Yellow leaves on sweet potato vines, a symptom known as chlorosis, signal that the plant is under stress. Chlorophyll, the pigment responsible for the plant’s green color and its ability to perform photosynthesis, is either not being produced sufficiently or is breaking down faster than it can be replaced. Because sweet potatoes are vigorous growers, their yellowing leaves are an urgent sign of an imbalance that needs prompt identification. Diagnosing the specific pattern and location of the yellowing is the most effective way to determine the root cause, which can range from improper cultural practices to nutrient deficiencies or biological attacks.
Water, Temperature, and Light Stress
Sweet potatoes require specific environmental conditions to thrive, and deviations from these can quickly manifest as chlorosis. The most common issue is improper water management, as both too much and too little water cause leaf yellowing. Overwatering or soil with poor drainage starves the roots of oxygen, leading to root stress and failure to absorb nutrients. This often causes the lower, older leaves to turn yellow first.
Conversely, underwatering causes the plant to cannibalize the chlorophyll in its lower leaves to conserve resources, resulting in wilting and yellowing. The tropical nature of the sweet potato means it is highly sensitive to temperature fluctuations. Cold temperatures, especially below 55°F, will cause growth to slow or stop, and leaves may turn yellow or purple due to chilling stress.
Extreme heat, particularly when coupled with low humidity, can also cause leaves to scorch and yellow along the margins. The plant requires full sun exposure, a minimum of six to eight hours of direct light daily, to produce the energy needed for healthy leaves. Insufficient light results in pale, weak, and yellow-green foliage because the plant is unable to produce adequate chlorophyll.
Identifying Key Nutrient Deficiencies
When environmental factors are ruled out, the pattern of leaf yellowing becomes a map for identifying specific nutrient deficiencies. Mobile nutrients, like nitrogen and magnesium, are relocated by the plant from older leaves to support new growth. Therefore, their deficiency symptoms appear first on the lower, older foliage. Nitrogen (N) deficiency is characterized by a uniform light-green or yellow-green color across the entire leaf, starting at the base of the plant.
Magnesium (Mg) deficiency also begins on older leaves but displays a distinct interveinal chlorosis. The tissue between the leaf veins turns yellow while the veins themselves remain a contrasting green. Immobile nutrients cannot be easily moved from older to newer tissue, so symptoms show on the youngest leaves first. Potassium (K) deficiency often presents as yellowing that begins along the edges or margins of the older leaves, sometimes followed by browning or necrotic spots.
The availability of nutrients is highly dependent on soil pH. Even if a nutrient is present in the soil, an incorrect pH level—either too acidic or too alkaline—can chemically “lock up” the element, preventing the sweet potato roots from absorbing it. A soil test is the best approach to confirm a deficiency and adjust the soil’s chemistry to make existing nutrients accessible.
Biological Threats
When yellowing is accompanied by other signs like spots, sticky residue, or distorted growth, the cause is likely a pest or disease. Sap-sucking insects like aphids, whiteflies, and spider mites feed on the plant’s juices, causing localized damage. This feeding results in tiny, spotty yellowing known as stippling, or the yellowing of leaf tips and margins, a symptom sometimes called “hopperburn” when caused by leafhoppers.
Inspection of the leaf undersides will often reveal these pests, which can be managed with a strong jet of water or horticultural soap. Diseases, often fungal or viral, typically present more severe and systemic symptoms. Fungal pathogens like Fusarium or Verticillium wilt cause leaves to yellow, often starting at the base, and are accompanied by wilting or a general decline in the plant’s vigor.
Viral infections, such as Sweet Potato Virus Disease, are harder to treat. They can cause distinct patterns like mottling, vein banding, or general chlorosis alongside stunted or distorted leaves. Since viral diseases are often spread by insect vectors, controlling pests is an important preventative measure. If a disease is suspected, removing and destroying the affected plant material is often the only way to prevent its spread to nearby healthy plants.