Sweet potato vines are frequently used as an accent plant in potted arrangements, or as a bedding plant. Their heart-shaped leaves come in black, dark purple and chartreuse green, adding a punch of color wherever they are planted. Browned leaves are likely caused by environmental conditions, such as drought or cold weather, but occasionally the plants may succumb to disease or insect infestations.
Sweet potato vines are native to the tropics and are not at all frost hardy. Even a mild late spring storm may cause the leaves to brown. If the leaves are brown, but not mushy, the plant will probably rebound and begin to produce new growth. Blackened or mushy leaves indicate that the plant has frozen and will not recover. Pull it up and discard it. Plant sweet potato vine after the last expected frost.
Root knot nematodes are tiny worms that feed on the roots of the sweet potato vine. In addition to brown or discolored leaves, you may notice stunted growth, especially during dry weather. As the nematodes feed, the plant is unable to take up water and nutrients, and eventually dies. There is no chemical cure for nematodes. To prevent nematodes, plant sweet potato vines in a clean pot with new potting mix each year. Rotate sweet potato vines grown in the ground with other plants, such as marigolds, which reduce nematode numbers.
Root rot and fusarium wilt may cause sweet potato vines to develop yellow or brown leaves that wither and drop. These diseases are more common in edible sweet potato crops than in ornamental sweet potato vines. Prevent the disease by planting healthy plants and giving them enough space so air circulates freely. Water the plants using a drip system, rather than overhead sprinklers, and water early in the morning so the leaves dry quickly. Amend the soil so the soil pH is between 6.5 and 7.0 and use nitrate nitrogen rather than ammonium nitrogen to reduce the incidence of fungal diseases. There is no cure for these diseases. Pull diseased plants to prevent spreading the disease.
A single ornamental sweet potato vine is not likely to succumb to insect attacks, but a large planting of the vines in a flower bed may attract bugs. Cucumber beetles, sweet potato flea beetles, grubs and leafhoppers all eat the tuberous roots of the plant. Most of the damage is evident on the roots, although the leaves may become distorted, yellowed or brown. Look for signs of insect infestation, such as eggs or insects on the undersides of the leaves, or a sticky substance known as honeydew secreted by leafhoppers and other leaf sucking insects. Consult a county extension agent to positively identify the pest and offer pesticide recommendations, such as insecticidal soap, Neem extract or rotenone.