Bacterial wilt (brown rot of potato): how to recognise and treat it

Disease

Symptoms

The youngest leaves wilt first, then the whole plant wilts quickly in warm weather while the leaves stay green, even though the soil is moist.
A cut stem is brown inside; drops of white or cream bacterial ooze may appear on the cut surface.
Streaming test — put a freshly cut piece of the lower stem in a glass of clear water; within 3–5 minutes milky threads stream from the cut end. Fusarium and Verticillium wilts do not do this.
Potato tubers have a brown ring under the skin when cut, with creamy ooze appearing on it after a few minutes; ooze can also come out of the eyes, so soil sticks to them.

In the EU and UK this is a quarantine disease — report suspected cases to your national plant health authority (in England and Wales, APHA) and follow their instructions.
Remove and destroy all infected plants, roots and tubers; do not compost them or leave potato waste lying around.
Clean soil off tools and disinfect them, for example with a household bleach solution, so you do not carry the bacteria to other beds.

There is no chemical control for bacterial wilt; it cannot be cured, only prevented.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Plant only certified seed potatoes and healthy transplants; never plant potatoes bought for eating.
Rotate for at least three years with crops such as corn, beans or cabbage — not with pepper, eggplant, potato, sunflower or cosmos — and remove volunteer potatoes and weed hosts.
Do not water with river or ditch water where watercourses are known to be contaminated; woody nightshade growing in water can carry the bacteria.
Where the soil is infested, grow tomatoes, peppers and eggplants on resistant grafted rootstocks or in a separate, clean bed.

Figures checked against these sources.