Powdery mildew is a common fungal disease that affects a wide variety of plants, including vegetables, fruits, flowers, and trees. It is caused by various species of fungi in the order Erysiphales. Powdery mildew is easy to identify by the white to gray powdery spots it causes on the leaves, stems and fruits of affected plants. While not usually fatal, powdery mildew can significantly reduce plant vigor and yield. Proper treatment and control measures are important to managing this disease in home gardens and commercial plantings.

What Causes Powdery Mildew

Powdery mildew fungi require living plant tissue to grow. They obtain nutrition by sending threadlike growths (hyphae) into the plant’s epidermal cells, where they absorb nutrients. The powdery appearance comes from the mycelium and spores formed on the leaf surface.

Several factors influence powdery mildew development:

Humidity – Powdery mildew thrives in humid conditions. Long periods of leaf wetness from rain, irrigation, or high humidity promote spore germination and infection.
Temperature – Most powdery mildews favor cooler temperatures in the 60-80°F (15-27°C) range. Warm weather may slow disease development.
Plant Age – Young, succulent growth is more susceptible than older, hardened-off tissues.
Plant Nutrition – Excess nitrogen fertilization leads to soft, succulent growth that is more easily infected.
Plant Spacing – Crowded plants create higher humidity levels around foliage favoring disease.
Shade – Shaded areas often have higher humidity and poorer air circulation which encourages powdery mildew growth.
Poor Air Circulation – Stagnant air allows spores to accumulate on leaf surfaces.
Stress – Drought, poor nutrition, and other stresses weaken plants and make them more susceptible.

Symptoms of Powdery Mildew

Powdery mildew first appears as white to gray powdery spots on leaves and stems. These powdery spots enlarge and spread to cover the entire leaf surface. Older spots often turn brown or tan.

Severely infected leaves may yellow, wither and die. Young shoots may become twisted and distorted. Fruits may develop sunken lesions.

Powdery mildew symptoms vary somewhat between plants:

On vegetables like squash, pumpkins, melons and cucumbers, small, circular whitish spots form first on older leaves. The spots enlarge to cover the entire leaf, which eventually withers and dies.
On roses, new shoots are stunted and distorted. Leaves are covered with white powdery growth, turning purple, red or tan as they age. Flower buds may fail to open.
Apple and cherry trees develop white to gray spots that can cover the fruit surface, resulting in cracking and distortion. Leaves develop circular mildew spots.
Grapes infected with powdery mildew have poor fruit set and overall reduced yields. Berries often crack open. White powdery growth covers older leaves.

Preventing Powdery Mildew

Powdery mildew is very difficult to eradicate once established, so prevention and early control are critical:

Plant resistant varieties – Choose vegetable, fruit and ornamental varieties bred for powdery mildew resistance when available.
Avoid excessive nitrogen fertilization – Use the minimum amount of nitrogen needed for good growth.
Prune and thin plantings – Improve air circulation by thinning overcrowded plantings and pruning to open up plant canopies.
Water early in the day – Allow ample time for foliage to dry before nightfall to reduce humidity levels.
Use drip irrigation – Avoid wetting foliage by using drip irrigation, soaker hoses or furrow irrigation.
Avoid overhead watering – Watering from sprinklers or hoses wets foliage and increases humidity.
Clean up plant debris – Remove infected leaves and prune out affected shoots. Promptly dispose of debris to reduce inoculum.
Use mulch – Maintain even soil moisture with organic mulch to reduce drought stress.
Avoid excessive shade – Grow plants in full sun when possible to promote good air circulation.

Treatment Options for Powdery Mildew

If preventive measures fail, there are several effective organic and conventional treatment options:

Organic Powdery Mildew Treatments

Baking Soda – Mix 1-2 teaspoons baking soda and 1/2 teaspoon of insecticidal soap or horticultural oil per gallon of water. Spray leaves, particularly undersides, every 7-10 days. This changes leaf pH, inhibiting fungal growth.

Milk – Mix 1 part milk with 2 parts water and spray plants. The milk proteins disrupt fungal growth. Reapply every 7-10 days.

Neem Oil – Use neem oil products according to label directions. Neem oil coats fungal structures and inhibits growth. Requires reapplication within 7-10 days.

Sulfur – Apply sulfur dust or spray to coat leaf surfaces. Sulfur inhibits fungal enzymes and growth. Reapply after rain or as directed on the product label, usually every 7-14 days.

