Also called the dumbcane plant, the dieffenbachia is a houseplant that grows up to 6 feet tall and 3 feet wide. The dieffenbachia is beloved for its large, tropical leaves and easy adaptation to indoor environments. Many different problems can cause dieffenbachia leaves to turn yellow, including disease, insect infestations and cultural problems. You must first diagnose what's causing the yellowing leaves in order to properly treat the problem.
Study the dieffenbachia's leaves to find yellowing leaf margins. Leaf burn around the edges, along with stunted new leaves, can indicate over-fertilization. Sometimes yellowing leaves and stunted, small new leaves can be symptoms of root rot, insufficient light, pot-bound roots or a nutrient deficiency.
Look for bottom leaves yellowing and dropping from the dumbcane plant to diagnose a nutrient deficiency or pot-bound roots.
Look for small yellow or yellowish-orange spots on the diffenbachia's leaves to diagnose bacterial leaf spot. The yellowish spots will have translucent centers and grow to 1/2 inch in size. The infected leaves tend to wilt and die.
Look for yellowish leaves with a speckled appearance on your dieffenbachia to diagnose a spider mite infestation. Spider mites can also leave fine webbing near the leaf axils.
Repot the dieffenbachia into well-draining potting soil mix and a container with plenty of drainage holes in the bottom, if you suspect that the yellow leaves are due to root rot. Water the dumbcane plant thoroughly only when the top 1 inch of potting soil feels dry to the touch, and don't over-water.
Feed your dieffenbachia monthly or once every two months while it's actively growing with an all-purpose houseplant fertilizer, if you suspect that the yellowing leaves are caused by a nutrient deficiency. Fertilize only once or twice during winter, following the dosage and application instructions on the label.
Pour water into the potting soil until it drains freely from the bottom of the pot to leach out the salt and fertilizer buildup, if over-fertilization is the problem. Ensure that the dieffenbachia is potted in a loose, well-draining soil mix to help prevent salt and fertilizer-deposit buildup. Perform the leaching procedure at least once every month.
Remove the dieffenbachia from its pot and study the roots to determine whether the plant's yellowing leaves are caused by being pot-bound. If the roots circle around to fill the pot and little soil is left, repot the dumbcane plant into a container that's 1 to 2 inches deeper and wider than the current pot.
Control spider mite infestations by applying a least-toxic insecticidal product made for houseplants. Consult an agricultural extension service for the best treatment options.