Powdery scab infection. Light micrograph of a section through a potato infected with powdery scab (Spongospora subterranea), showing pustules and spores. S. subterranea is a cercozoan protist that infects and destroys tubers. It causes raised brown pustules (red) on the tuber skin, which breaks and curls back over the pustule to form a brown powdery mass of spore balls (brown). Here, the lesions have formed a cork layer under the epidermis (skin, blue). Magnification: x7 when printed 10 centimetres wide.
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