Given my recent borderline obsession with these fries covered in cheese, bacon and scallions served at the restaruant downstairs from my office, this should have been one of the parasites featured in the week of parasites of plants I love (May 10-17). Spongospora subterranea is a parasite of potatoes that causes a disease with the funny name of "powdery scab." It used to be called a slime mold and grouped with fungi, but is now considered part of the phylum Cercozoa. Free-swimming zoospores invade the roots of the potato and induce galls. There they produce the characteristic lesions that look a lot like scabs, which when they rupture and the spores are shed, look like, well, powdery scabs. Because the spores stay in the soil in enormous numbers, they can be really difficult to eliminate from a field. These parasites can be found throughout Europe, but some believe that they originally came from South America. Spongospora subterranea is the vector for a virus as well. We'll meet that parasite soon.
Photo is from this page.