Powdery mildew is one of the most troublesome plant fungal diseases. It is rarely lethal, but it spreads easily. It can affect most plants from perennials to trees and shrubs. While some plants are more susceptible than others to this fungal disease, no plant is immune to it.
Despite the unpleasant way in which it affects the plants, this mildew is not that dangerous and it can easily be dealt with. Nonetheless, to properly rid your plants of it, you must correctly identify it from an early stage and apply the proper remedies.
Like always, we urge you to use chemical-free remedies, so that you don’t risk damaging your plants. Chemical fungicides can also harm beneficial pollinators that might be attracted to your plants.
What is powdery mildew?
Powdery mildew is a common fungus. There are different types of fungi that can cause this disease. It is most common in dry and warm climates. This is unlikely for most fungi that prefer wet and moist environments. Powdery mildew does need a good amount of humidity to thrive but it usually gets it from condensation or dew. As such, it is most comfortable in areas with warm days and cool nights. This is why it usually rears its ugly head in late spring and early summer.
There are also other external factors that can encourage its development. Such is the case of poor air circulation or insufficient natural sunlight. In the long term, powdery mildew robs plants of water and nutrients.
This type of mildew develops directly from live spores which spread on the plants. They can spread through direct contact, but can also be spread by wind, insects, or splashing water. Under certain favorable conditions, they can also survive during the wintertime in the soil, in mulch, or in compost.
How to identify powdery mildew
The tricky thing with powdery mildew is that it can be hard to identify in its early stages. It presents itself as a grey or white powder that sets on the leaves and the stems of plants. At first sight, it can be confused with dust. You can sweep it away with your fingers but it will quickly return.
Powdery mildew usually appears on the leaves, but it actually starts spreading from underneath the leaves. It can also appear on the stems, on the flowers, or even on fruits. New plant growth is more susceptible to mildew damage, but if it is left untreated, it can end up covering the entire plant.
While they can initially be confused with dust, upon a closer inspection, the fungi spores are round and fuzzy. Sometimes the spores can be slightly raised. Powdery mildew can be harder to spot on plants with natural variegation. For example, some types of melons and zucchinis have a powdery variegation pattern. The difference lies in the texture.
Mildew is fuzzy and it can be wiped off, whereas natural variegation stays as-if. If you spot the powder underneath the leaves, it is a clear sign of infestation. A severely infected plant will look as it has been dusted in flour. A heavy layer of mildew will also impede photosynthesis, which will lead to a slow but certain death of an infested plant.
Due to the fact that it robs the plants of nutrients, powdery mildew will also exhibit other identifiable signals. Aside from the obvious dusty appearance, it can cause yellow, withered leaves or distorted leaves. It causes overall stress on the plant which can lead to fewer blooms, slower growth, or even a complete lack of growth. Stressed plants are also more susceptible to other diseases as well.
What plants are most and least likely to be affected by powdery mildew?
As we already mentioned, powdery mildew does not discriminate. It attacks edibles as well as ornamental plants or shrubs. Some plants are more vulnerable to this fungal disease. Powdery mildew usually attacks some of our favourite summer and fall vegetables like zucchinis, butternut squash, pumpkins, melons, cucumbers, tomatoes, beans, eggplants, kale, lettuce, peas, peppers, cabbage, cauliflower, broccoli, and sunflowers.
As far as ornamental flowers go roses, begonias, hydrangeas, peonies, lilacs, dahlias, hibiscus, mums, verbena, zinnias, and bee balm seem to be more heavily affected by this fungal disease. Overall, this disease can affect over 1000 species of plants.
On the other, certain plants seem to have a natural immunity to powdery mildew. Such is the case of citrus fruits, avocados, figs, and passion fruit. Aromatic herbs are also less likely to be affected and so are most strongly scented spices like garlic, ginger, turmeric, or guava. Succulents and cacti are also immune to it.
Since powdery mildew can also be found on fruit, you are probably wondering if it is safe to eat fruit and vegetables that present mildew spores. Well, unless you are particularly sensitive to common fungi, powdery mildew is not a danger for humans. Just make sure to wash the fruit and vegetables properly before eating them. Moreover, try to avoid heavily infested areas.
How to prevent powdery mildew
Like always, prevention is the best cure. So, before we dive into the best remedies, let’s explore a few practices that will decrease your chances of dealing with powdery mildew. Even if powdery mildew occurs, these practices will prevent it from spreading all over your garden.
Start with plants that have a lower risk of infestation
For starters, you need to be extra careful if you live in an area where this fungal disease is more common, like the Central Coast of California. In this case, you might want to fill your garden with plants that have a natural immunity to this disease. As your garden grows, combine plants with mildew immunity with plants that are more susceptible to this disease. Companion planting will also help your plants battle other diseases and pests.
