Powdery mildew is a common problem for gardeners in temperate zones. An unsightly issue, it can affect pretty much every plant in the garden. Typically causing white or gray spots to appear on foliage, if left untreated these grow and develop. Eventually the spots develop, covering the entire area with blight. Rarely fatal, the disease can harm both growth and fruit production.
If you want to learn more about this common garden problem as well as how to create an effective powdery mildew treatment, this guide will explain everything that you need to know.
An unsightly if rarely fatal issue, a powdery mildew treatment is easy to apply.
What is Powdery Mildew?
Before we discuss effective powdery mildew treatment options, we will first take a few moments to explore exactly what this unsightly disease is.
Powdery mildew is the name given to diseases caused by Erysiphales fungi. Its name is inspired by the white-gray talcum-like powder that appears on the foliage of affected plants. This powdery substance is a combination of spores and fungal threads known as mycelia.
This disease thrives by harvesting nutrients from a host plant. It is unusual because in order to stay alive, powdery mildew needs the host plant to also remain alive. Consequently, while unsightly, this disease rarely kills plants.
Cases tend to be more severe in warm, dry weather.
Like other fungi these pathogens can reproduce both sexually and asexually. These two processes look different. Spores from the asexual stage are first to appear, these spread the disease.
Later, the sexual stage produces fruiting bodies, known as chasmothecia. These are important for overwintering, enabling disease to live on for another year.
Most strains of this disease produce fungal threads that spread on the surface of leaves. These fungi, known as haustoria, send out specialised structures that inhabit the plant cells, harvesting their nutrients.
Initially appearing as small, white marks, the disease soon spreads to cover wide sections of the plant.
Without treatment, powdery mildew can affect a range of plants. However, just because one type of plant has the disease it doesn’t mean that others, even those growing close by will develop the disease. This is because different strains of the disease affect different plants. Luckily, whatever strain your plant develops, the powdery mildew treatment method is the same.
Ideal Powdery Mildew Conditions
Humidity is necessary for powdery mildew spores to germinate. This disease is at its worst when humidity levels are high at night and low in the day. Additionally, spore dispersal is accelerated in low humidity.
While water in the air is bad, free water, such as rain or water from a hose, doesn’t cause the disease to form. In fact a blast from a hose actually washes the spores from the foliage, killing them in the process.
Moderate temperatures, typically between 70 and 80 ℉, can also encourage infection. This means the disease is at its most common in the spring or fall.
Spores can be spread by the wind or by insects, such as wooly aphids, visiting numerous plants. Overcrowding or planting in shady and damp areas also encourages the disease.
Typically infections start with the emergence of white or gray spots on the top of leaves or new growth. These can look like a dusting of flour or talcum powder.
The disease typically affects the lower leaves first. As the spots grow, the mildew can spread through the entire plant.
Flowers, young fruit, stems and buds can all be infected. In warmer climates the fungi can also overwinter in buds.
Eventually, the fungi produce fruiting structures, roughly the size of a pinhead. Initially white, as they develop the fruiting structures turn a shade of yellow-brown and then black. Affected leaves can become twisted or distorted before wilting and dying away. This, in turn, can cause fruit to become sunburnt.
As the disease develops foliage browns and curls.
This disease can affect a number of different plants. As I have already noted, powdery mildew is a catch all term. Each strain of the disease affects a certain type of plant. So, just because your sedums develop a problem it doesn’t necessarily mean that your parsnips or dahlias will.
Flowers that are commonly affected by this disease include:
- Bee Balm,
If planted too closely together, the dense foliage of some flowers can develop unsightly white blotches.
The disease can also affect many types of fruit and vegetables, including:
Herbs, including parsley, cilantro and rosemary can also develop this disease as can some succulents. Sedums, should they develop the issue, develop brown, scabby spots. Kalanchoe develops corky brown spots.
Indoor plants such as poinsettias and African violets can also develop this disease.
Conversely, some plants such as annual phlox and Belladonna delphiniums have a natural resistance. Hybrids also tend to have more resistance than non-hybrid or heirloom plants. Increasingly you can now find varieties that are resistant to powdery mildew. Resistance is usually marked on the seed packet or plant label.
How to Prevent Powdery Mildew
Infections are usually mild. Healthy plants can recover on their own as the weather changes. Many gardeners don’t bother to apply a powdery mildew treatment unless the problem shows signs of becoming particularly severe.
There are also a few things that you can do to reduce the chances of the disease developing or to help your plants withstand an infection.
Caring correctly for plants is one of the easiest ways to prevent the disease. Before planting, use a soil test kit to measure your soil’s pH level. A soil test kit can also be used to test your soil for any deficiencies. Make any necessary amendments before planting.
You should always try to plant specimens in a suitable position. Meeting the plant’s light and sun requirements can help to prevent not only this disease but many other common problems.
When planting, be sure to space your plants out correctly. This helps to improve airflow around the plants. Maintaining good airflow is one of the easiest ways to prevent disease. Damp or crowded foliage is far more likely to succumb to disease. As your plants develop they may start to overcrowd each other. Regularly prune and stake the plants to maintain good airflow.
When planting, correctly space your specimens out so that air can freely circulate.
After planting establish a good water and fertilization routine. When watering, use a watering can to keep the foliage as dry as possible. Alternatively, water early in the morning. This gives the foliage time to dry out.
Avoid fertilizing plants with fertilizers rich in nitrogen from late summer onwards. Too much nitrogen causes new foliage to emerge. New foliage is more susceptible to powdery mildew.
A soil moisture meter is a great, easy to use little gadget if, like me, you struggle to know when to water your plants. A 3 in 1 meter, such as the Alkey Soil Moisture Meter also measures how much light your plants receive and the pH level of your soil. This is incredibly useful information if you are trying to grow healthy, productive plants.
