You have a problem: Plants are losing their leaves and failing to thrive. Upon closer examination, you find a white substance on the leaves. This white material could be one of two things: mealy bugs or powdery leaf mildew. Mealy bugs are soft, white insects that suck the juices out of plants and multiply rapidly. Leaf mildew is a fungal disease usually caused by high humidity and poor air circulation around plants. Both can be eradicated by following simple instructions.
Determine the type of white substance on your houseplant. Fungal disease will begin as circular white spots on the leaves and usually can be partially removed by rubbing the leaves. Mealy bugs will be found on the leaves where they join the stems or on the leaf veins.
For either problem, remove and destroy any severely damaged or infected leaves to avoid spreading.
For mealy bugs, mix a solution of 4 parts water to 1 part rubbing alcohol in a spray bottle. Spray one or two infected leaves heavily and watch the plant for a few days for any poor reactions to the alcohol, as some plants can be sensitive to this chemical. If no reaction is found, continue to spray infected leaves one time per week for several weeks or until the problem no longer persists.
For fungal leaf disease, set up a dehumidifier in the room with the infected plants and separate multiple plants to increase air circulation.
Spray the fungus on the leaves with a fungicide that contains neem oil, an extract from the tropical neem tree.