Synchytrium endobioticum

Synchytrium endobioticum is a chytrid fungus that causes the potato wart disease, or black scab.[1] It also infects some other plants of the genus Solanum, though potato is the only cultivated host.

Synchytrium endobioticum
Scientific classification
Kingdom:
Fungi
Division:
Chytridiomycota
Class:
Chytridiomycetes
Order:
Synchytriales
Family:
Synchytriaceae
Genus:
Synchytrium
Species:
S. endobioticum
Binomial name
Synchytrium endobioticum (Schilb.) Percival 1909

Systematics

Traditionally, Synchytrium endobioticum has been placed to the subgenus Mesochytrium, but it has been suggested that on the basis of the mode of germination it should be transferred to the subgenus Microsynchytrium. It was first identified and studied by Vera Charles. The New Zealand scientist Kathleen Maisey Curtis also studied Synchytrium endobioticum for her doctoral thesis that, in 1919, resulted in her being recognised as producing groundbreaking research on the organism's pathology.[2]

At least 18 pathotypes of the fungus exist, most of them with quite limited ranges in Central Europe. The most widely distributed is the pathotype 1.

Morphology

Like some other Chytridiales, Synchytrium endobioticum develops no mycelium. The fungus produces a thick walled structure known as a winter sporangium. It is 25-75 µm in diameter and contains 200-300 spores. Sporangia are clustered into thin-walled soruses. The motile life stage, zoospore is about 0,5 µm in diameter and has one posterior flagellum.

Life cycle

In spring, at higher temperature and moisture, overwintering sporangia germinate to release motile zoospores which infect suitable host epidermal cells. In infected cells, the summer sporangia develop, which quickly release new populations of zoospores. The infection cycle may be repeated as long as infection conditions are suitable. The infected plant cells swell, divide and surround the dividing zoospores resulting in the wart.

Under certain stress conditions some zoospore pairs fuse, resulting in a zygote. The zygote bearing host cells divide, forming eventually the walls of a new winter sporangium. In autumn, the warts rot and disintegrate, releasing new thick-walled resting spores of the fungus into the soil. The diploid resting spores (pro-soruses) undergo a dormancy period and before germination (probably) a meiotic division and several mitotic divisions, becoming a sorus.

Ecology

Synchytrium endobioticum is an obligate parasite that infects several plants of the genus Solanum. The most favourable conditions for its development are warm temperatures (but not over 20 °C) with enough humidity. Winter sporangia can remain viable for up to 20–30 years. It can survive at depths of 50 cm in the soil. Three different fungi have been observed to parasitize the resting sporangia.

S. endobioticum originates from the Andean region of South-America, with now almost worldwide distribution in areas where potatoes are cultivated (absent in most of tropical Africa, Middle East, most of Canada, Japan and Australia).

Environment

Cool and wet soils are conducive to potato wart disease development. While there are discrepancies in the reported temperatures for maximal potato wart infection, summers with an average temperature of 18 °C or less, winters below 5 °C, and annual precipitation of 700 mm or more are considered to be favorable for the disease.[3] Soil pH does not appear to be associated with disease occurrence; S. endobioticum infections have been reported in soils with pH ranging from 3.9-8.5.[4]

Overwintering sporangia of S. endobioticum are extremely resilient. Dry sporangia can survive at 100 °C for 11–12 h,[5] composting for 12 days at 60– 65 °C, pasteurization for 90 min at 70 °C,[6] the digestive system of animals feeding on infected tubers, as well as treatment in 1% formaldehyde and 0.1% mercuric chloride for one hour and three hours, respectively.[7] A soil temperature of at least 8 °C and water is required for sporangia germination and the dispersal of zoospores.[8] The microelements B, Cu, Zn, and Mo, have been reported to induce the germination of sporangia following overwintering.[9]

Management

The resilient nature of the resting spores, the overwintering sporangia, is the principal challenge in managing the disease. Several means of control have been explored, including chemical[10][11] and biological,[12] but have been largely ineffective, impractical, or neglected. Cultivation of resistant varieties is the best management approach. However, the development of resistant varieties is challenged by the discovery of novel pathotypes and the polygenic nature of resistance to potato wart.[13] Despite these challenges, legislative action has been taken that leverages the use of resistant varieties and has effectively curtailed the spread of potato wart. This action enforces the demarcation of contaminated plots and safety zones and the disposal of infected potato material. Safety zones are areas where only resistant varieties can be grown, and potatoes cannot be grown in sites until the absence of sporangia can be demonstrated. These legislative efforts also include measures that forbid the trade of infected potatoes and the presence of the disease in potato seed production.

Pathogenesis

Very little is known about the pathogenesis of S. endobioticum at a molecular level. Indeed, this is true of chytrids more generally, excepting a few well-studied species.[14] However, recent genome sequencing and annotation of S. endobioticum has shed light on the potential molecular mechanisms of pathogenesis.[15] For example, this analysis demonstrated the coding capacity of S. endobioticum to process complex sugars, which may include cellulose and starch. However, genes for cell wall degrading hemicellulases are reduced in S. endobioticum relative to the closely related, saprophytic chytrid, S. microbalum. The lack of hemicellulases may allow the pathogen to evade defense responses triggered by damage-associated molecular patterns from cell wall degradation.[16] Several regions within the genome have been identified as possible effector coding regions, but further work will need to be done to verify this. S. endobioticum did not contain genes coding for enzymes that are crucial to the biosynthesis of purine and pyrimidine. It is probable S. endobioticum exacts purine and pyrimidine from its host.

