Powdery scab is a disease of
potato tubers. It is caused by the
cercozoa
Cercozoa is a phylum of diverse single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, and are instead defined by molecular phylogenies of rRNA and actin or polyubiquitin. They were the first major eu ...
n ''Spongospora subterranea''
f. sp.
''Forma specialis'' (plural: ''formae speciales''), abbreviated f. sp. (plural ff. spp.) without italics, is an informal taxonomic grouping allowed by the International Code of Nomenclature for algae, fungi, and plants, that is applied to a par ...
''subterranea'' and is widespread in potato growing countries.
Symptom
Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign for example may be a higher or lower temperature than normal, raised or lowered blood pressure or an abnormality showin ...
s of powdery scab include small
lesions in the early stages of the
disease, progressing to raised
pustules containing a powdery mass. These can eventually rupture within the tuber
periderm.
[Maldonado, M. Loreto Hernandez, Richard E. Falloon, Ruth C. Butler, Anthony J. Conner, and Simon R. Bulman. "''Resistance to Spongospora Subterranea Induced in Potato by the Elicitor β-aminobutyric Acid.''" Australasian Plant Pathol. Australasian Plant Pathology 44.4 (2015): 445-53. Web. 21 Oct. 2015.] The powdery pustules contain resting spores that release anisokont
zoospores (asexual spore with two unequal length
flagella
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have f ...
) to infect the root hairs of potatoes or tomatoes. Powdery scab is a cosmetic defect on tubers, which can result in the rejection of these potatoes. Potatoes which have been infected can be peeled to remove the infected skin and the remaining inside of the potato can be cooked and eaten.
[Dennis A. Johnson and Thomas F. Cummings. ''Effect of Powdery Scab Root Galls on Yield of Potato.'' (2015).Plant Disease 99:10, 1396-1403]
Disease cycle
In general, not a lot is known about the life cycle of ''Spongospora subterranea'' f.sp ''subterranea'' (Sss). Most of the currently-proposed life cycle is based on that of ''
Plasmodiophora brassicae'', a closely related and better-studied
protozoa
Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
n. It has been proposed, due to this similarity, that there are two distinct stages that Sss can exist as; the asexual and sexual stages.
Asexual Stage: A
zoospore infects root tissue and becomes a
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