Molds and fungi live all around us. In the garden, they serve a constructive purpose, eating away at decaying matter. Inside the house, though, they are unable to move on to the next job; they're stuck on the houseplant they came in with. Troubleshooting plants with mold and fungus problems requires a little knowledge and considerable patience. Some infections require quick response and some are just a nuisance. There are some actions, however, that can minimize them all.
Nip problems in the soil by sterilizing all soil before potting houseplants. Even "soilless" mixtures can carry slime mold spores that grow in yellow, orange or brown patches on the surface of the soil. Cover soil in a large pan with aluminum foil and bake at 180 to 200 degrees for an hour before use. When repotting plants, dip their roots into a pan of warm water and remove all of the old soil before replanting.
Water soil rather than foliage of houseplants to avoid fungal diseases of plant surfaces. Provide humidity with a mist around (not on) plants from a spray bottle or set plants on a tray of pebbles. Plants with red blotch or leaf spot develop the red or brown splotches first on outside edges of leaves where water sits. Water plants only when the soil surface is dry and put houseplants in pots with drainage holes to prevent mold growing in "sour" fermenting soil at the bottom of the pot.
Deal with fungi on leaves and branches immediately: Botrytis or gray mold, fungal leaf spot and powdery mildew will spread to neighboring leaves and plants. Treat plants with a houseplant fungicide or homemade spray with a teaspoon or two each of baking soda and mineral oil in a spray bottle filled with room-temperature water. Spray all areas of the plant, particularly young shoots and flower buds.
Keep plants clean by removing dead or affected leaves. Fungus thrives in decaying matter. After spraying plants with "upper fungal infections," take affected leaves off the plant and clean up any that have fallen on the soil. Keep leaves off the soil to allow air to circulate around the soil, too.
Use only soil that drains well. Most potting mixes need amendments of sand, vermiculite or perlite to drain properly; they stick together and cake when they get dry. The tendency is to keep watering the soil, which puts plants in a swamp sealed under a hardpan surface---the perfect place to develop crown or root rot.