Powdery mildew is a fungus that grows on plants and looks exactly as it sounds, a white powder that is present on leaves, making them look like they have mildew on them. This disease can wreak havoc in a container garden because it is highly contagious. The 'powder' is carried over the air, and can easily spread to other plants.
Conditions for Growth:
As the name suggests powdery mildew thrives in warm humid conditions particularly when humidity reaches 90%. Places with poor air circulation and with a climate of around 60 to 80 degrees Fahrenheit (15 to 26 degrees Celsius) are ideal. The fungus can resist cold winters, however, by residing in debris and dead leaves, and can be present in black spots shown amidst the white powder. Once the temperature is warmer the black spots will 'burst' up in the air causing a spread of the infection.
The Damage Done:
Powdery mildew does not attack the host, and can live happily on leaves, stems and buds spreading slowly. As the infection spreads however, buds may fail to open, leaves turn yellow and drop, and stems are distorted. If left untreated, the plant will eventually succumb to this disease, as it slowly takes all the nutrients from the plant leaving it stunted.
Prevention and Control:
As luck would have it, powdery mildew is quite limited to its hosts, and if it is present on one type of plant it does not necessarily mean that it will spread on a different plant. This is favorable in a container garden if different plants are present.
It is preferable that the plants have good air circulation in order to keep the plant dry. For this reason, overhead watering should be avoided as much as possible. Having said this, spraying the plants daily with a watering host reduces the risk of infection, because even though the fungus thrives in humid conditions, it hates water. The trick is timing, to let the plant dry.
Any leaves, stems, debris or dead leaves that are infected should be removed, and not turned into compost. If the plant is too far gone, it would be best to remove it completely to avoid the risk of it infecting the other plants.
There are some plants which are naturally resistant to powdery mildew, such as some rose and lilac cultivars. Other plants, such as succulents, are very susceptible to the disease, so care must be taken where to place these plants, so as not to favor growing conditions.
There are fungicides which can be bought off the shelf to control the spread of powdery mildew; however, there are also recipes which can be made at home. Mixing a gallon of water and 1 tablespoon baking soda, for example, is a good way of stopping the spread. Adding a dash of liquid soap to the mix also helps. It is not a cure however, just prevention against further spread.