I have 2 different varieties of dieffenbachia and as you can see they both have yellowing leaves. I'm not sure what the problem is and the leaves are turning yellow at a much higher rate than normal. I've had them about 5 months and they did great in the beginning with no leaves turning yellow but in the last 2 or 3 weeks, it is usually several leaves all the time. The bigger one just looks droopy. I know the backside of their leaves are supposed to be white, but now they get greener and that's when I know they'll be yellow soon. On some of the leaves, the underside looks almost oily or wet? I know they can get like this when they're maturing, with too much/too little sunlight/water/humidity. I'm not sure what it could be.
Just a thought! I had a dief come into the plant clinic this morning that looks much like the pic of yours. Have you checked the base of the leaf, where the leaf ends and the stem begins? In that valley (sort of) look for tiny yellow to white blobs that look like cotton. Check all the little crevasses for these. If you find these little critters they are mealy bugs and are sucking the sap out of your plant. Cotton swabs dipped in rubbing alcohol will kill them. Depending on it's size you may want to just take it out side and spray it with half alcohol and half water. Let me know what you find. barb
