White Powder on Plants in Terrarium

• The humid and still air in terrariums attracts powdery mildew fungi. Ryan McVay/Lifesize/Getty Images

  A terrarium, which is a small enclosed space for growing plants, encourages the fungal disease known as powdery mildew because of the humidity and lack of air circulation inside. Powdery mildew begins by forming white-to-grayish powder on plants. Several methods exist to control the disease.

Symptoms

• Powdery mildew fungi first produces spots of white-to-grayish growth that resembles talcum powder. Some people misidentify powdery mildew as dust. The fungi spreads easily in the humid, still air of a terrarium, forming patches of mildew. Sometimes the fungi overwinter and appear as tiny spherical structures; they are first white, then turns to yellow-brown and black. They usually attach themselves to the upper surface of leaves, but could also affect other parts of plants. Infected leaves could become distorted, turn yellow and fall prematurely. Infected buds could fail to open. The fungus could suck nutrients from the plants and decrease vigor and growth rate.

Cultural Control

• Early cultural control efforts could eliminate the need for fungicides. Remove the areas of the plants with the mildew spots from the terrarium to prevent them from spreading. Reduce the humidity of the terrarium and allow air to circulate. For example, use a dehumidifier or heater, place the plants further apart from each other and don't water the plants from above. Avoid adding a nitrogen fertilizer, which could lead to the production of succulent tissues, which are more susceptible to powdery mildew.

Chemical Control

• Fungicide sprays can effectively eliminate powdery mildew problems. Use products that contain sulfur, neem oil, triforine, Bacillus subtilis or potassium bicarbonate as an active ingredient. Repeat application every seven to 14 days to provide continuous protection for your terrarium plants. Ensure that the product you use is authorized for home use in your area and follow the instructions on the product label. Combine the use of fungicides with cultural control methods for best results.