Answer: Based on your description I suspect your dieffenbachia may be suffering from overwatering. The soil should be slightly damp like a wrung out sponge, not sopping wet. Water thoroughly, then let the soil almost begin to dry before you water again. Empty the saucer under the plant after watering so it does not sit in water. Give this plant bright but indirect light. Fertilize spring through fall with a water soluble fertilizer for foliage plants such as 10-10-10 plus minors. Follow the label directions for how much to use. Also, this plant can yellow if exposed to cold. The minimum temperature should be about 65 degrees. In winter, give it ample humidity for best results.
Also check it for signs of spider mites as these can cause yellowing. Spider mites are tiny but they leave a very fine webbing on the undersides of the leaves or where the leaf joins the stem. If you find this, spray with a commercially formulated insecticidal soap per the label directions.
Rubber trees or rubber plants do quite well in relatively small pots so do not rush to repot. When you do it, use a soil mix that matches the existing soil and use only a slightly larger container. Spring or early summer would be a good time to repot when it is needed.
Give this plant bright but indirect light also. Keep the soil slightly moist. Fertilize spring through fall with a water soluble fertilizer for foliage plants such as 10-10-10 plus minors. Follow the label directions for how much to use.
Enjoy your plants!
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