Powdery mildew is a common fungal disease that affects a wide variety of plants. It is caused by several species of fungi in the order Erysiphales. Powdery mildew presents itself as white or gray powdery spots on the leaves, stems, and fruits of affected plants. While not generally fatal, powdery mildew can seriously damage your plants if left uncontrolled. Fortunately, there are several effective methods to prevent and treat powdery mildew on plants.
What Causes Powdery Mildew
Powdery mildew fungi thrive in warm, humid environments. The disease is spread by wind-borne spores that land on plant surfaces and send out threadlike roots to extract nutrients. Factors that encourage powdery mildew growth include:
- High humidity – Powdery mildew thrives when relative humidity is over 90%.
- Poor air circulation – Stagnant, enclosed environments promote mildew.
- Moderate temperatures – 65-80°F is ideal for mildew.
- Shaded areas – Mildew flourishes out of direct sunlight.
- High nitrogen fertilizers – Excess nitrogen makes plants more susceptible.
The fungi mostly affect young, succulent growth rather than older foliage. Different powdery mildew species target specific plants. For example, one mildew fungus infects cucurbits like squash, while another attacks roses. However, some common hosts include cucumbers, melons, zucchini, roses, and zinnias.
Signs of Powdery Mildew
Powdery mildew first appears as white or gray powdery spots on leaves and stems. These spots expand into blotches that coat the surface in a mat of moldy fibers. Severely affected leaves may curl, distort, turn yellow, or die off completely.
Other symptoms include:
- White powdery coating on leaves, stems, flowers, and fruits
- Distorted, stunted growth
- Premature leaf drop
- Failure of fruit to properly ripen
Check young leaves and the undersides of mature leaves often for early signs of infection. Powdery mildew usually starts there first before spreading to the rest of the plant.
How to Get Rid of Powdery Mildew on Plants
If you suspect powdery mildew in your garden, act quickly before the fungi have a chance to spread. Here are some effective treatment options:
Improve Air Circulation
One of the easiest ways to combat powdery mildew is to improve air flow around your plants. You can:
- Space plants farther apart to encourage air circulation.
- Prune and thin dense foliage to open up the canopy.
- Use poles, stakes, or trellises to raise foliage off the ground.
- Remove weeds that crowd your plants.
- Add fans to blow stagnant air out of enclosed growing areas.
Avoid wetting foliage when watering, and water early in the day so plants dry out by nighttime. Powdery mildew needs humidity to thrive.
Apply Fungicides
Several conventional and organic fungicides are very effective against powdery mildew:
- Sulfur – This natural mineral inhibits spore production. Apply as a dust or spray every 7-10 days.
- Neem oil – Derived from the neem tree, it disrupts the fungus life cycle. Coat all leaf surfaces for best results.
- Bacillus subtilis – This beneficial bacterium fights mildew and boosts plant immunity. Use weekly as a preventative.
- Potassium bicarbonate – The baking soda in this spray raises pH to inhibit fungus growth. Apply twice a week.
- Copper fungicides – Copper salts kill powdery mildew on contact while acting as a protectant. Follow label instructions carefully.
For best results, start spraying at the first sign of mildew or as a preventative before disease onset. Ensure thorough coverage of upper and lower leaf surfaces and repeat applications as directed on the product label. Combine with cultural controls for a one-two punch against mildew.
Use Milk Spray
Believe it or not, plain milk makes an effective DIY treatment against powdery mildew. The milk proteins inhibit the growth and spread of the mildew fungi. Simply mix 1 part milk with 9 parts water and spray liberally over affected plants. Reapply every week or after rain. Discard leftover mixture within 24 hours.
Remove Severely Infected Plant Parts
Prune off badly infested leaves, stems, and shoots well below signs of infection. This helps prevent the disease from spreading to healthy plant tissues. Dispose of the pruned material immediately – do not add it to compost piles. After pruning, spray with neem oil or fungicide to protect newly exposed areas.
Apply Baking Soda Spray
The sodium bicarbonate in baking soda raises the pH on plant surfaces to make conditions less hospitable for fungal spores. Mix 1 tablespoon baking soda with 1 quart water and 1/2 teaspoon liquid soap. Spray liberally over affected plants, coating all leaf surfaces. Reapply every 7-10 days until mildew clears. This works best as a preventative spray.
