Fungus in houseplant soil is a common issue. Indoor conditions are the ideal environment for the spread of fungal spores. Consistent watering of houseplant soil increases and encourages the growth of fungi. Some fungi varieties are innocuous, while others can damage plant growth and potentially kill houseplants when left unchecked.
According to Tom Volk of the University of Wisconsin, a common fungus found in houseplant soil is Leucocoprinus birnbaumii, or the yellow mushroom. Rich, warm soil is the favorite breeding ground of this variety, so it is common in greenhouses and indoor soil. Leucocoprinus birnbaumii often comes in potting soil and will not damage the plant.
Fungal diseases are common in houseplants and can create a variety of problems and symptoms. Fungal leaf spots create small, brownish spots on leaves with yellow margins around the edges. Anthracnose is a disease caused by the fungi Colletrotrichum and Gloeosporium. This causes leaves to turn yellow, tan then fall off. Root rot is also caused by fungus. The fungi Rhizoctonia, Pythium, Botrytis, Phytophthora, Alternaria and Sclerotinia cause plant roots to turn brown and decay, preventing the plant from absorbing water and nutrients from the soil.
Use only clean, sterilized soil when planting a houseplant. A sterilized pot is also required. Cleaning a recently used pot with a bleach-and-water combination prevents fungi from traveling from old potting soil to new soil. Adding a fan near the plants will circulate the air and help prevent fungi from creating a mold on top of the soil. Watering the plant when it is dry also prevents the multiplication of fungal spores.
Treatment of fungi depends on the variety and the condition of the plant. Fungi that appears on the leaves of a houseplant will travel to the soil and multiply if the leaf is not removed. Remove and destroy infected plant material to prevent spreading. Root rot is rarely treatable unless it is caught early. If caught, the roots of the plant require dipping in a solution of one part bleach, nine parts water for 30 minutes. Once sterilized, the plant should be repotted in sterilized soil.
Fungicides are available from most home and garden stores, as well as online. Fungicides are applied according to the instructions on the label. Identifying the fungus before buying the pesticide is essential for proper treatment. Some university extensions provide a fungi identification service, requiring only that you send in a sample of the fungus. Fungicides, however, can cost more than a new plant.