Why Is My Potato Stem Rotting Black at the Base?

A potato plant that wilts, yellows, and then collapses — where pulling the stem reveals a black, slimy, foul-smelling rot at the base of the stem and at the stolon attachment point — has blackleg disease. Blackleg is caused by bacteria in the Pectobacterium and Dickeya genera (previously all called Erwinia carotovora) and it is one of the most clear-cut diagnoses in the potato garden: the black, wet rot at the very base of the stem is unmistakeable. Understanding how it spreads and what to do when you find it prevents significant crop losses.

How blackleg spreads

The primary route of blackleg infection is through infected seed potatoes. The bacteria live latently in apparently healthy tubers, then activate and spread into the developing plant when conditions favour them — particularly warm, wet soil with poor drainage. Once established in a plant, the bacteria cause the soft wet rot in the stem that you see above and below ground. The bacteria also move through soil water, meaning neighbouring plants in the same wet, contaminated soil can become infected. Unlike fungal diseases, blackleg bacteria do not produce persistent spores in soil, so it is primarily seed-borne rather than soil-borne.

Identifying blackleg symptoms

The tell-tale sign is a distinct black to dark brown wet rot starting at the base of the stem and often extending several centimetres upward. The affected tissue is soft, slimy, and foul-smelling — a strong, unpleasant rotting odour is characteristic. The plant above the rot wilts and yellows, with the yellowing often starting at the leaf tips. Unlike blight, which starts at the top and moves down, blackleg starts at the base and moves up. The stem at soil level is visibly darkened and collapses. Dig down and you will often find the mother tuber has completely rotted.

What to do with infected plants

Remove infected plants immediately — stems, roots, and any remaining mother tuber — and dispose of them away from the vegetable garden. Do not compost them. Do not handle healthy plants immediately after touching infected ones without washing your hands. Check neighbouring plants regularly over the following weeks, especially if the season has been wet. There is no fungicide or bactericide treatment for blackleg — removal is the only response. Healthy neighbouring plants usually escape because the bacteria do not persist long in dry, well-aerated soil.

Prevention for future seasons

Buy certified seed potatoes from a reputable supplier each year — this is the most reliable protection because certified seed is tested for the most serious diseases. Improve drainage in your potato beds: blackleg is much more severe in wet, waterlogged conditions than in well-drained ground. Plant in raised ridges rather than flat drills. Avoid planting into very cold, wet soil in spring. Rotate potato ground on a three to four year cycle. If you save your own seed, only use tubers from plants that showed no disease symptoms at all through the season.