Powdery mildew is caused by a fungus and is seen as a light gray or whitish powder on the surface of leaves. It occurs following warm days and cool nights often being seen in the fall and spring. Indoors, powdery mildew thrives best in cool, damp conditions with limited air circulation. The disease is considered more unsightly than harmful. Death of the plant is rare. 

Symptoms and Diagnosis

Powdery mildew appears on many ornamental plants including, but not limited to, beebalm, lilac, ninebark, oak, peony, phlox, sunflower, zinnia, and many vegetable crops. Indoors, plants such as African violets, begonias, ivy, jade, kalanchoes, poinsettias, and rosemary are susceptible. 

Superficial powdery patches appear on leaf surfaces, young stems, flowers, and even fruit. The powder is composed of mycelium and colorless chains of spores on upright stalks. Later there may be dark "pepper-like" spots among the powder (the spots are spore-producing bodies). Photosynthesis is impaired and growth may be stunted. Buds fail to mature. The plant declines in growth and vigor and eventually becomes unsightly. As the disease progresses, leaves may be dwarfed, curl, turn yellow, and drop off. Flowers may be deformed. Fruit crops may be reduced, with the fruit misshapen and covered with powdery patches. The powdery patches can be partially removed by rubbing the leaves but this will not eliminate the fungus.

Life Cycle

Powdery mildew commonly winters over as mycelial mats in dormant buds or on plant stems and fallen leaves. During humid and warm spring weather (with cool nights), infected buds open and spread conidia (spores) to new host tissue. These conidia do not require free moisture in order to germinate as many fungi do. A new generation may be produced every 72 hours if conditions are right. 

Even though each species of powdery mildew attacks only a narrow range of hosts, there are 11,000 species of powdery mildew fungi. The species of fungi that causes powdery mildew on African violets is not the same species that causes powdery mildew on rosemary. Therefore, powdery mildew on one plant may not spread to a dissimilar plant. Even given this, both plants may develop powdery mildew from different species of fungi if favorable conditions exist for disease development. Several species of powdery mildew can be common both indoors and out. 

Integrated Pest Management Strategies

For outdoor plants-  

1. Purchase resistant varieties. This is the best strategy for avoiding powdery mildew. 

2. Pruning. Prune out diseased tissue and destroy it. Gather and destroy dead leaves and stems in the fall. This is the source of next season’s inoculum. 

3. Keep plants in good vigor. Space the plants for good air circulation. Give the plants plenty of sun, as too much shade seems to make the problem worse. Try to avoid overhead watering and do not handle the plant when the foliage is wet as this will spread the disease. Water early in the day. 

4. Fungicides. Use fungicides as a preventive treatment before rain for maximum protection. Spray both sides of leaves and flowers well with sulfur, mancozeb, maneb, chlorothalonil (Daconil), cinnamaldehyde (Cinnamite), or copper-containing fungicides. 

Organic Strategies 

Strategies 1, 2, and 3 are strictly organic approaches. Of the fungicides mentioned in Strategy 4, consult the Organic Materials Review Institute (OMRI™) for appropriate organic copper products. 

For indoor plants- 

1. Choose healthy plants and purchase mildew-resistant cultivars if possible. 

2. Sanitation. At the first sign of infection, isolate the plant. Remove and dispose of all infected plant parts. Discard severely infected plants. 

3. Evaluate and modify the growth environment. Keep plants in a well-ventilated area and do not overcrowd. Improve air circulation around plants. Avoid wetting leaves when watering, do not water from above. 

4. Don’t over-fertilize. Avoid high nitrogen fertilizers since powdery mildew attacks new succulent growth. 

5. Treat with baking soda. Some gardeners recommend applying a baking soda solution to prevent the fungi from establishing. This changes the pH of the leaf surface. Test on a few leaves first as this may damage some plants. Some may feel this treatment may look as unsightly as the mildew disease itself. 

6. Use a commercial fungicide. Valuable plants or crops may require the application of an appropriate fungicide labeled for indoor use, preferably one that is systemic. Be sure to read the label and follow directions. 

Organic Strategies 

Strategies 1, 2, 3, and 4 are strictly organic approaches. 

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against powdery mildew. Not following the pesticide label before usage is a violation of federal law.