may be suffering from root mealy bugs. These root mealy bugs are found only on the roots of infested plants and do considerable damage to the plant. This lead to the plant rotting where the damage allows fugal to enter the plant tissues, due to this, they produce powdery white deposits in the soil. These bugs are tiny pinkish brown woodlice can be seen in the soil and roots. To get rid of this mealy bugs use regular applications of insecticide (Read carefully the label before you use any insecticide) watered into the soil, or immerse the plant pot up to the top of the soil in a bucket of insecticide with a couple of drops of washing-up liquid to help wetting the soil. Make sure the plant need to be carefully dried out after a soaking. Even you can add moth balls to the potting mix, but the chemicals in the moth balls can cause damage to the plastic pots. It is best used with clay pots for better results.