The correct answer is B. Phytophthora infestans.
Late blight of potato is a devastating disease caused by the oomycete Phytophthora infestans. It is a major threat to potato production worldwide, and has caused widespread famines in the past. The disease is characterized by the appearance of brown, water-soaked lesions on the leaves, stems, and tubers of potatoes. The lesions can quickly spread and kill the plant.
Phytophthora infestans is a water mold that requires free water to survive and spread. It is most active in cool, wet weather conditions. The pathogen can overwinter in infected tubers or in soil debris. It can also be spread by wind-blown spores or by contaminated irrigation water.
There is no cure for late blight, but there are a number of methods that can be used to control it. These include crop rotation, using resistant varieties, and applying fungicides.
A. Alternia solanii is a fungus that causes early blight of potato. It is a less serious disease than late blight, but it can still cause significant damage to potato crops.
C. Albugo candida is a fungus that causes white mold of potato. It is a relatively minor disease, but it can cause cosmetic damage to potatoes.
D. Fusarium is a genus of fungi that can cause a variety of diseases in potatoes, including Fusarium wilt and Fusarium rot. These diseases can be serious, but they are not as widespread as late blight.