Conventional Fungicide Options

Several conventional fungicides are also very effective against powdery mildew when applied preventively according to label directions:

Triforine (Funginex)
Myclobuanil (Spectracide Immunox)
Propiconazole (Banner Maxx, Ortho Rose Pride)
Tebuconazole (Folicur)
Potassium bicarbonate (Milstop)

Powdery Mildew Treatment and Control by Plant Type

The specifics of treating and controlling powdery mildew often depend on which plants are affected. Here are some key points for different plant categories:

Vegetable Powdery Mildew Control

Vegetables commonly affected by powdery mildew include cucumbers, melons, squash, and pumpkins. Treatment focuses on preventing spread of the disease:

Apply bicarbonate products or sulfur preventatively when vines begin to run or the first symptoms appear. Reapply every 7-14 days.
Improve air circulation and reduce humidity by trellising vines, thinning leaves, and widening plant spacing.
Remove badly infected leaves or entire plants to reduce fungal inoculum.
Rotate crops – avoid planting cucurbits in the same location for successive years.
Plant resistant varieties when available, like many powdery mildew resistant cucumbers.

Fruit Tree Powdery Mildew Management

Apples, cherries, peaches, nectarines and other fruit trees are prone to powdery mildew fungal infections. Effective control involves:

Apply lime sulfur or sulfur sprays at bud break to protect developing leaves and fruit. Reapply every 10-14 days up until the day before harvest.
Improve air circulation through the canopy by pruning and training branches. Thin crowded branches.
Remove and destroy fallen leaves and prunings harboring fungal inoculum.
Plant resistant varieties when available. Many newer apple and cherry varieties have powdery mildew resistance.

Ornamental Powdery Mildew Solutions

Roses, bee balm, phlox, zinnias and many other flowers commonly develop powdery mildew. Treatment focuses on prevention:

Apply sulfur dusts or sprays to new growth as it appears in spring. Reapply every 7-14 days.
Improve air circulation through regular pruning and thinning of plants.
Water in the morning to allow time for foliage to dry before night.
Remove and destroy badly infected leaves and flowers to reduce spread.
Use drip irrigation or soaker hoses to avoid wetting foliage.
Plant resistant varieties when available.

Grapevine Powdery Mildew Management

Grape vines are very susceptible to powdery mildews. Effective treatment involves:

Apply sulfur or copper-based fungicides at bud break. Reapply every 10-14 days up until veraison (onset of ripening).
Improve airflow by pruning off excess shoots and foliage. Position shoots to allow air movement.
Remove water sprouts and old leaves regularly to reduce disease carryover.
Thin bunches to improve spray coverage and air circulation.
Plant resistant wine grape varieties when available, like many newer Vitis vinifera hybrids.

6 Key Questions about Powdery Mildew Answered

1. Is powdery mildew harmful to humans?

No, powdery mildew fungi that infect plants are not harmful to humans. Only plant tissues are infected. The spores may cause allergy symptoms in sensitive individuals if inhaled in large quantities.

2. How does powdery mildew spread?

Powdery mildew spreads via wind-dispersed spores. Overwintering structures in infected plant debris also initiate new infections each spring. The fungi grow superficial mycelium on plant surfaces which then form new spores. Spread is promoted by splashing water.

3. How do you distinguish powdery mildew from downy mildew?

Downy mildew causes pale yellow leaf spots rather than white powdery growth. Downy mildew requires extended leaf wetness and infects leaf undersides first. Powdery mildew thrives in drier conditions and is more superficial.

4. When is powdery mildew most active?

Powdery mildew fungi favor moderately warm, humid conditions. Infections develop fastest at temperatures around 68-77°F (20-25°C) with high humidity. Hot, dry weather slows growth. Overwintering structures initiate new infections in spring.

5. What environmental conditions favor powdery mildew?

The fungi thrive in humid, shaded environments with poor air circulation. Still, stagnant air allows spores to accumulate. Overhead irrigation, crowded plants, nitrogen excess, drought stress and cool nights also encourage powdery mildew.

6. How can you eradicate powdery mildew once plants are infected?

Eradication is very difficult. Severe infections require removal and destruction of badly infected plant parts. Protect new growth with preventative fungicide applications. Improve growing conditions to avoid recurrence. In some cases, severely affected plants may need to be removed entirely.

Conclusion

Powdery mildew is a troublesome fungal disease in home and commercial plantings worldwide. Identifying factors that encourage disease development is key to prevention. When infections occur, prompt treatment with organic or conventional fungicides limits spread. Improving growing conditions to avoid powdery mildew favorable circumstances also helps restrict this disease. With proper integrated treatment and control methods, powdery mildew can be successfully managed to minimize impacts.

