Indoor plants sometimes operate at a bit of a disadvantage because they are living in a "foreign" environment that does not always give them all the advantages of sunlight, air circulation and fresh air. They are, however, in a position to be more closely monitored by you than their outdoor relatives, so you can often catch and diagnose indoor plant diseases in time to take action before your plant dies.
Check your plant for mites and scale bugs. Mites are small, red or black bugs that are about the size of grains of ground pepper. You may spot them on the insides of leaves, but you are more likely to see the webbing the leave in the crooks of stems and in areas near the base of the plant. If you have scale bugs, then you may see them or you might note a black coating on the plant where mold or mildew is growing thanks to their honey dew excretions.
Look for spots on the leaves. A number of fungal infections in house plants manifest by leaf spotting. Shot-hole disease creates yellow spots with black centers that eventually rot away, leaving holes in the leaves. Anthracnose leaves blotchy, dead areas, while other leaf-spot diseases can cause yellow, purple, brown or black spots and holes in the leaves.
Monitor your indoor plant's response to watering. If it appears wilted and parched but does not perk up with water, then it probably has a "wilt," meaning that a fungal infection has attacked its roots and is preventing the uptake of water and nutrients.
Look closely for black, gray, white or pinkish coatings on the leaves, stems or flowers. These coatings can be powdery mildew or other forms of mold. They tend to occur when the foliage is wet and does not receive adequate air circulation.