Why Do My Sweet Potato Leaves Have a White Powdery Coating?

A white or greyish powdery coating on the upper surface of sweet potato leaves — spreading outward from initial patches to cover large areas of the leaf — is powdery mildew, caused by fungi in the genus Erysiphe or related genera. Unlike most plant diseases, powdery mildew does not need wet conditions to spread — it thrives in warm (20–28°C), dry weather with high humidity at night. It is most common in late summer and early autumn as nights cool down.

Identifying powdery mildew

Look for a white, powdery, talc-like coating on the upper surface of leaves. In early stages, small circular white patches appear on isolated leaves. As the infection spreads, the patches merge to cover most of the leaf surface. Severely affected leaves turn yellow and die. Unlike other diseases, the powdery coating can be rubbed off with a finger — this is the massed fungal spores. Lower leaves and older growth are usually affected first, with the infection spreading upward through the plant.

Why it appears late in the season

Powdery mildew thrives in the temperature and humidity combination of late summer: warm, dry days and cool, humid nights. It spreads by airborne spores and can travel significant distances on the wind. It is more severe in dense plantings where air circulation is poor and in plants that are stressed by drought, overwatering or other problems. A plant in good health with good air circulation resists infection more effectively than a stressed, crowded one.

Treatment options

For light to moderate infection: remove and destroy the most heavily affected leaves. Spray with a sodium bicarbonate solution (1 teaspoon bicarbonate of soda + a few drops of liquid soap in one litre of water) applied to the upper leaf surfaces every seven to ten days. This raises the pH on the leaf surface, creating conditions hostile to the fungus. Diluted milk spray (40% milk to 60% water) has also shown effectiveness in research trials. Sulphur-based fungicide is an organic option for severe cases.

Impact on the harvest

Light powdery mildew in late summer, when tubers are nearly mature, has minimal impact on final harvest yield — the tubers are already formed and are completing their size development. If mildew appears early in summer and covers large areas of leaf, photosynthesis is significantly impaired and tuber development may be reduced. In either case, harvest is not affected by the appearance of the leaves — check tuber size with a test dig and harvest at the right time regardless of foliage condition.