How to Stop Powdery Mildew From Spreading: 11 Pro Tips
Powdery mildew is an unruly fungal pathogen that can wreak havoc on your garden! It converts healthy green leaves to gray, white, and fuzzy ones. Though some infections are expected, you can mitigate their spread with these 11 pro tips from seasoned grower Jerad Bryant.
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What is powdery mildew? It’s a fungal pathogen that’s spread by tiny spores. The disease overwinters on garden debris over the winter, and it infects living plant tissue during the growing season. It’s most common with warm, humid weather that appears in spring and fall.
Where I live in the Pacific Northwest, powdery mildew is a fact of life! Its presence in the garden is inevitable, and managing its spread is more effective than trying to eradicate it. Even if I eradicate it, spores will blow in from elsewhere and cause another infection!
Don’t lose hope, though, as mitigation and prevention are extremely effective in preventing the spread of this fungal condition. Even if infections occur, most vegetables will still produce a harvestable crop despite the presence of mildew.
So don’t worry when you see the white-gray fungal patches forming—try these 11 pro tips instead!
Prune Infected Leaves
Pruning infected leaves works to remove the biggest spore sources. Without big, infected patches to source spores, the disease can’t spread through the air onto your nearby garden plants. You’ll lower the risks of further infections in the garden.
Though pruning is effective in removing infected leaves, it may not work well as a solo solution. Pair it with other methods like mulching, spacing, and watering consistently. Excessive pruning can weaken your plants so they lack the energy to thrive and yield a large crop.
In my garden, squashes, hops, and grapes are the common plants that see infections. I know to trim them as needed to remove the fuzzy patches and reduce further spread. If you know you have plants that see infections annually, watch them closely and keep pruners nearby to tackle the issue before it spreads out of control.
Sanitize Garden Tools
Sanitation is key in reducing further infections! When you prune an infected plant, the spores may land on your pruners and stick to their parts. As you prune other plants, you spread the spores onto their wounds and risk infecting them.
An easy way to sanitize your tools is with rubbing alcohol. Use a mix that’s 90% alcohol or higher, and coat all metal parts with it. Let it evaporate and dry, then proceed with your pruning regimen. Sanitize the tools after making cuts on any new plant, before moving onto another.
Other tools to sanitize include herbal snips, trowels, and weeders. Any tool that comes into contact with the spores can spread them. Consider cleaning, sharpening, and sanitizing your tools each spring and fall.
Clean Up Debris
As said earlier, garden debris is where the mildew spores overwinter. When you clean up this debris, you lower the chances of the disease’s presence in your yard. Dead leaves, stems, and fruits are all possible overwintering sites. Remove them in the fall to prevent the spores from popping up in spring.
You may remove garden debris using a few different methods. Hot composting is one solution you can do in the backyard. The compost pile reaches hot temperatures that kill the overwintering pathogen. You may also bury the scraps a foot or more deep underground to let them decay out of sight.
If you lack the space for these methods, consider throwing the scraps in your garden waste or trash bins. It’s especially important to remove leaves with plenty of fuzzy patches, as they’ll provide possible overwintering sites when they sit on the ground during the cold months.
Switch Irrigation Methods
Some watering methods may cause more harm than good! What I will say is this—use whichever method is best for you and your garden. Sometimes overhead watering is the only viable solution for your space. Though convenient, it’s also a decent spreading mechanism for powdery mildew.
Overhead watering causes droplets to fall on your plants. The droplets can also splash soil particles onto the leaves, and soil has the potential to spread diseases to your plants. The standing water on the foliage creates a humid environment conducive to fungal growth.
Instead of overhead watering, consider other simple techniques like drip irrigation, soaker hoses, or a hose nozzle pointed toward the soil rather than the leaves. Keep the soil moist to ensure your plants stay healthy, vigorous, and strong enough to resist disease pressures.
Give Plants Enough Space
Spacing is key in preventing the spread! Both planting guidelines and proper pruning ensure that air channels exist in the garden. These air pathways ensure the leaves aren’t crowded and overgrown; the more packed they are, the more likely they are to spread the spores onto each other.
Many seed packets offer spacing guidelines on their label. These aren’t a joke! They may seem large, though they account for the mature size of the vegetative or fruiting plant. Proper spacing ensures that each specimen has enough space from the others so they grow well without pests, diseases, or cultural issues.
