Blackleg

What you are looking at

One potato plant in a row is wrong. The shoot came up, then yellowed, stunted and rolled its leaves upwards while the plants either side grew away normally.

Follow the stem down to the soil. The bottom few centimetres are inky black, soft and slimy, and the whole plant lifts or topples with almost no resistance because the stem has rotted through at the base. It smells foul.

Underground, the seed tuber has usually turned to a wet cream or grey mush, and any new tubers rot from the stolon end inwards.

The other place it appears is in the sack. Tubers that looked sound at lifting collapse into slime weeks later, and one takes several of its neighbours with it.

It came in on the seed tuber

That is the fact the rest of the page hangs from. This bacterium lives inside apparently healthy potatoes, and it travels from one season to the next in planting stock rather than in the ground.

It does not persist well in soil. Without potato debris to live in it dies out within a season or two, which makes it far less intractable than clubroot or white rot and means a normal rotation helps.

What it needs to cause damage is moisture and a lack of oxygen. Cold wet soil around a seed tuber, a waterlogged bed, a cut face on a tuber, or a polythene bag in a damp shed all supply that. Plant a clean tuber into warm ground that drains, and lift and cure the crop dry, and the same bacteria that would have destroyed the crop do nothing at all.

Certified seed is the whole of the prevention. It is grown under inspection in regions chosen for low disease pressure, and it is why blackleg is a nuisance in British gardens rather than a catastrophe. Saving tubers from a crop that had it reproduces the problem faithfully.

What to do about it

Buy certified seed potatoes each year rather than saving your own or planting supermarket ones.

Plant them whole. Do not cut them to make the bag go further.

Wait for the soil to warm and to drain rather than planting to a date.

Pull any affected plant as soon as you see it, with its seed tuber and its new tubers, and bin or burn the lot.

Lift the crop on a dry day, dry the tubers off, and cure them in the dark for a fortnight before storing.

Sort ruthlessly and store nothing that is cut, marked, soft or slug-holed.

Use paper or hessian sacks somewhere cool, dark and airy rather than polythene.

Check the store every few weeks and take out anything soft.

Pull volunteer potatoes out of the rotation.

What it gets mistaken for

Blight begins on the leaves as brown blotches with pale margins, works down the plant, and turns the tubers a firm reddish-brown from the outside in. Blackleg starts underground and rots wet and soft.

Potato scab roughens the skin and stops there.

Waterlogging kills plants across a whole bed together, on the wettest ground, without blackening the stem base.

Slug damage bores holes into tubers, and the soft rots that follow into those holes are the same family of bacteria arriving by a different route.

Frost damage on early shoots blackens the leaves and growing tips above ground and leaves the stem base sound.