9 Tricks to Defeat Powdery Mildew in the Late Summer Garden
Powdery mildew shocks garden plants during late summer. This fungal disease wreaks havoc on tender leaves and may reduce crop yields if left unchecked. Garden writer Jerad Bryant breaks down tricks from Epic Gardening’s Jacques in the Garden to defeat it for good!
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Powdery mildew is an annoying fungal infection that spreads through spores. It covers leaves on both sides with fuzzy, white-gray growth. Splotches spread and creep, then produce more spores to spread onto new hosts. This may seem like one disease, but it’s a diverse mix of many fungi from different genera. The type differs depending on the host, with dozens of plant species offering refuge for these fungi.
These late-season tricks help you defeat powdery mildew before it spreads out of control. Small infestations are easy to remedy, and it’s better to catch them while they’re manageable. Don’t worry if your plants are covered with mildew, as there are easy solutions to stop it.
Prevent this infection early in your growing season by using resistant varieties. Seed labels will say “powdery mildew resistant.” Some crops, like watermelons, don’t have resistant varieties as they are naturally resistant. Keep your plants healthy and happy to offer a final line of defense. Your crops will grow strong and resist disease with proper water, nutrition, and sunlight.
These nine tricks offer further guidance for your mildew problems. Worry no more; we’ll rid your garden of this disease before summer’s end. We’ll cover a foolproof remedy from Jacques in the Garden of Epic Gardening alongside other trustworthy treatment solutions.
First Defense: Give Your Plants Space
Spacing your plants offers a multitude of benefits beyond preventing powdery mildew! It first prevents disease by keeping plants off of each other. When near each other, infected species pass along spores to nearby healthy ones. Proper spacing creates wind channels between plants, further discouraging spores from landing on your crops.
If your plants are in the ground already, there’s not much spacing you can do. Prune excess leaves off crops like cucumbers, tomatoes, and watermelons to facilitate airflow between close plants. Trees and shrubs like maples, rhododendrons, and oaks can also get this disease; space these specimens out using their future mature size, not their size at planting.
Are you wondering what other plants are susceptible to powdery mildew? This chart offers an easy breakdown of host species and their symptoms:
Host Species and Symptoms
| Plant Host | Infection Symptoms |
| Sunflower | Lower leaves develop white splotches. |
| Wheat | Streaks of white-gray growth start on lower leaves. |
| Barley | Streaks of light yellow growth, like wheat, form on the foliage. They’re more linear than wheat infections. |
| Legumes | White powder starts on leaves—the foliage shrivels and dies as the powder spreads. |
| Grape | Look for white or greenish patches on the undersides of leaves. Infections start there, then spread to cover fruit, stems, and flowers. |
| Onion | White fungal patches form on long onion leaves. |
| Stone Fruits | Look for white-gray growth on ripening fruit and new foliage. Affected leaves shrivel and curl. |
| Melon | Leaf centers start with white, splotchy growth. It spreads, covering entire leaves with time. |
| Lilac | Lower leaves start forming white growth on tops. |
| Strawberry | Infected leaves have powdery spots on the leaf undersides. This eventually spreads, leading to purple blotches on both leaf surfaces. |
| Woody Trees | Both sides of tree leaves can receive infections. Look for white splotches on tender shoots and buds. |
| Hydrangea | Hydrangea leaf surfaces grow white, then purple lesions, before the entire leaf shrivels and browns. |
| Roses | Reddish spots that turn white develop on leaves and upper stems. They turn purple-black with age. |
| Tomatoes | Tomato powdery mildew starts as bright white, yellow, or greenish spots on foliage that grow gray and purple over time. |
Avoid Overhead Watering
If powdery mildew has already taken hold of your garden, overhead watering may spread it. This common irrigation practice is convenient, especially with modern sprinkler systems. The danger with overhead watering lies in its ability to bolster humidity around plants. The combination of summer heat and high humidity creates the perfect conditions for this pathogen.
Overhead watering also wastes water, as many droplets remain on the leaves rather than reaching the soil. Plants can drink water through their leaves, but they’re much more efficient at using their roots. A lot of the moisture evaporates before they can suck it in, especially during high-heat days.
Keep your plants strong and healthy by watering low to the ground around their base. Drip irrigation is one easy way to have consistent moisture without too much humidity. Set on a timer, this system removes the headache of remembering to water. Another simple solution is to supplement your watering with olla pots. These terra cotta structures leach water into soils over time, so your plants stay well watered no matter the temperature.
Finally, avoid water blasts to treat pests on plants that are susceptible to powdery mildew or in soils where the fungus has been present. Use pruning, biological controls, and organic pesticides instead.
