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Common scab is a plant disease of root and tuber crops caused by a small number of '' Streptomyces'' species, specifically '' S. scabies'', '' S. acidiscabies'', '' S. turgidiscabies'' and others. Common scab mainly affects
potato The potato is a starchy food, a tuber of the plant ''Solanum tuberosum'' and is a root vegetable native to the Americas. The plant is a perennial in the nightshade family Solanaceae. Wild potato species can be found from the southern Unit ...
(''Solanum tuberosum''), but can also cause disease on
radish The radish (''Raphanus raphanistrum'' subsp. ''sativus'') is an Eating, edible root vegetable of the family Brassicaceae that was domesticated in Asia prior to Roman Empire, Roman times. Radishes are grown and consumed throughout the world, be ...
(''Raphanus sativus''),
parsnip The parsnip (''Pastinaca sativa'') is a root vegetable closely related to carrot and parsley, all belonging to the flowering plant family Apiaceae. It is a biennial plant usually grown as an annual. Its long taproot has cream-colored skin an ...
(''Pastinaca sativa''),
beet The beetroot is the taproot portion of a beet plant, usually known in North America as beets while the vegetable is referred to as beetroot in British English, and also known as the table beet, garden beet, red beet, dinner beet or golden beet ...
(''Beta vulgaris''), and
carrot The carrot ('' Daucus carota'' subsp. ''sativus'') is a root vegetable, typically orange in color, though purple, black, red, white, and yellow cultivars exist, all of which are domesticated forms of the wild carrot, ''Daucus carota'', nat ...
(''Daucus carota''). This plant disease is found wherever these vegetables are grown. Common scab symptoms are variable and can range from surface russeting to deep pits in root and tuber vegetables. This disease does not usually affect yields, but it can greatly reduce quality of the harvested vegetables and make them unsuitable for sale. Root and tuber vegetables are susceptible to infection by ''Streptomyces'' species as soon as the root or tuber forms, but, because this disease only affects root and tubers, the symptoms are not usually noted until harvest. Dry soils increase disease incidence and severity, therefore proper irrigation can aid in control of this disease. Common scab is suppressed if the
soil pH Soil pH is a measure of the acidity or basicity (alkalinity) of a soil. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics. pH is defined as the neg ...
is lower than 5.2, although scab lesions may still form in low pH soils due to physiological stresses or ''S. acidiscabies''.


History

The first known reference to common scab dates back to 1825 when it was included in
John Claudius Loudon John Claudius Loudon (8 April 1783 – 14 December 1843) was a Scottish botanist, garden designer and author. He was the first to use the term arboretum in writing to refer to a garden of plants, especially trees, collected for the purpose of ...
''Encyclopaedia of Agriculture''. It was not known what caused it then, it was not thought to have a biological cause, but was noted to be associated with soils treated with alkaline materials. In 1884,
Worthington George Smith Worthington George Smith (25 March 1835 – 27 October 1917) was an English cartoonist and illustrator, archaeologist, plant pathologist, and mycologist. Background and career Worthington G. Smith was born in Shoreditch, London, the son of a ...
suggested that it was caused by grit in soil irritating potato tubers as they grew. The theory gained much support, as it was known to be more common on light gritty soils. In 1890 however,
Roland Thaxter Roland Thaxter (August 28, 1858 – April 22, 1932) was an American mycologist, plant pathologist, botanist, and entomologist, renowned for his contribution to the insect parasitic fungi—Laboulbeniales. His college education was completed at Har ...
isolated a microbe that could cause common scab lesions, naming it ''Oospora scabies''. Over the years, this species was renamed several times, now being known as '' Streptomyces scabies''. In 1977, the acid-tolerant '' S. acidiscabies'' was found to also cause common scab, and then in 1996, '' S. turgidiscabies'' was isolated.


Hosts and symptoms

Common scab is a prevalent plant disease that is caused by the bacteria Streptomyces and it can affect a wide range of taproot crop hosts. Some examples of Streptomyces hosts are potatoes, beets, carrots, parsnips, radishes, rutabagas, and turnips. There are a few different symptoms that can arise from common scab. In some hosts, brown spots or lesions can appear on the surface of the root, and the severity can vary from mild spotting to deep pits on the surface. These lesions can cause secondary infections or attract nearby organisms and cause a lot of damage to the root. These symptoms are the same across the whole range of hosts. Although the disease does not affect the overall harvest, the appearance of infected plants is ruined. In crops such as potatoes, this can mean an unsellable crop and cause an extensive amount of loss for a farmer.


Disease cycle

The life cycle of Streptomyces scabies or common scab in potato starts out as the pathogen overwinters in tubers left behind in the soil. As spring comes around, some of the hyphal like growths from mycelium develop cross walls that break into asexual spores and disperse through wind, rain, or movement of soil. The spores infect developing roots/tubers through natural openings or wounds. Mycelia grow through the external cell layers and allow the pathogen to feed on plant tissue. As the pathogen feeds, it produces lesions that are seen with common scab. This process repeats after the pathogen overwinters the following season.


Management

Many different management styles can be used to lessen the severity of common scab. One of these can be crop rotation with non-host crops. This is a worthwhile strategy if there is enough land to work with. By rotating fields from root crops to non-root vegetables such as soybeans, the strength of the pathogen is reduced. If soil moisture is kept above 90% for the initial growth of root crops, this is another effective way of reducing the pathogenicity of streptomyces. Acidic soil (lower than pH 5.4) is also beneficial in eliminating the growth of streptomyces. By adding acidic fertilizer to a field over the course of a few years, the pH will not be suitable for the pathogen to grow. Finally, growing scab resistant cultivars will greatly improve crop appearance in the presence of streptomyces by decreasing the number of infected cultivars.


See also

* List of potato diseases


References


External links

*http://plantclinic.cornell.edu/factsheets/commonscabpotato.pdf *{{cite journal , last1=Babcock , first1=M. J. , last2=Eckwall , first2=E. C. , last3=Schottel , first3=J. L. , title=Production and regulation of potato-scab-inducing phytotoxins by Streptomyces scabies , journal=Journal of General Microbiology , date=1 July 1993 , volume=139 , issue=7 , pages=1579–1586 , doi=10.1099/00221287-139-7-1579 , doi-access=free *https://projects.ncsu.edu/cals/course/pp728/Streptomyces/Streptomyces_scabies.htm *https://cropwatch.unl.edu/potato/common_scab Potato diseases