Why Do My Potatoes Have Dark Spots Inside When Cut?

You cut open a potato that looks perfectly normal from the outside and find dark brown or reddish-brown spots, arcs, or blotches inside the flesh. This internal discolouration has no visible external sign, which makes it particularly frustrating — there is no way to sort affected tubers from healthy ones without cutting each one. There are two quite different causes of internal browning that look similar from the outside but have different origins and different prevention strategies.

Heat necrosis

Heat necrosis — also called internal heat necrosis or jelly end rot — is a physiological disorder caused by high soil temperatures and water stress during the tuber bulking phase. When the soil temperature exceeds around 25°C during periods of drought, the internal tissue of developing tubers can die, creating brown or grey necrotic spots and patches, often concentrated around the stolon end of the tuber. The damage is done during the growing season and cannot be reversed or treated after harvest. Prevention focuses on maintaining consistent soil moisture and mulching heavily to keep soil temperatures down during heat waves. Planting times that avoid the hottest soil temperatures during active tuber growth also help.

Spraing (tobacco rattle virus)

Spraing produces very distinctive brown curved arcs or circular rings inside the potato flesh — sometimes described as looking like rusted arcs or brown zebra stripes when a slice is cut. The external skin looks completely normal. Spraing is caused by tobacco rattle virus (TRV), transmitted by free-living nematodes (particularly Trichodorus species) in the soil. The nematodes carrying the virus are most common in light, sandy, or peaty soils and in ground that has been converted from old pasture or lawn. There is no treatment for an infected plant, and there is no cure for affected tubers — they are safe to eat but the affected flesh has an unpleasant texture and bitter taste.

Managing spraing

Crop rotation is important — avoid planting potatoes in ground where spraing has occurred for at least four years. Some potato varieties have better tolerance of TRV than others, though no variety is fully immune. Maris Piper and King Edward are known to be particularly susceptible; Desiree is somewhat less so. Watering to maintain consistent soil moisture reduces nematode activity (they are most mobile in very dry and then very wet soils). If spraing is persistent and severe on your plot, soil analysis to identify nematode populations may be worthwhile before choosing your next variety.

Distinguishing the two causes

Heat necrosis tends to produce scattered, irregular brown spots concentrated toward the stolon end and often appears watery or gelatinous. Spraing produces the characteristic curved arcs or rings in the flesh, often visible as a pattern when you cut a thin slice across the width of the tuber. If you see arcs and rings, the problem is TRV/spraing. Irregular spots without a pattern, especially in a hot summer, point to heat necrosis. Both are safe to eat with the affected areas removed, though neither tastes pleasant if the discolouration is extensive.