Give Leaves Sufficient Sunlight
Shady conditions facilitate powdery mildew’s spread. Plants that prefer full sun catch spores more often in partial shade. By situating your annuals and perennials in areas with enough light, you’re helping them battle pests and pathogens that target weak, sickly species.
Some species catch powdery mildew in the shade despite preferring to grow in shady conditions. One example, the hydrangea, easily grows white splotches in late summer. Overhead watering, shade, and high temperatures exponentially speed the powder’s ability to grow. Give hydrangeas and similar species some sunlight to help them out—partial shade means they need between two to six hours of sun a day.
Shaded vegetable beds also readily attract disease, as our annual food crops are often susceptible to pathogens. If a tree or shrub shades the garden, consider pruning some branches off to allow light to reach your garden beds. Most vegetables need full sun, from six to eight hours of direct light daily.
Add Mulch Or Compost
Mulch acts in two ways to prevent and defeat powdery mildew. This disease often resides in soils, and water droplets splash it onto leaves. Mulch keeps soil hidden below a thick covering of organic matter. When water falls, the spores struggle to jump up on your plants.
Organic matter prevents infection by keeping your garden specimens strong, turgid, and resilient. As organic mulches decay, they add beneficial bacteria, fungi, and bugs that battle the bad ones. Healthy microorganism populations inhabit soils, making it difficult for pathogens to claim territory.
A mulch-like compost also preserves moisture during drought and facilitates drainage during rainstorms. It’ll decay over time, meaning you’ll want to add a steady supply throughout your growing season. Compost is the best mulch you can add. Other beneficial options are leaf mold, wood chips, and straw.
Fertilize The Correct Amount
Fertilizers influence plant growth by providing an abundance of nutrition. When applied correctly, they are a part of a healthy ecosystem. Excess nutrients may harm plants and make them more susceptible to diseases.
One example is nitrogen—this element helps plants grow leaves, and its presence or absence has drastic effects. Too much nitrogen leads to excess foliage. A lot of leaves create hospitable conditions for mildew, both by offering more host sites and by reducing airflow.
Avoid adding extra nutrients to your soils, and use a soil test to know exactly which ones you’re missing. If you apply compost consistently for years, your soil may not need extra fertilizer. It’s best to add a little now, as you can always add more later.
Prune Off Infected Leaves
If infections are already on your ornamentals, pruning keeps them from spreading. I use this strategy each autumn as powdery mildew sets in on squash, tomatoes, and melons. Snip off leaves with small infections and dispose of them away from your garden. Be careful not to shake the leaves while pruning, as this may spread spores to your other plants.
Plants with few leaves may not benefit from this strategy. Only prune healthy plants with abundant leaves—without leaves, they’ll struggle to grow new ones, produce fruit, and battle pathogens. Decaying leaves on the soil surface allow the disease to continue thriving, so remove them along with any foliage you prune off.
Apply Organic Treatment Sprays
When all else fails, spraying organic solutions kills and prevents mildew growth in healthy plant tissue. Powdery mildews are fungi, meaning natural fungicides harm or defeat them after continuous applications. The diseases evolve to resist treatments—vary your approach each year so they don’t build resilience to your sprays.
You’ll also want to spray in the early morning or at dusk. Leaves pull in moisture when thirsty, so they’ll suck in sprays during the day. This may cause damage, and the foliage may wither or die. Always spray before the day’s warmth arrives, and you’ll avoid accidentally hurting your garden ornamentals and vegetables.
When you mix these solutions, use them within a day or two. If left in storage, they’ll lose potency or grow toxic. Make small amounts, and you won’t have to deal with excess treatment solutions.
These five sprays work to prevent, defeat, and kill powdery mildew.
Potassium Bicarbonate
Jacques in the Garden uses this treatment option in his garden for superb treatment! This organic powder doesn’t harm pollinators and is easy to mix. Apply it in the afternoon after the sun has faded away, and direct it towards infected leaves and stems. Insects are less active, and your plants won’t suffer damage when you spray them.
Potassium bicarbonate mixes well with a garden-safe soap like castile. The soap works as a surfactant and helps the powder stick to leaves. Here’s a basic recipe:
- 1 tablespoon of potassium bicarbonate powder
- 1 teaspoon of garden-safe soap
- 1 gallon of water
Shake or stir the solution well, then spray it directly onto infected plant parts. Cover the leaves with the solution to ensure you destroy all spores. Potassium bicarbonate doesn’t prevent future infections, so remove extremely infected leaves before you apply it. It kills small infections before they rampantly spread.
