Why Is My Dieffenbachia Turning Yellow?

Improper Sunlight

• Improper sunlight exposure may cause the foliage of the dumb cane plant to turn yellow. Although it easily adapts to different growing conditions, providing consistent light levels all year-round reduces chances of leaf discoloration. Dieffenbachias thrive in bright, direct light in fall, spring and summer as long as it is filtered by a light curtain. During the winter when light levels are low, remove the curtain to allow direct sunlight to the plant. This practice provides consistent, balanced sunlight all year and prevents harsh fluctuations that cause yellowing foliage.

Watering

• Over- and under-watering tropical dieffenbachias can cause yellowing foliage along with black leaf tips or edges and possibly premature leaf drop. Grow the plant in quality, well-draining potting mix to prevent water standing around its roots. Under-watering a dieffenbacia dehydrates it and causes the leaves to turn yellow. Conversely, over-watering prevents the roots from absorbing and transporting oxygen to the leaves, causing them to turn yellow and eventually drop. Ideally, water the houseplant thoroughly until excess water drains out through the holes, and let the soil dry to a depth of 1 inch before irrigating again.

Pests

• Pests such as aphids and spider mites attack dieffenbachias and sap essential juices from their leaves, causing infected parts to turn yellow and wilt. Aphids are tiny green or black pests that measure less than 1/8 inches in length, while spider mites are small pests that leave fine webbing near leaf axils. The pests puncture the foliage with their piercing mouth parts and sap juices. Rub infected leaves with cotton dipped in rubbing alcohol, or spray insecticidal soap or neem oil over pest infestations.

Disease

• Improper growing conditions increase the dieffenbachia's susceptibility to diseases that cause yellowing leaves, softening stems and stunted plant growth. Bacterial leaf spot causes tiny orange, yellow or yellowish-orange spots with translucent centers on the leaves. Growing 1/4 to 1/2 inches in width, the spots prevent the leaves from photosynthesis, causing them to turn yellow, wilt and eventually die. Fungal leaf spots cause lesions on young leaves and yellowing foliage. Snip off infected parts to inhibit the disease from spreading, avoid overhead irrigation and control temperature and humidity.