Houseplants aren't immune to fungus diseases and, if you don't eliminate the disease, some may claim the lives of your plants. The first step in treating these diseases is identifying them. Most diseases can be controlled through changing conditions in the plant's immediate environment and applying a fungicide. In severe cases of fungus, especially if root rot is involved, plants may need to be repotted or discarded entirely.
Put on a pair of rubber or vinyl gloves. Your should have at least one pair per plant so that you do not inadvertently spread disease between plants as you touch the leaves to look at them. Change gloves after touching a plant, before you touch another.
Check plant foliage for white or grey, dust-like areas. Rub the area lightly with your finger. If some of the substance comes off, this may be powdery mildew. According to the Plant Clinic at Cornell University, this is a very common fungus disease, and actually several kinds of fungi can be responsible for powdery mildew. Treatment includes moving the plant to a drier location of the home, removing badly affected areas of the plant and spraying it with a fungicide for indoor plants.
Look for yellow or brown spots on leaves; these spots may or may not be peppered with tiny black spots. According to Purdue Cooperative Extension Services, the probable cause is fungal disease (many type of fungus can cause this). They recommend removing affected leaves, if only a few are affected. If the problem seems widespread, you should spray with a fungicide. As with plants affected by any fungus, move these to a drier location.
Observe the plant for signs of root rot. The University of Minnesota Extension Service lists plant wilting--which is unchanged by watering it--and yellowing lower leaves as two signs of root rot. To fully diagnose root rot, remove the plant from the pot and check the roots. Roots affected by fungus will appear weak and often discolored and slimy. Fungus that cause root rot are usually present in the soil; you'll have to completely repot the plant in fresh, sterile soil if it is to have a chance at surviving.