Apply Compost Tea
Compost tea introduces beneficial microbes that can outcompete and kill off powdery mildew fungi. Place high-quality compost in a burlap sack and soak it in a bucket of non-chlorinated water for 3-7 days. Stir the bag gently every day. Once ready, use the compost tea as a foliar spray immediately. Reapply every 1-2 weeks. The compost soak water also contains available nutrients that strengthen plant defenses against infection.
Switch to Drip Irrigation
Overhead watering like sprinklers splashes spores from leaf to leaf. Switch to drip irrigation at soil level to help keep foliage dry and limit disease spread. Water first thing in the morning so leaves dry out before nightfall. Drip irrigation also prevents soil erosion and nutrient leaching compared to sprinklers.
Control Humidity and Temperature
Since humidity and warmth foster powdery mildew growth, do your best to keep humidity below 60% RH and temperatures between 60-80°F in your growing area. Improve airflow with fans and vents, reduce irrigation, and avoid dense planting. Greenhouses are notorious for powdery mildew issues due to the warm, stagnant conditions.
How to Prevent Powdery Mildew
Once powdery mildew infects a plant, it can be difficult to fully eradicate, so prevention is critical. Here are some key tips to avoid powdery mildew in your garden:
- Select resistant varieties when available – Certain cultivars have genetic resistance.
- Allow for sufficient spacing between plants to encourage air flow.
- Prune plants to open up dense foliage.
- Water early in the day, avoiding wetting leaves.
- Apply neem oil or Bacillus subtilis weekly as a protective spray.
- Remove and destroy infected plants to limit spread.
- Avoid excessive nitrogen fertilization.
- Monitor for early symptoms and treat immediately.
- Stake or trellis plants to lift leaves off the soil.
- Improve airflow with fans in enclosed growing areas.
Careful cultural practices and preventative sprays will help safeguard your garden from destructive powdery mildew. Be vigilant in checking for early signs of infection. At the first white spots, take action to contain the disease before it damages your plants any further. With vigilance and prompt treatment, you can successfully keep powdery mildew at bay in your garden.
Frequently Asked Questions About Powdery Mildew on Plants
What does powdery mildew look like on plants?
Powdery mildew presents as white or gray powdery spots and blotches on the leaves, stems, flowers, and fruits of infected plants. These powdery spots expand to coat the surface with a mat of moldy fungal fibers that resembles talcum powder.
What causes powdery mildew on vegetable plants?
Powdery mildew fungi thrive in warm, humid conditions with poor air circulation. Several species infect vegetables, including Podosphaera xanthii on cucurbits and Golovinomyces cichoracearum on lettuce and endive. Overhead watering and crowded planting encourage disease spread.
How do you treat powdery mildew on roses naturally?
Effective organic options for powdery mildew on roses include neem oil, milk sprays, potassium bicarbonate, compost tea, and Bacillus subtilis. Improve airflow by thinning and spacing plants. Prune out heavily infected canes. Remove fallen leaves and petals to reduce spore sources.
How do you get rid of powdery mildew on cucumbers?
Use resistant cucumber varieties when available. Improve airflow and reduce leaf wetness. Spray plants weekly with neem oil as a protective barrier. Remove badly infected leaves to limit spread. Apply sulfur, Bacillus subtilis, or potassium bicarbonate fungicides at first onset. Ensure thorough coverage.
How do you treat powdery mildew on zucchini?
Space zucchini plants to encourage air circulation and prune dense growth. Apply neem oil as a preventative before mildew appears. Spray sulfur, copper, or baking soda solutions at first sign of infection. Improve airflow with fans if growing under cover. Use drip irrigation to reduce leaf wetness. Remove severely infected leaves.
Can I still eat vegetables with powdery mildew?
In most cases, vegetables infected with powdery mildew are still safe to eat as long as you wash or peel them first. Simply trim off any affected areas. However, mildew can negatively impact flavor. It’s best to prevent infection and treat at first onset to protect your harvest. Fruits with dense mildew growth are unappetizing and best discarded.
Conclusion
Powdery mildew is a persistent fungal disease, but not an impossible one to overcome. With vigilance and integrated disease management combining cultural practices and organic fungicides, you can successfully prevent and treat powdery mildew in your garden. The keys are improving airflow, reducing humidity, monitoring routinely for early infection, treating promptly at first onset, and using resistant varieties when available. Consistent prevention and treatment will keep powdery mildew from gaining a foothold and damaging your vegetable, fruit, and ornamental plants. With a proactive management approach, you can enjoy a healthy, productive garden free of powdery mildew issues.