Yellowing of the entire plant
Yellowing may result from too little light, too little fertilizer, insects or mites. Most often yellowing is a symptom caused by over-watering. As above, check for drainage in the pot, examine the roots and if most are dark, soft and dead, it is better to discard the plant.
Yellowing, browning, death of lower leaves, new growth looks OK
Yellowing and death of the lower leaves is generally caused by nitrogen deficiency. Pot-bound plants are especially susceptible. Regular application of fertilizer when the plant is growing vigorously and re-potting periodically in fresh soil avoids this problem.
Leaves are smaller, plants are tall and spindly
Too little light will cause plants to be pale, tall and weak. Consult a reference for light requirements for specific plants and ways to provide supplemental light indoors.
Sticky substance on leaves, sometimes accompanied by a black growth which can be washed off
Look for insects such as whiteflies, mealybugs, scales, or aphids. The black mold is called ‘sooty mold.’ It is growing on the sweet honeydew produced by the insects and is only secondary to the insect infestation, the the primary cause of the damage.