Black Spots on Ornamental Sweet Potato Vine Leaf
Growing ornamental sweet potato vines (Ipomoea batatas) is not for the faint-of-heart. Perennials at U.S. Department of Agriculture plant hardiness zones 9 through 11 and annuals elsewhere, 6- to 12-foot sweet potato vines trail from containers and scramble over bare ground with leave. Their purple-black, chartreuse or variegated foliage and late-season trumpet flowers make bold visual statements, until the unsightly black fungal spots that follow pest infestations ruin the show.
Sweet Potato Vine Pests
Tiny, sap-sucking aphids and whiteflies feed on sweet potato vine leaves. Pear-shaped aphids leave the foliage curly, wilted and discolored. Melon or cotton aphids, frequently located on sweet potatoes, soften from pale or dark green in existence to yellow as temperatures rise. Whiteflies have white-winged, yellow bodies covered with white, powdery wax. In massive numbers they cause the leaves to yellow, wither and drop. Both insects congregate and feed on the foliage undersides. The waste that they secrete is what links them directly into black, fungus-spotted leaves.
Aphids and whiteflies consume more sap when they need for survival. They remove the rest as a gooey, carbohydrate-dense waste called honeydew. Huge populations of the insects drench a sweet potato vine using the clear, sticky substance. Ants and black sooty mold fungi generally feed on the honeydew. Sooty-mold infested vines have a velvety black patches on their leaves.
Because sooty mold fungi feed on honeydew, they don’t penetrate or directly damage sweet potato leaf tissue. Left unchecked, sooty mold may collect enough to keep sunlight from reaching the leaves. Inadequate sun exposure interferes with the vine’s photosynthesis, possibly limiting its development. The fungus-coated leaves sometimes fall and age prematurely. Even without these issues, having sooty mold on a vine grown mainly because of its ornamental foliage is less than desirable.
Ants worth honeydew as a food source so highly that they kill aphids’ and whiteflies’ natural predators to secure their supply. Eliminating ant colonies close to your sweet potato vine shields these predators, including ladybugs, lacewings and parasitic wasps. Control the issue with slowly acting boric acid ant bait put near the vines for the ants to transport back to their nests.
Together with the removal of honeydew-secreting insects, sooty mold fungi finally starve and wither away. If predators are not keeping aphids and whiteflies in check, a blast in the garden hose or pruning and disposing of heavily infested stems may remove light populations. Removing adult whiteflies using a handheld vacuum, and freezing the dust cup bag and its contents before disposal, may work. Avoid excessive fertilizing with nitrogen-based fluid which encourages tender, aphid-attracting new development.
Insecticidal soap, horticultural or neem oil, sprayed heavily enough to saturate both surfaces of the foliage, suffocate aphids and mealybugs. These treatments only kill the insects present during application, so you may need to repeat the spraying. Spray when the plants are well nourished and the temperature is below 80 degrees Fahrenheit. Always use these remedies depending on their tag specifications.