Should any problems or damage develop, prune away and destroy infected areas. Do not place infected foliage on the compost pile. This can cause the disease to spread.
Regularly weed and keep the area around your plants debris free. There are a number of weeding tools you can use to help keep your garden neat and weed free.
Finally, clean your garden tools before and after using them.
Easy to Apply Powdery Mildew Treatment Options
Healthy plants can withstand infections. Mild cases often go away on their own or as the weather changes. However, you can also treat and cure the problem yourself. The following powdery mildew treatment suggestions and methods are some of the easiest ways to treat the disease.
A Milk Based Powdery Mildew Treatment
A popular organic solution, spraying milk onto affected foliage is an easy way to control the disease, as long as you don’t mind the smell.
This powdery mildew treatment is particularly effective if applied during the early stages of the disease. Appropriate to use and effective on a range of plants, a milk based powdery mildew treatment is particularly effective on:
To make this treatment, dilute milk with water, creating a 1:10 solution. Spray on the foliage at the first sign of an infection. If infections are severe, apply a mixture that is even parts milk and water.
Any type of milk, including powdered or liquid milk can be used. Many people believe that raw or organic milk is best, assuming that the fats in whole or 2% milk impede treatment. However any type of milk can make an effective powdery mildew treatment.
After diluting the milk, pour the solution into a spray bottle. Cover all infected areas including the underside of leaves and stems. Continue to apply the solution twice a week until results are visible. If it rains the treatment may be washed away. Reapply the solution once the rain stops.
A milk solution can also be applied to plants to prevent the disease from developing. Apply a preventative powdery mildew treatment once a week.
Sulfur Based Fungicides
Sulfur based fungicides are common garden treatments for a range of issues. They also provide an effective and cheap preventative powdery mildew treatment. Spray once a week onto plants that are prone to developing the problem. Again, remember to reapply the solution after rainfall.
Avoid applying sulfur based fungicides to plants already treated with a horticultural oil. Allow for 2 weeks to pass before applying a new treatment. You should also avoid applying this solution if temperatures are forecast to pass 80 ℉.
An effective preventative, start to apply sulfur based fungicides once the signs of infection are visible.
As well as sprays, you can also dust or brush sulfur based fungicide products onto plant foliage. Be careful if you are applying the solution in this way, the powder can irritate sensitive skin, eyes and the throat. Wear appropriate protection such as gloves and a long sleeved top or goggles. Always read the label before applying any product.
Sulfur based fungicides aren’t suitable for some plants, such as grapes, apples, melons and squash.
Another common fungicide that can be an effective powdery mildew treatment or preventative, copper based fungicides can also be used to control and treat some bacterial infections. While some gardeners prefer the sulfur based fungicide, believing it to be more effective than copper fungicides, unlike sulfur fungicides copper based powdery mildew treatment solutions are organic.
Low concentration copper octanoate sprays such as Bonide Liquid Copper Fungicide are typically copper based treatments bound to soap. These are ready to use and can be easily sprayed onto the plants as a preventative measure.
For the copper fungicide to be effective it should be applied as soon as you notice any symptoms. Continue to treat the affected foliage every 7 to 10 days. Remember when treating plants to spray both the top and underside of leaves and to reapply if it rains.
Copper fungicides can be slightly toxic. Be careful when using. Follow the instructions and wear the appropriate, protective clothing. Copper fungicides are also toxic to fish. Avoid using the products near ponds or streams.
Baking soda is present in almost every kitchen cupboard. But it is also useful in the garden. Combine baking soda with water and a liquid non-detergent soap, which helps the mixture cling to the leaves, to make an effective powdery mildew treatment or preventative. Spray onto plants that are prone to developing the disease such as lilac
once a week from early in the growing season. Remember to reapply the solution if it rains. .
To make the preventative solution, combine 1 tbsp of baking soda, half a teaspoon of liquid, non-detergent soap and a gallon of water. Mix thoroughly and pour into a sprayer. Evenly coat all areas of the plant. Any unused solution should be discarded. It rarely keeps well and quickly loses its effectiveness.
Water your plants a few days before applying the solution to prevent foliage burning. Never apply the solution when plants are in full sun.
Before applying, test the solution by spraying it onto a small test area. The following day, check the sprayed area. If it is fine, and hasn’t developed any burning or scorching, it is okay to apply the solution to the rest of the foliage.
Potassium Bicarbonate Solutions
Thought by some to be a more effective powdery mildew treatment than baking soda, mix 1 tbsp or potassium bicarbonate and half a teaspoon of liquid soap into 1 gallon of water. Decant into a pump sprayer and apply to the affected plants. Remember to test the solution on a small area before applying to the entire plant.
An unsightly disease, an effective powdery mildew treatment is easy to create and apply.
Finally, a blast of water from a garden hose is a quick and effective way to wash away any spores. If you are washing away the spores, remember to do it early in the morning on a dry day. This gives the foliage plenty of time to dry before the cooler evening temperatures arrive.
Neem oil is also a popular, natural insecticide. You can mix neem oil into any of the homemade powdery mildew treatment methods outlined above to further boost their effectiveness.
While this disease may be an unsightly disease but with a little time and planning on your part it is easy to prevent. Should one of your plants develop the disease, there are a number of easy to use, safe powdery mildew treatment options that you can apply.
Jen is a master gardener, interior designer and home improvement expert. She has completed many home improvement, decor and remodeling projects with her family over the past 10 years on their 4,500 sf Victorian house. She is also a passionate farmer who keeps goats, chickens, turkeys cows and pigs on her farm, and an instructor for her community’s Organic and Sustainable Farming project.