Brown Spots & Yellow Leaves on a Dumb Cane Houseplant

Dumb cane (Dieffenbachia spp.) is a houseplant named for the toxic sap its canelike stems exude when cut or broken. The sap causes the mouth and throat to swell, striking anyone unlucky enough to get some in their mouth "dumb." The plant is favored for its ability to grow in a range of light conditions and its large, attractive leaves that may be deep green, variegated white, cream or gold with blotches, stripes or spots. If the plant develops brown spots and the leaves start to yellow, take steps to address the underlying problem and save your dumb cane plant. Planted outdoors, dumb cane grows well in U.S. Department of Agriculture plant hardiness zones 10b through 11.

Dumb Cane Culture

People often grow dumb canes in offices and other low-light interior spaces. The plants can tolerate the low light, but grow slowly. They thrive in bright filtered light in spring, summer and fall and bright light in winter. Aside from good light, your dumb cane needs to be placed where the temperature is 60 to 75 degrees Fahrenheit in an area free from drafts. They require water when the top inch of potting soil is dry; water the plant to a depth of one inch. A general houseplant fertilizer from spring through early fall keeps the plant growing well. With proper light, water and nutrients, the dumb cane quickly grows 3 to 6 feet tall.

Cultural Brown Spot Causes

Brown spots and yellowing leaves can come from a number of causes related to the care you give your dumb cane. Brown spots or patches at the margins of the leaves can be caused by over-fertilization -- the leaves can be burned by the soluble salts in the product. Plants should be watered until water comes from the bottom of the pot to leach excess fertilizer from the potting mix. On the other hand, leaves that turn yellow and are smaller and closer together on the plant's stems indicate poor nutrition, particularly low nitrogen. When dumb cane wilts and turns yellow from the bottom up, it needs to be watered more regularly.

Insect Damage

Mites are the most problematic insect pests to disturb dumb cane. The two-spotted spider mite can infest the plants at any time of year. These are tiny insects that suck sap from the leaves. They appear as small flecks and are often not noticeable until leaves yellow and are heavily speckled with the mites. The plant can die in a heavy infestation. Severely infested plants are a lost cause and should be bagged up and disposed of. Catch the problem early enough, and you can wash the plants down periodically with insecticidal soap or horticultural oil to eliminate the pests.

Bacterial and Fungal Diseases

Dumb cane is prone to several bacterial and fungal diseases resulting in brown spots and yellowing leaves. Xanthomonas leaf spot is a bacterial disease that causes dark brown spots surrounded by yellow halos. The entire leaf can collapse when the spots enlarge enough to hit a vein. The disease is promoted by over-fertilization. Anthracnose is a fungal disease with a similar look, though the spots can develop raised black fruiting bodies. Avoiding overhead watering to keep foliage dry, which helps prevent this problem.