House plants can develop many problems but most have environmental or cultural causes. If something is actually attacking a house plant it is usually an insect. Diseases are not common on most house plants.
Insects most often encountered are mites, aphids, mealybugs, scale, whitefly, and various soil insects. Some pesticides which control insects are harmful to the plant. Be sure to read the pesticide label and follow all instructions. Any single pesticide does not usually kill all pests and repeat applications are usually necessary.
Mites or spider mites are hard to see. They are extremely small and a magnifying glass is usually needed to see them. Plants infested with mites lose their green color and appear bronzed or washed out. In severe cases, the mites will form a fine webbing that may cover all or part of the plant. Once a plant is infested with mites, control will be difficult. Mite infested leaves may have a gritty feel, or look like they are covered with a fine coat of ashes.
Aphids are common on house plant insects but fortunately, are easily controlled. Aphids suck sap from the plant and can cause new growth to be distorted. Aphids are found on new growth and the undersides of the leaves. Heavy infestations cover the plants with a sticky syrup called honeydew. Aphids can be controlled with most commonly available house plant insect sprays.
Mealybugs look like little white tufts of cotton so are often mistaken for a disease. They are normally found on the undersides of leaves or on stems at the point where a leaf joins. The white, waxy coating protects the insects from sprays, making control difficult. Add 1/2 teaspoon of household detergent to each gallon of spray solution to ensure the mealybugs are wet.
The adult whitefly is a small white "fly" while the immature stage is scale-like and doesn't move. Moving infested plants causes the adults to fly away. Controlling whitefly is difficult and repeated sprays will be needed. Avoid plants such as fuchsia that are favored by the insect.
Scale insects often build up to large numbers because they go undetected. Their shell protects them from pesticides. Scales are usually found on stems and the undersides of leaves but can be on top of the leaves. Small infestations can be removed by touching each insect with a cotton swab dipped in alcohol.
Soil insects are noticed when brought to the surface during watering. Adults and larvae of several insects may fly or crawl around on the soil surface. In most cases they do no real harm to the plant. Large populations can cause wilting and poor plant growth due to minor root pruning. Unfortunately, pesticides used to control soil insects may be as harmful to the plant as the insects.
House plant diseases are not seen as often as insects. Diseases such as powdery mildew or various root and stem rots are encountered, but can be controlled to some extent with proper plant care. Most problems resembling diseases are the result of improper care.