Phytophthora infestans is a water mold that causes the disease known as late blight, primarily affecting potatoes and tomatoes. It thrives in cool, moist conditions and can spread rapidly, leading to significant crop losses. The pathogen produces spores that can be carried by wind or water, making it highly contagious. This organism was responsible for the Irish Potato Famine in the mid-19th century, which resulted in widespread starvation and emigration. Effective management strategies include using resistant plant varieties, crop rotation, and fungicides to control its spread and minimize damage to affected crops.