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There are black marks inside my homegrown potatoes

Val Bourne / 20 November 2018

A reader wonders why their potatoes have dark patches inside.

Stored potates


My homegrown potatoes look fine from the outside, but when I cut into them there are often dark patches inside, some small but some quite large. What could be causing it?


There could be a variety of reasons for this. If you’re storing homegrown potatoes it may be that they've got slightly frosted. This will produce grey black patches inside what appears to be a perfect potato. It's really important to find a frost-free position and, in cold winters, a garden shed doesn't offer enough protection.

Lots of potato diseases can occur especially if potatoes are planted too early on waterlogged soil in cold conditions. Most of these diseases affect main crop potatoes so it's a good tip to wait until mid-April before planting these. There is a myth that potatoes need planting on Good Friday, a religious superstition that cropped up in Elizabethan times. Potatoes, which weren't mentioned in the Bible, were thought to be the devil's food. Potatoes are high-altitude plants from South America and they're prone to frost damage so Easter is usually too early.

It's mainly maincrop potatoes that suffer from disease, because they're in the ground for a longer time. Potato blight, a fungal disease that usually occurs in August when the weather’s warm and wet, is very common. Yellow lesions appear on the foliage and it flops very quickly. If you spot diseased foliage on any potato cut it away, making sure not to put it on the compost heap. Blight can get it into tubers, making them inedible.

You can get blight-resistant varieties, but these tend to be rather tasteless. The following maincrops, ‘Cara’ and Valor’ are known for their disease resistance and they perform well in gardens or on allotments.

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The opinions expressed are those of the author and are not held by Saga unless specifically stated. The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.