Milk Spray
Who knew dairy could be used in the garden? Milk spray is one effective treatment method to defeat powdery mildew and prevent its spread. The salts and amino acids in milk destroy spores, preventing existing growth from growing out of control.
Make milk spray by mixing one part of milk with two parts of water. The only caveat with this treatment is its smell—if you garden where summers are hot and dry, applying milk spray will lead to rotten smells. Continue using milk if you don’t mind, but try another treatment option if you’re sensitive to the smell.
Bacillus Subtilis
The bacteria Bacillus subtilis destroys mildew growth before it spreads! This is a common ingredient in antifungal sprays. It’s certified for use in organic agriculture and safe in recommended doses for your garden.
Apply this spray out of direct sunlight when the weather is mild. The bacteria will battle it out with the fungi, and your plants will be spore-free. You may continue applying it as recommended if powdery mildew spreads—if it doesn’t work, the strains may be resistant to B. subtilis. Use this spray and others as a part of a complete treatment regimen.
Neem Oil
Neem oil is a common organic antifungal. It contains an extract from the neem tree that harms bugs, fungi, and bacteria. Unlike potassium bicarbonate, this spray may also harm beneficial pollinators in your garden. It also works less as a treatment and more as a preventative measure. Neem oil prevents spores from spreading but struggles to eradicate infestations.
Use neem oil as part of your prevention techniques if you consistently struggle with powdery mildew. Apply it early in the morning or at dusk to avoid harming beneficial bugs. Avoid spraying flowers, as the neem oil can remain in them and hurt bees on their hunt for pollen.
Sulfur
Sulfur spray works like potassium bicarbonate. It’s a common antifungal application for organic gardening. It damages powdery mildew strongholds and prevents their spores from spreading. You’ll find it at stores as a powder or spray. Powders are usually more cost-effective, as you can make a homemade spray with water and sulfur dust.
Combine a tablespoon of sulfur dust with a gallon of water for DIY sulfur spray. Apply it to infections weekly until growth disappears. If leaves have lots of powdery white growth, prune them off before you spray. This prevents spores from spreading onto healthy leaves while you apply the sulfur.
Remove Infested Plants
Sometimes, treatment methods fail our tender ornamentals and vegetables. Most trees and perennials continue thriving despite powdery mildew infections, which usually occur from midsummer through fall. This means some plants thrive in the other seasons and trudge on through fungal infestations.
Vegetables like tomatoes, squash, and watermelons may not fare as well. Annuals have limited areas where they have the luxury of living through multiple seasons. If disease spreads throughout their stems, they struggle to flower and fruit. This means you’ll dedicate space in your garden to unproductive plants.
Remove overly infected plants from your garden. Pull them up or chop them at their base to avoid disturbing the soil. Throw them away, and do not compost them. Plant new seeds or transplants in their place to make use of the space. If there’s still time, sow a disease-resistant variety of the same crop. Otherwise, sow autumn and winter crops like beets, chard, and spinach.
Choose Resistant Varieties For Next Year
Add an extra layer of defense with disease-resistant crops. If your zucchini grew white fuzz all over its leaves, try ‘Emerald Delight,’ a powdery mildew-resistant summer squash. Choose vegetables like this one, and you’ll avoid powdery fungal infections next year.
Some species, like watermelons, lack resistant varieties. Watermelons have a seeming built-in resistance to powdery mildew. Other plants need more prevention and treatment techniques than their resistant counterparts. Here are a few other resistant types that thrive despite a strong fungal presence:
Frequently Asked Questions
Can powdery mildew spread to other plants?
Yes, it can! When the powdery white growth starts producing spores, they may splash or waft onto other plants. If the other plants are weak or susceptible, the spores infect their leaves and grow white, fuzzy blotches.
Will powdery mildew go away?
Winter weather resets gardens and destroys current fungal growths. However, spores can overwinter in soil. In warm winter climates, powdery mildew may spread out of control. It’s important to apply prevention and treatment methods to completely defeat powdery mildew.
What does powdery mildew look like?
You’ll notice white, gray, or black fuzzy blotches on leaves, stems, flowers, and fruit. On grassy species, growths look like streaks rather than circles. Large infections spread spores, so you’ll notice this growth on multiple leaves where they’ve taken hold.
Why do I keep getting powdery mildew?
Recurring infections are usually a result of improper growing conditions, and spores present in your soil. Full sun-loving plants are prime targets in the shade. Crowded gardens, overhead watering, and heat create perfect fungal growing conditions. To defeat the disease, fix these cultural issues first, and powdery mildew occurrences will go down over time.
