Why Do My Potato Leaves Have Brown Spots?

Brown spots and lesions on potato leaves can be caused by several different problems, from fungal disease to nutrient deficiency, and telling them apart matters because the responses are very different. The size, shape, and pattern of the spots, along with what time of year they appear, usually give you enough information to make a confident diagnosis. Here is how to work through the main causes.

Early blight (Alternaria solani)

Early blight causes distinctive dark brown spots with concentric rings inside them, creating a target-like pattern. The spots are typically angular with a yellow halo around the brown centre, and they start on the lower, older leaves first before progressing upward. Early blight is caused by the fungus Alternaria solani and is most common in hot, dry weather with occasional humid periods — a classic summer weather pattern. It rarely kills the plant outright but reduces vigour and can affect tuber quality. Removing heavily affected lower leaves, avoiding overhead watering, and ensuring good airflow around plants all help. Copper-based fungicides applied at the first signs can slow its spread.

Late blight (Phytophthora infestans)

Late blight is far more serious than early blight and needs urgent action. It causes pale green to brown, water-soaked patches — not neat circular spots but irregular, spreading blotches — that develop a white furry border on the underside of the leaf in humid conditions. Late blight spreads extremely rapidly in warm, wet, or foggy weather and can destroy an entire plant in days. If you see these symptoms, you are dealing with the same disease that caused the Irish potato famine. Remove infected leaves immediately, apply a copper-based fungicide to unaffected foliage, and prepare to harvest tubers early if the haulm collapses. Do not compost any affected material.

Magnesium deficiency

Magnesium deficiency produces yellowing between the leaf veins of older leaves, which can turn brown and crispy as the deficiency worsens. Unlike fungal disease, the damage follows the vein pattern clearly and there are no water-soaked edges or concentric rings. Heavy rain leaches magnesium from sandy soils rapidly. A foliar spray of Epsom salts (magnesium sulphate) at 20g per litre, applied to the leaves, is a fast and effective corrective treatment and is particularly useful mid-season when you need a quick response.

Physical damage and sunscald

Potatoes exposed to very intense direct sun, especially after a period of cooler, overcast weather, can develop pale brown bleached patches on the upper surface of leaves. This sunscald is not infectious and does not spread — the damage is localised and the plant recovers. Similarly, physical impact from heavy rain, hail, or overhead irrigation can cause small brown spots that look like disease but heal without spreading. If spots are only on the upper surface, limited to one area of the plant, and not progressing, weather damage is the likely explanation.