Houseplants can be subjected to many different stressors. Drafts near windows and heat coming up through vents can create difficult conditions for plants. Humid or damp conditions in your house can lead to fungus infestations. One fungus is powdery mildew.
According to the book "The Houseplant Expert" by Dr. D. G. Hessayon, it often appears as a white fuzzy substance on the leaves and stems of the plant. It can eventually cover the plant and is sometimes misidentified as dust. According to the Cornell University Fact Sheet, the mildew spreads by spores that enter the air from the white substance on the leaves.
According to the Ohio University Extension Fact Sheet, a powdery mildew that affects roses is not the same one that would affect lilacs, as an example. The majority of ornamental plants can suffer from some form of powdery mildew.
Powdery mildew can suffocate the plant by blocking the leaves from receiving sunlight. It's not necessarily fatal but can retard plant growth. The leaves that are infected can become curled, weighed down and misshapen during a heavy infestation. The entire plant could become engulfed. The mildew is also unsightly. The plant might not bloom as often.
Differentiating Between Mold and Mildew
Mold and mildew are both types of fungus, so they can appear similar. You need to be able to differentiate among different fungi so that you can treat the problem correctly. Powdery mildew can easily be confused with gray mold (botrytis), as powdery mildew can appear gray as well as white.
According to "The Plant Expert," gray mold appears during humid and cool conditions, and commonly targets begonia and cyclamen as well as Gloxinia and many different African violets. Gray mold will also appear puffier and thicker, while powdery mildew is flatter. Also, gray mold is more apt to cover the entire plant, while powdery mildew usually starts first on the leaves and then spreads to other parts of the plant.
At first, you might not notice anything wrong with the plant, except some white or gray circular patches on the leaves. After a couple of weeks however, you will notice the white powdery substance has spread to more leaves and possibly to the stem. The longer the plant goes untreated, the more the leaves will appear bent and curled. Sometimes the leaves will appear dried or brown.
Powdery mildew can occur at any time of year. Often, plants that are brought indoors for the winter ("wintering over") are more susceptible to powdery mildew. Changes in temperature and air quality can also trigger the disease. Plants that are close together can spread the disease. Also, the smaller the plant and the larger its infestation, the greater the chance the houseplant might not be able to survive the fungus.
Prevention and Treatment
Fungus can multiply quickly. Therefore, it's important to take care of the situation as soon as you notice something amiss.
Always keep infected plants away from healthy plants to prevent further problems.
The "Houseplant Expert" recommends removing the leaves that are highly infected. With this type of fungus, a systemic fungicide or dinocap might be the best solution. "The Houseplant Expert" also recommends lightly dusting the leaves with sulfur alternately. As a final step to a cure, the book mentions improving ventilation around the plant to prevent further infection.
Follow the directions on the fungicide you choose, including how often to reapply the fungicide and how much to spray as well as where to spray. Being faithful to the instructions can ensure your plant's full recovery. When treating the plant, make sure to wash your hands before touching other plants, so as to avoid inadvertently spreading the fungus.
Another good rule of thumb is to buy plants from reliable stores, rather than discount stores or shops that don't specialize in houseplants. Also, when re-potting houseplants, use fresh soil.