When planted properly, in the right soil, provided preferred light, and watered and cared for properly, most houseplants will have few if any serious disease problems. That said, even when everything is done right there can be disease.
Here's some disease prevention and control tips:
Look for disease. With the exception of powdery mildew, which appears as a white powdery substance on the surface of leaves, if a disease happens to pop up on a houseplant, or a leaf is damaged, remove any infected foliage and discard.
Soil moisture. If a plant looks diseased or discolored, check soil moisture. More houseplants are die or are killed due to improper watering than anything else. The rate of water loss and thus the need to water your houseplants depends upon temperature, humidity, and light levels as well as the type of plants you have. Thus, it is hard to set a strict watering schedule. Know the requirements of your plants and use the "touch method" to evaluate soil moisture and the need to water, or not. Press the tip of your finger down about 1/4" into the soil. A cool, damp feeling indicates there is still adequate moisture in the soil and a dry feeling indicates that you should water. If you have a lot of house plants, consider getting a water moisture meter to test soil.
Keep plants clean! Dust and dirt on leaves block light and reduce photosynthetic activity. This causes decreased vigor and gradual decline in the plants. There are many ways to clean your houseplants. Larger plants can be put directly in the sink or shower and sprayed with water. Smaller and more delicate plants can be turned upside down (use your hands to hold back the soil) and dunked and swirled around in a sink or bucket of water. Let the leaves dry completely before exposing them to direct sunlight. A wet cloth or sponge can be used to wipe foliage of plants with very large leaves. For a glossier surface, wipe the leaves with a piece of soft cheesecloth or even easier, spray the leaves with a Plant Shine product.
Avoid splashing. Water plants at the base to avoid splashing water on leaves.
Mulch is good. Mulch under and around plants to avoid leaf to soil contact.
Soil is important! Plant houseplants in well-drained soils to avoid root rot, leaf spot and other plant diseases caused by wet feet. Unless a specialty fertilizer is required, as is with African violets, cactus and orchids, I usually go with a lightweight, premium or professional potting mix. Avoid those cheap, dollar-a-bag potting soils you find at the box stores.
Let them breathe. Space plants properly in the home instead of overcrowding. Overcrowding doesn't allow for good air circulation; one of the best preventive measures for disease and fungus.
Homemade Remedies for Treating Fungal Diseases
Try these homemade recipes as a possible remedy and prevention for disease - they cant hurt:
Mix two tablespoons of baking soda into a quart of water. Pour into a spray container and spray affected areas. Repeat this process every few days until problem ceases.
Powdery Mildew: Mix equal parts milk and water and spray on infected plants. Three treatments a week apart should control the disease.
Insects and Fungal Diseases: Combine one tablespoon of cooking oil, two tablespoons of baking soda and a few drops of Ivory soap into a quart of water. Pour into a spray container and spray foliage.