How did late blight affect potato tubers as well as potato leaves?

Late blight, caused by the water mold Phytophthora infestans, impacts potato plants by causing distinct damage to both foliage and tubers.1

Effects on Leaves

When a plant is infected, lesions appear on the leaves, petioles, and stems. These lesions are round or irregularly shaped areas that range in color from dark green to purplish black, resembling frost injury. Additionally, a whitish growth of spore-producing structures may develop at the margins of these lesions on the undersides of the leaves.1

Effects on Tubers

Potato tubers develop rot that can reach up to 15 mm (0.6 inch) deep. This initial damage often facilitates secondary infections by fungi and bacteria, such as Erwinia species. These secondary invaders cause further rotting, leading to significant losses during storage, transit, and marketing.1

This rapid destruction of the crop was the primary cause of the Great Famine in Ireland, as the lack of genetic diversity in the potato population allowed the pathogen to spread unchecked.2