Age-Old Wisdom meets Modern Tools
How to Identify and Control Powdery Mildew
Wondering about that white fungus on your plants? The fungal disease powdery mildew affects a wide variety of plants and takes away a plant’s nutrients. This causes the plant to bloom less and become weaker.
What is Powdery Mildew?
There are many different species of the fungal disease powdery mildew, and each species attacks a range of different plants. Unlike many other fungal diseases, powdery mildew thrives in warm, dry climates, though it does require fairly high relative humidity (i.e., humidity around the plant) to spread. In cooler, rainy areas, it does not spread as well. That being said, it is capable of infecting your plants under a wide variety of conditions.
When the fungus begins to take over one of your plants, the mildew that forms is made up of many spores. These spores carry the infection to other plants through the wind. Powdery mildew can slow down the growth of your plant. In some cases, if the infection is severe enough, powdery mildew can kill your plants.
How to Identify Powdery Mildew Damage
- Plants infected with powdery mildew look as if they have been dusted with flour.
- Powdery mildew usually starts off as circular, powdery white spots, which can appear on leaves, stems, and sometimes fruit.
- Powdery mildew usually covers the upper part of the leaves, but may grow on the undersides as well.
- Young foliage is most susceptible to damage. Leaves turn yellow and dry out.
- The fungus might cause some leaves to twist, break, or become disfigured.
- The white spots of powdery mildew will spread to cover most of the leaves or affected areas.
- The leaves, buds, and growing tips will become disfigured as well. These symptoms usually appear late in the growing season.
Powdery mildew first appears as small white spots on the upper part of the leaves. Photo Credit: The Regents of the University of California, UC Davis.
Control and Prevention
How to Control Powdery Mildew
- Remove all the infected plant parts and destroy them. Remember, do not compost any infected plant, as the disease can still be spread by the wind and persist in the composted materials.
- Spray infected plants with fungicides. Effective organic fungicides for treating powdery mildew include sulfur, lime-sulfur, neem oil, and potassium bicarbonate.
Prevent Powdery Mildew
- Choose plants that are resistant or tolerant to powdery mildew. Many mildew-resistant varieties of cucurbits (melons, cucumbers, squash, etc.) have been developed and can be bought from major seed suppliers.
- Avoid watering plants from overhead in order to reduce relative humidity.
- Selectively prune overcrowded areas to increase air circulation; this also helps to reduce humidity around your plants.
- Spray your plants with the fungicides mentioned above according to the directions included with the products.
- If you don’t want to use fungicides, try spraying your plants with a bicarbonate solution:
- Mix 1 teaspoon baking soda in 1 quart of water. Spray plants thoroughly, as the solution will only kill fungus that it comes into contact with.