Ho i have a diffenbachia and i noticed white spots on the leaves . The spots feel like cotton ? Could you please suggest the reason for this please.
Your dieffenbachia has in insect called mealy bugs. They attack plants that have been weakened by the air being too dry or from being overwatered or underwatered. Insect do not usually attack healthy plants.
This site has excellent advice on the care of this plant: http://plantcaretoday.com/how-to-care-for-a-diffenbachia-plant-growing-the-dumb-
But first you need to get rid of the mealy bugs.
Mealy bugs are white, soft-bodied insects that suck plant juices. They are covered with white, waxy threads that protect them from predators. They are normally found where leaves join stems or along leaf veins, and their feeding will cause leaves to turn yellow and drop.mealy bugs
Plants most commonly affected by mealy bugs include aglaonema, most ferns, some palms, ficus, some draceanas, pothos, philodendron, Norfolk Island pine, schefflera, dieffenbachia, jade plant, African violets, streptocarpus and yucca.
Control mealy bugs by pruning infested branches and removing and destroying heavily infested plants. These pests may also be controlled with insecticidal soap. For heavy infestations, spray a mixture of 50 percent rubbing alcohol and 50 percent water directly onto the insects. For smaller infestations, dab the insects with a cotton swab dipped in rubbing alcohol. Repeat this procedure every week until the bugs are gone. Make sure to test the soap and alcohol mixtures on a small portion of the plant prior to full application because some plants may be sensitive to soap or alcohol.
Repeat the spray twice a week for 4 weeks. Consistent spraying is the key to getting rid of this insect as eggs can hatch out and produce additional mealy bugs for at least 3 weeks.
For more information, see the following Colorado State University Extension fact sheet(s).
Insect Control: Soaps and Detergents
If you have more questions feel free to write again. Good luck!