Added to the USA's federal bioterrorism list for agricultural plant pathogens in 2002 in accordance with the Public Health Security and Bioterrorism Preparedness Response Act.

References
  1. Professor Alan Gemmell Gardening, The Mastermind Quiz Book (Editor: Richard Morgale)
  2. Thomson, A. D. "Lady Kathleen Rigg, pioneer plant scientist". New Zealand Science Monthly Online. Webcentre Ltd. Archived from the original on 14 January 2015. Retrieved 19 March 2015.
  3. Weiss, Freeman (July 1925). "The Conditions of Infection in Potato Wart". American Journal of Botany. 12 (7): 413–443. doi:10.1002/j.1537-2197.1925.tb05846.x. ISSN 0002-9122.
  4. Weiss, Freeman (July 1925). "The Conditions of Infection in Potato Wart". American Journal of Botany. 12 (7): 413–443. doi:10.1002/j.1537-2197.1925.tb05846.x. ISSN 0002-9122.
  5. GLYNNE, MARY D. (February 1926). "The Viability of the Winter Sporangium of Synchytrium Endobioticum (Schilb.) Perc., the Organism Causing Wart Disease in Potato". Annals of Applied Biology. 13 (1): 19–36. doi:10.1111/j.1744-7348.1926.tb04250.x. ISSN 0003-4746.
  6. Steinmöller, Silke; Bandte, Martina; Büttner, Carmen; Müller, Petra (2012-02-09). "Effects of sanitation processes on survival of Synchytrium endobioticum and Globodera rostochiensis". European Journal of Plant Pathology. 133 (3): 753–763. doi:10.1007/s10658-012-9955-y. ISSN 0929-1873. S2CID 14648345.
  7. Weiss, Freeman (1928). "Factors of spread and repression in potato wart". Technical Bulletin. 56: 13.
  8. Chisnall Hampson, M. (December 1993). "History, biology, and control of potato wart disease in Canada". Canadian Journal of Plant Pathology. 15 (4): 223–244. doi:10.1080/07060669309501918. ISSN 0706-0661.
  9. Tarasova, VP (1969). "Role of environment in the decay of warts and germination of zoosporangia of Synchytrium endobioticum on potato". Byull VN 1 1 Zaschity Rastenii.
  10. Hampson, MC (1977). "Screening systemic fungicides for potato wart disease". Can Plant Dis Surv. 57: 75–78.
  11. Reigner, N (2006). "Potential chemical controls and crop protection industry contacts for recovery from establishment of select agent plant diseases in U.S. agriculture". Report from the Croplife Foundation Washington DC.
  12. ROACH, W. A.; GLYNNE, MARY D.; BRIERLEY, WM. B.; CROWTHER, E. M. (May 1925). "Experiments on the Control of Wart Disease of Potatoes by Soil Treatment with Particular Reference to the Use of Sulphur". Annals of Applied Biology. 12 (2): 152–190. doi:10.1111/j.1744-7348.1925.tb02261.x. ISSN 0003-4746.
  13. Maris, B. (November 1961). "Races of the potato wart causing fungus Synchytrium endobioticum (Schilb.) Perc. and some data on the inheritance of resistance to race 6". Euphytica. 10 (3): 269–276. doi:10.1007/bf00039096. ISSN 0014-2336. S2CID 19403203.
  14. van de Vossenberg, Bart T.L.H.; Prodhomme, Charlotte; van Arkel, Gert; van Gent-Pelzer, Marga P.E.; Bergervoet, Marjan; Brankovics, Balázs; Przetakiewicz, Jarosław; Visser, Richard G.F; van der Lee, Theo A.J. (2019-05-22). "The Synchytrium endobioticum AvrSen1 triggers a Hypersensitive Response in Sen1 potatoes while natural variants evade detection". dx.doi.org. doi:10.1101/646984. S2CID 182330467. Retrieved 2020-12-07.
  15. van de Vossenberg, Bart T. L. H.; Warris, Sven; Nguyen, Hai D. T.; van Gent-Pelzer, Marga P. E.; Joly, David L.; van de Geest, Henri C.; Bonants, Peter J. M.; Smith, Donna S.; Lévesque, C. André; van der Lee, Theo A. J. (2019-06-17). "Comparative genomics of chytrid fungi reveal insights into the obligate biotrophic and pathogenic lifestyle of Synchytrium endobioticum". Scientific Reports. 9 (1): 8672. Bibcode:2019NatSR...9.8672V. doi:10.1038/s41598-019-45128-9. ISSN 2045-2322. PMC 6572847. PMID 31209237.
  16. Choi, Hyong Woo; Klessig, Daniel F. (2016-10-26). "DAMPs, MAMPs, and NAMPs in plant innate immunity". BMC Plant Biology. 16 (1): 232. doi:10.1186/s12870-016-0921-2. ISSN 1471-2229. PMC 5080799. PMID 27782807.
  • EPPO data sheet on S.endobioticum
  • Resting-spore germination in Synchytrium endobioticum
  • Germination and parasitation of the resting sporangia of Synchytrium endobioticum
  • About potato wart disease
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