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-fertilisation. 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 centres 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 overwater.
Feed your dieffenbachia monthly or once every two months while it's actively growing with an all-purpose houseplant fertiliser, if you suspect that the yellowing leaves are caused by a nutrient deficiency. Fertilise 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 fertiliser build-up, if over-fertilisation is the problem. Ensure that the dieffenbachia is potted in a loose, well-draining soil mix to help prevent salt and fertiliser-deposit build-up. 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.
If you still aren't sure what's causing your dieffenbachia's leaves to turn yellow, contact your area's agricultural extension office. Extension offices can usually diagnose the problem by the plant's symptoms and recommend the safest, yet most effective, treatment.
Don't treat your dieffenbachia with a fungicide or other chemical if you suspect it has bacterial leaf spot. No widely-available treatments exist for this plant disease, but you can prevent it by keeping temperatures around the plant at 60 to 75 degrees Fahrenheit and no cooler. Also, ensure that the dieffenbachia isn't crowded together with other plants.