After planting, some plants may crowd others. Prune your perennial herbs, remove weeds, and cut out shade-giving branches that hang over your beds. With regular airflow, your veggies and fruits will thrive from spring through fall.
Increase Light Levels
Light is crucial for healthy plants! Without it, mildew pathogens spread rapidly and readily from species to species. Partial and full shade are ripe conditions for the spreading of spores. Stop them from harming your tender annuals by blasting them with sunlight.
When vegetables, flowering shrubs, and fruiting specimens grow with full sun, they stay strong and resilient with ample healthy stems. It’s never a good idea to put a sun-loving plant in a shady site. The mildew will fill your yard, causing powdery patches wherever you look.
Ensure your sun-loving crops have access to direct sunlight before the condition occurs for the best results. If powdery patches appear, increasing sunlight levels can help prevent them from spreading further.
Add Mulch
Mulch covers the soil, so how can it prevent mildew from spreading? When you place a layer of mulch down, it covers the soil and any fungal spores on it. Covering them up prevents them from splashing onto the leaves when you water your yard.
Any mulch will do, so long as it consists of organic matter and decays well into the ground. Compost, leaf mold, and straw are some viable options. Other possible mulches include wood chips, fallen leaves, and grass clippings.
Leach Excess Fertilizer
Fertilizer feeds plants, but too much of the stuff can cause growing issues! Excessive amounts of fertilizer, especially those rich in nitrogen, can cause excessive leafy growth that’s a ripe breeding ground for powdery mildew.
If you suspect your soil has too much fertilizer, try leaching it out with a steady trickle of water. Set a hose on low, then place it over the soil so water moves through the ground. After an hour or two, the water should have leached enough nitrogen from the site.
A soil testing kit is a surefire way to know if your soil is overfertilized. It’ll tell you the soil pH, the nutrient levels, and how much organic matter is present. It won’t test for nitrogen, though, as the nutrient is incredibly mobile in the soil. Instead, watch for signs of excessive nitrogen, like ample leafy growth that’s dark green, and few flowers or fruits.
Spray Potassium Bicarbonate
If you try the above methods and the mildew is still spreading, consider using a spray that’ll stop the spores in their tracks. Potassium bicarbonate is one such solution, as it creates an inhospitable environment for mildew. The powdery patches won’t form or spread with repeat applications.
Potassium bicarbonate is also ideal because it doesn’t harm pollinators. Other sprays, like neem oil and soapy solutions, can harm bumblebees, butterflies, and moths. Rather than creating a deadly garden for pollinating insects, use a spray that prevents the disease without harming local wildlife. Apply it in the morning or afternoon when the insects aren’t active to further protect them from harm.
For perfect application, use this recipe from expert gardener Jacques in the Garden. He uses potassium bicarbonate annually to reduce mildew infections. Here it is:
- 1 tablespoon of potassium bicarbonate powder
- 1 teaspoon of garden-safe soap
- 1 gallon of water
Mix the solution well, then release it to your plants using a pressurized sprayer for precise applications.
Apply an Anti-Fungal
Other anti-fungal solutions can work to prevent the mildew from spreading further. Sulfur, Bacillus subtilis, and milk spray are viable products. Follow the instructions on the products’ labels, and apply them on calm days without high winds so they don’t waft away.
Milk spray, though it seems unlikely, is a possible solution for powdery mildew! Milk contains salt and amino acids that suppress fungal growth. Make a milk spray by mixing one part of milk with two parts of water.
Though milk works, it can grow smelly in hot gardens! Avoid applying it when temperatures are high and the weather is dry, as it’ll cause rotten smells throughout the landscape.
Plant Resistant Varieties Next Year
After a particularly challenging battle with this plant disease, you’ll definitely want to avoid it during the next growing season! One way to reduce its occurrence is with resistant plant varieties that’ll thrive despite the fungal presence.
Resistant cucumbers, squash, apples, and peas exist for home gardeners. Look at the seed packets’ labels to see if they say they’re powdery mildew resistant. If it doesn’t say, it isn’t resistant.
With resistant varieties, healthy growing practices, and a ready management solution, this condition will be a worry of your past. Adapt your strategy each year as you learn more about how it shows up in your space.
