Final answer:
The causal agent of late blight is Phytophthora infests, managed today through synthetic pesticides and resistant potato varieties, combined with integrated pest management strategies.
Step-by-step explanation:
The causal agent of late blight is Phytophthora infestans, an oomycete that attacks potato plants, leading to decay of stalks and stems into black slime. This pathogen was responsible for the Irish potato famine in the nineteenth century. Managing late blight today involves using synthetic pesticides and cultivating resistant varieties of potatoes. However, the pathogen often evolves to overcome the plant's defenses, making continuous management and innovation in agricultural practices essential. Modern methods also include integrated pest management strategies that combine the use of pesticides with other practices to reduce the reliance on chemicals alone.