Bacterial wilt is caused by a soil-borne bacterium named Ralstonia solanacearum (formerly known as Pseudomonas solanacearum). Potato wilt bacterium mainly inhabits the roots, and enters the root system at points of injury caused by farm tools or equipment and soil pests.

Procedure: Cut a 2-3 cm piece of stem from the base of the suspected plant. Discard the lower and upper parts; Tie the piece with the string provided; suspend the piece of stem horizontally in the glass full of clean clear water as illustrated in Figure 38 and wait for about 15 minutes. If the plant was infected with bacterial wilt, it will exude white smoky liquid / milky threads down wards from one or both ends of the cut stem.

https://snv.org/assets/explore/download/PotatoTraining%20Handbook%20WEB.pdf