Promote air circulation
A natural airflow will also create less favourable conditions for the development of mildew. As such, make sure to respect the adequate spacing requirements of each plant and to prune the plants when they need it.
Use a drip irrigation system
Hoses and sprinklers are not only wasteful, but they can also encourage the development of fungal diseases. It is important to water the soil underneath the plants, not their leaves. Wet leaves can favour the development and spread of fungi. A drip irrigation system will also prevent dehydration. Hydrated plants are generally healthier and less likely to get sick.
Improve the overall health of your plants
Healthy plants are stronger against pests and diseases. You can increase the immunity of your plants by regularly enriching your garden soil with organic matter and aged compost. Compost tea is also a great health booster.
Check your plants regularly
A good gardener knows that plants need to be properly inspected regularly. While a visual inspection can give you a general idea of the plant’s well-being, you need to use your hands to properly check the plant. Check under the leaves and on all sides of the stem. This will help you prevent not only powdery mildew but also all types of pests and diseases.
Natural ways to get rid of powdery mildew
Remove infected areas
You might be tempted to clean the powdery mildew. However, keep in mind that fungi spores are very small and light. As such, they can easily spread through the air when you wipe off an infected leaf, landing on other parts of the plant. If the infestation is not severe, it is more effective to simply cut off the infected parts. Try to shake the infested parts as little as possible in the process and clean your pruning shears when you are done.
Neem oil spray
We know that neem oil is recommended for almost every plant disease or pest. The truth is that it really does have amazing healing properties. However, when it comes to powdery mildew, it only works in the early stages of the infestation. Otherwise, it will simply slow down the infestation, but it won’t be strong enough to eradicate it altogether. Never apply neem oil directly on the plants. It can damage them, and it can be irritating for humans as well if they consume plants that have been treated with undiluted neem oil. To prepare a neem oil spray mix one gallon of water with two tablespoons of concentrated neem oil. Add one teaspoon liquid soap, and a few drops of essential oil.
Baking soda spray
Sodium bicarbonate, more commonly known as baking soda is the best natural alkalizer. Fungi prefer a neutral pH, so if you change the pH of your plant’s leaves, you will create less hospitable environments for the pesty spores. Simply mix 2 tablespoons of baking soda with a gallon of water. To make the solution more effective, add one tablespoon of liquid soap. Diluted liquid soap is also a known remedy for plant diseases but it also helps the soda stick better to the plants. You can also add one tablespoon of vegetable oil to this mixture. Spray the plants heavily with this mixture, both on top of the leaves and on their undersides.
Since we already established that fungi spores don’t like alkaline environments, you might think that an acidic solution would be counterproductive. Nonetheless, vinegar can indeed rid your plants of fungi. Still, you must be very careful when working with vinegar. If you don’t dilute it properly it can severely damage the plants. We advise you to mix 4 tablespoons of vinegar with a gallon of water. Spray the solution heavily on the infected plants. Re-apply it every three days until the infection disappears.
You can buy garlic oil or you can make your own. It contains a high amount of sulfur which is very effective against fungi. Crush 6 garlic cloves and add them to a mixture of 30 grams neem oil 30 grams rubbing alcohol. Let it sit for two days and then strain the garlic and set the liquid aside. Use the crushed garlic again and let it sit in a cup of water for a day. Strain it again and dispose of the crushed garlic. Combine the garlic water with the garlic oil and add them to one gallon of water.
Sulfur and copper
Sulfur is an essential mineral for plants, but it can also be used as a fungicide. The same is true for copper, and these two minerals can even be mixed together to create a powerful fungicide. However, you need a high concentration of sulfur and copper for the remedy to be strong enough to kill fungi. This will also affect the plant tissue and beneficial insects and soil organisms. As such, avoid using this solution unless you have a big crop that is severely affected by powdery mildew. Try the organic methods first. They are rarely inefficient.
Throw away infected plants
When natural remedies fail you, your last resort is to throw away the infected plants before they make more victims. Under no circumstance are you to use the infected plants as compost. As we mentioned above, fungi spores can survive for long periods of time, only to resurface in a hospitable environment. They can overwinter in aged compost only to wreak havoc in the following summer.
Powdery mildew, like most plant diseases, can easily be dealt with, especially if it is caught in an early stage. It is rarely lethal, but it can cause serious damage if it is left untreated. It is more common amongst edible plants, but it can affect all types of plants.
Whether you are dealing with edible or ornamental plants, don’t use chemical remedies. Choose organic remedies for the well-being of your garden and the overall environment. Chemicals will affect the quality of your crops and will harm pollinators and other beneficial insects and organisms. If you are persistent with the treatment, organic remedies can be efficient, without harming the environment.