Which of the following diseases is being described? This disease forms dark spots on the base of stems and roots in cabbage plants. The plants turn yellow, wilt and die. The disease is spread by rain, wind and human activity.

The disease described is indeed associated with Blackleg, which specifically affects cabbage plants and is caused by the pathogen Leptosphaeria maculans. The characteristic dark spots on the base of stems and roots are a primary symptom of this disease. Additionally, Blackleg leads to yellowing and wilting of the plants, ultimately resulting in their death.

Blackleg can spread efficiently due to its distribution mechanisms; rain can splash spores to new healthy plants, wind can carry spores over distances, and human activities such as farming or movement of infected plant material can further disseminate the disease.

The other options do not align with the symptoms and spread mechanism described. Bacterial wilt primarily affects the vascular system rather than forming dark spots at the base of stems and roots. Bacterial spot generally presents as leaf spots rather than root or stem base issues, and Potato scab is a disease affecting tubers, not stems or roots in cabbage. Thus, the traits mentioned in the question clearly point to Blackleg as the correct answer.