Powdery Mildew: Treatment and Control on Plants

Powdery mildew is a common fungal disease that affects a wide variety of plants. It is easy to identify by the white to gray powdery spots it causes on leaves, stems and fruits. Proper treatment and control measures are important to managing this disease in home gardens and commercial plantings.

What Causes Powdery Mildew

Powdery mildew fungi require living plant tissue to grow and obtain nutrition. The powdery appearance comes from spores and mycelium formed on leaf surfaces.

Factors that influence powdery mildew development include:

Humidity – wet conditions promote spore germination
Temperature – most mildews favor cooler 60-80°F temperatures
Plant age – young tissues are more susceptible
Plant spacing – crowded plants have higher humidity
Poor air circulation – allows spores to accumulate
Plant stress – drought, poor nutrition increase susceptibility

Symptoms of Powdery Mildew

Powdery mildew first appears as white to gray powdery spots on leaves and stems. Spots enlarge to cover the entire leaf surface. Older spots often turn brown or tan.

Severely infected leaves may yellow, wither and die. Young shoots may become twisted. Fruits may develop sunken lesions.

Symptoms vary between plants:

On vegetables, small circular whitish spots form on older leaves, enlarging to cover the whole leaf.
On roses, shoots are stunted and distorted. Leaves are covered in white powdery growth.
On fruit trees, white-gray spots cover fruits, causing cracking. Leaves develop circular mildew spots.
On grapes, infected berries often crack open. Leaves are covered in white powdery growth. Poor fruit set occurs.

Preventing Powdery Mildew

Prevention and early control of powdery mildew are critical:

Plant resistant varieties when available
Avoid excessive nitrogen fertilization
Prune and thin plantings to improve air circulation
Water early in the day to allow foliage to dry
Use drip irrigation to avoid wetting foliage
Avoid overhead watering which wets foliage
Remove infected plant debris promptly
Use mulch to maintain even soil moisture
Grow plants in full sun when possible

Treatment Options

Effective organic treatments include baking soda, milk, neem oil and sulfur. Conventional fungicides like triforine, myclobutanil and propiconazole also provide good control.

Powdery Mildew Control by Plant

Control methods depend on the affected plant:

Vegetables: Apply bicarbonates or sulfur preventatively. Improve air circulation. Remove infected plants. Rotate crops. Plant resistant varieties.

Fruit trees: Apply sulfur or lime sulfur sprays at bud break. Improve canopy airflow. Remove fallen leaves. Plant resistant varieties.

Ornamentals: Apply sulfur dusts/sprays to new growth. Improve air circulation. Water in mornings. Remove infected leaves. Plant resistant varieties.

Grapes: Apply sulfur or copper fungicides at bud break. Improve airflow. Remove water sprouts and old leaves. Thin bunches. Plant resistant varieties.

Key Questions about Powdery Mildew

Is powdery mildew harmful to humans?

No, the fungi that cause powdery mildew only infect plants. The spores are not harmful, though may cause allergy symptoms if inhaled in large amounts.

How does powdery mildew spread?

Via wind-dispersed spores. Overwintering structures also initiate new infections. The fungi grow superficial mycelium on plants which form new spores. Spread is promoted by splashing water.

How do you distinguish powdery mildew from downy mildew?

Downy mildew causes pale yellow leaf spots rather than white powdery growth. Downy mildew requires leaf wetness and infects leaf undersides first. Powdery mildew thrives in drier conditions.

When is powdery mildew most active?

Powdery mildew fungi favor moderately warm, humid conditions around 68-77°F with high humidity. Hot, dry weather slows growth. Overwintering structures initiate new infections in spring.

What conditions favor powdery mildew?

Humid, shaded environments with poor air circulation favor powdery mildew. Overhead irrigation, crowded plants, nitrogen excess, drought stress and cool nights also encourage disease development.

How can you eradicate powdery mildew infections?

Eradication is very difficult. Removing badly infected plant parts is needed. Fungicide applications protect new growth. Improving growing conditions helps avoid recurrence. Severely affected plants may need removal.

Conclusion

Proper integrated treatment and control methods allow successful management of troublesome powdery mildew fungi. Preventing disease development through management practices and fungicide applications is most effective. When infections occur, prompt organic or chemical control limits spread. With vigilance, impacts of powdery mildew can be minimized in home and commercial plantings.