Indoor plants are less susceptible to diseases because they're kept in a controlled environment where certain diseases cannot thrive. Many different plant diseases can infect houseplants, however, most of which are caused by harmful fungi. The critical time for indoor plants is when you first bring them into your home. This is when you're most likely to see signs of diseases that the plants have acquired at the nursery or garden store.
Diagnose the fungal diseases root rot and stem rot by looking for wilted stems and leaves, often with brown or black tissue girdling the stems at the soil level. Treat this disease by avoiding over-watering your indoor plants and by repotting them in sterilized potting soil.
Look for a white powdery substance covering the leaves of your indoor plant to diagnose powdery mildew. Caused by the Oidium species of fungi, powdery mildew occurs in humid conditions. Remove the diseased leaves, avoid over-watering the plant and move the plant into a drier location.
Identify fungal leaf spots by looking for small brown spots that have yellow margins, some with a ringed pattern or small black dots. Remove and destroy all the diseased growth from the indoor plant and avoid getting water on the foliage.
Look for water-soaked spots on the indoor plant's leaves with a yellow halo around them to identify bacterial leaf spots. The spots grow and blend together to cover the entire leaf during wet conditions, or they turn reddish-brown and appear speckled during dry conditions. Pick off the spotted leaves, isolate the plant from other indoor plants and don't get any water on the foliage.
Diagnose anthracnose in your houseplants by looking for yellowing leaf tips that turn tan, then dark brown, often spreading to the entire length of the leaf edges and causing the leaves to die. Control anthracnose by removing and destroying the diseased leaves on the indoor plant, and refraining from misting the foliage.