate on the leaf surface and draw nutrients to support their spread. This is what leads to the white, powdery coating they create on the surface of your foliage.

Weather Conditions

Environmental conditions influence the spread of this fungal disease.

The transmission and spread of mildew typically takes place in a specific type of weather. It’s airborne, rather than waterborne like many fungal spores. The ideal conditions for spread include dry weather, high heat, and increased humidity. 

These tend to be the conditions during late summer as temperatures peak, and rainfall is scarce. Warm days paired with cool, humid nights create the perfect environment for the spores to germinate. The temperature matters, with a range of 68 to 86°F (20-30°C) being ideal for this fungus to thrive. 

Likewise, periods of rainy weather tend to knock spores off leaves. In exceptionally hot weather, the growth of the fungus will slow down. It typically slows when the thermometer goes above 90°F (32°C). 

Stress

Stressed plants are more susceptible to problems.

Plant stress is another determinant of how serious your infection gets. Stress makes your plants more vulnerable because of lowered natural defenses. When the plant is weak to begin with, the fungus can take hold and do more damage, faster.

Stress comes from a variety of sources, many of which we can fix if we identify them in time. Drought causes stress, and since powdery mildew likes dry weather, this creates a bad combination. A nutrient imbalance, inconsistent watering, or low light are all factors that can cause stress in your plants. 

Crowding

Lack of airflow promotes spread.

Crowding is a major cultural cause of mildew, and it’s easy to fix. When your plants become crowded, airflow decreases. When the air cannot properly circulate, you end up with small pockets of humid air throughout. In these pockets, spores settle, germinate, and spread. 

Crowding also means the foliage of a plant or many plants touch. This promotes the movement of spores from one leaf to another and from one plant to another. Before you know it, you can end up with an entire section of the garden covered in white powdery dust. 

Weeds and Debris

Garden debris carries powdery mildew around the garden.

Weeds and garden debris can also contribute to the spread of mildew. For one thing, they compete for water and nutrients, which adds to plant stress. They can also serve as a host for the fungi, and then spread to your other plants when the conditions favor the spread. 

Garden debris from previous years can carry the spores for a long time. Many types of mildew can overwinter on plant material. They become dormant in the winter, but then reactivate when the weather warms up.           

Symptoms

White spots on leaves slowly spread.

The primary symptom of mildew is easy to spot. On infected plants, a fine white or gray powdery coating forms. It’s typically most evident on the upper surface of foliage. It typically starts out as small circular spots and gradually covers the entire leaf. 

Left untreated, the diseases can have some undesirable consequences. While it won’t commonly kill outright, it will cause a general decline in health without a powdery mildew fix. Infected leaves may curl and become distorted. 

The covering of spores inhibits the plant’s ability to photosynthesize, which leads to overall decline. Ultimately, the leaves will begin to turn pale and yellow, drying out and dropping prematurely. This weakens the plant overall.

Prevention

Prevention is the best form of defence.

Prevention is the best powdery mildew fix. It’s important to make your garden as inhospitable as possible. The spores are nearly always present, so creating conditions where they can’t proliferate is the best option. 

Space your plants properly to avoid overcrowding. This allows the air to circulate, drying out the leaves, where spores can thrive. Keeping things pruned and thinned out is another important habit to get into. This also increases the airflow between plants. 

Powdery mildew prefers shaded spots, so adequate sunlight is a good preventative measure. Always water at the base of your plants, rather than overhead, to avoid creating more humidity around and between the leaves. 

Avoid using high nitrogen fertilizers during the time of year when powdery mildew is most common. Nitrogen causes new, tender foliage to grow, and these new leaves are more vulnerable to the disease. 

Plant resistant varieties if you’re able. There are many plants that have resistant varieties. Rotate your crops yearly to avoid dormant spores from affecting the same type of plants. Keep weeds under control, and always clean up infected debris that can keep those spores present. 

Finally, in greenhouses, the spores find the ideal circumstances for growth. The still, warm, humid air is a breeding ground for spores. For this reason, it’s important to increase the airflow in your greenhouse. Use a fan and open any vents to help dry out the air.

Treatment 

There are several effective treatments that you can use to fix the powdery mildew issue. The simplest way to start out is by combating the fungus with the natural element that washes it away: water. 

Water

Water can resolve minor issues.

Simply rinsing the affected foliage with water for several days can fix a less serious powdery mildew case. However, this isn’t the most effective treatment, as it simply moves spores into the soil, providing a habitat that allows the fungus to proliferate.

Potassium Bicarbonate

A solution of potassium bicarbonate can reduce issues.

Another valuable fix for powdery mildew is a solution of potassium bicarbonate. Potassium bicarbonate acts as a contact fungicide when applied liberally, killing spores that are present. It also raises the pH of the leaf surface on healthy plant tissue, which makes it inhospitable for other spores to take hold.

Mix a gallon of water with one tbsp of potassium bicarbonate and a teaspoon of castile soap. Mix it well and drench the tops and undersides of your foliage with the solution. Reapply this solution every one to two weeks to combat the return and fix the issue while conditions favor powdery mildew.

Neem Oil

This works for several garden problems, including powdery mildew.

Neem oil seems to be a fix for a host of garden problems, and it works in this instance as well. Neem oil prevents the spores from germinating and infiltrating the leaf tissue. Therefore, it’s a better preventative than it is a treatment.  

Mix your neem oil according to the directions on the bottle. This product comes in different forms and concentrations. Coat the leaves top and bottom with the neem oil solution, repeating about once per week. 

Note that neem oil can be harmful to beneficial insects. Try to avoid spraying on flowers and during the day when pollinators are active. Spray your plants at in the early morning before pollinators are active. This will prevent killing those insects, as well as prevent your leaves from burning. Spraying while the sun is overhead can lead to burned leaves. 

Copper Fungicides

Use these fungicides as a last resort.

Copper sprays are another way to treat and fix powdery mildew. However, these can be toxic to humans and plants. They truly are more of a preventative method, so using them ahead of an infection is ideal. While they can be effective, these typically aren’t the best method for treating this particular disease.