Powdery scab

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Powdery scab
Sporosori (survival structure) of the powdery scab pathogen
Sporosori (survival structure) of the powdery scab pathogen
Scientific classification
(unranked):
SAR
(unranked):
Rhizaria
Phylum:
Cercozoa
Class:
Phytomyxea
Order:
Plasmodiophorida
Family:
Plasmodiophoraceae
Genus:
Spongospora
Species:
Spongospora subterranea
Trinomial name
Spongospora subterranea f. sp. subterranea
(
Lagerh.
1892

Powdery scab is a disease of

pustules containing a powdery mass. These can eventually rupture within the tuber periderm.[4] The powdery pustules contain resting spores that release anisokont zoospores (asexual spore with two unequal length flagella) to infect the root hairs of potatoes or tomatoes.[5] 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.[6]

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 protozoan. 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

zoosporangium, which eventually release more zoospores. This process can happen relatively quickly and can act as an important source of secondary inoculum within a field.[7]

Sexual Stage: This stage follows a similar pattern to the asexual stage, but with a few exceptions. It is hypothesized that two zoospores fuse together to form a dikaryotic zoospore (with two separate haploid nuclei, n+n) and then infect the roots. Once the infection occurs, the dikaryotic zoospore develops into a binucleate plasmodium (one pair on nuclei, n+n). Similar to the asexual stage, this plasmodium will also replicate its nucleus to create a multinucleate plasmodium (many pairs of nuclei, n+n). The second main different between stages occurs here. The pairs of nuclei (n+n) will fuse by karyogamy, and the plasmodium will quickly divide into numerous resting spores within a sporosori (spore sack, alternatively called cystosori). These resting spores have three-layered walls and are extremely resistant to the environment, allowing them to persist in the soil for longer than 10 years.[7]

As a reminder, most of the life cycle is still unclear. However, the presence of zoospores, plasmodia, zoosporangia, and resting spores have been observed in the field and lab.[8] The ploidy levels and karyogamy events are only theorized and have yet to be proven.

Environment

Spongospora subterranea pathogenesis is most effective in cool, damp environments, such as northern Britain, the

Columbia Basin of south-central Washington, and north-central Oregon.[6] The environmental condition is particularly critical during the release of infective agents (zoospores) into the soil-environment . Upon release from resting spores, zoospores require moisture to swim towards the host tuber or roots.[9] One study, found powdery scab was significantly more common on plants grown in constant dampness compared to plants grown with varying moisture levels.[10] In this same study it was concluded that disease risk was related more to the environment, or moisture level, than the level of inoculum present. Inoculum may be present but not able to disperse due to environmental conditions, and therefore does not reach host tissue to infect. Other environmental factors that affect Spongospora subterranea infection are directly related to agronomic practices. Increased use of fertilizers containing nitrate or ammonium nitrogen increase the incidence and severity of powdery scab.[11] It is thought that the fertilization increases root growth, and thus provides more tissue for infection and disease cycling to occur. Also, reduced cellulose
within the cell walls caused by excess nitrogen may increase susceptibility of host to infection. It is apparent that the environment can directly affect both the host susceptibility and the dispersal of the pathogen ultimately setting the pace for the disease cycle.

Pathogenesis

Potato tuber covered in powdery scabs

S. subterranea is an obligate parasite

russet skinned are somewhat resistant.[16] Russet skin is thicker and has higher levels of the LOX protein which is used as a marker for resistance.[6] There is little known about variation and sexual recombination within S. subterranea, therefore high priority is given to researching the variations within potato cultivars for researching host/pathogen relationships and management.[17]

Importance

Powdery Scab has important implications for commercial farming. Not only does the pathogen itself cause harm, but the pathogen is also a vector for

Potato Council funded diagnostic project discovered that as much as 82% of fields tested positive for soil inoculum.[6]

Management

S. subterranea currently has no effective chemical controls. Therefore, other cultural management techniques must be used. Using certified clean seeds and planting in fields that have been historically healthy is the best form of control.

beta-aminobutyric acid (BABA) in promoting potato resistance. BABA triggers a plants systemic acquired resistance (SAR), a natural plant defense mechanism. When potatoes are inoculated with BABA and then later inoculated with the pathogen, S. subterranea, they exhibit overall reduction in disease. While pathogen reduction has been experimentally supported, further experimentation needs to be performed.[4] Genomic data inclusive a genome draft became recently available, that might hold information that can be used to improve disease management.[21]

References

  1. ^ "Resistance to Powdery Scab in Potatoes" (PDF). USDA/ARS. Retrieved 2015-11-11.
  2. ^ "Potato scabs/RHS Gardening". www.rhs.org.uk. Retrieved 2015-11-11.
  3. PMID 29024322
    .
  4. ^ a b 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.
  5. ^ "Bulletin #2436, Powdery Scab of Potatoes | Cooperative Extension Publications | University of Maine". umaine.edu. Retrieved 2015-11-11.
  6. ^ a b c d e f Dennis A. Johnson and Thomas F. Cummings. Effect of Powdery Scab Root Galls on Yield of Potato. (2015).Plant Disease 99:10, 1396-1403
  7. ^
    S2CID 38001426
    .
  8. S2CID 8842628
    .
  9. S2CID 13658887
    .
  10. ^
    S2CID 29987047
    .
  11. S2CID 18388456
    .
  12. PMID 29024322
    .
  13. ^
    ISBN 9781840765083
    .
  14. ISBN 978-9401733021
    .
  15. ^ "Powdery Scab | AHDB Potatoes". potatoes.ahdb.org.uk. Retrieved 2015-10-22.
  16. ^ a b c d "How to Manage Pests." UC IPM: UC Management Guidelines for Powdery Scab on Potato. N.p., n.d. Web. 19 Oct. 2015.
  17. S2CID 37670848
    .
  18. ^ Gudmestad, Neil. "Potential Management of Powdery Scab and Mop Top Virus Using an Integration of Soil Fumigation and Genetic Resistance" (PDF). Department of Plant Pathology, North Dakota State University, Fargo, ND.
  19. ^ C. R. Brown, G. Vandemark, D. Johnson, Tom Cummings, Dallas Batchelor, Jeff Miller, Chris Olsen. Resistance to Powdery Scab in Potato. Web. 11 Nov. 2015.
  20. ^ Nitzan, Nadav, Dennis Johnson, Dallas Batchelor, and Chuck Brown. "Powdery Scab." Potato Country (2007): 6-7. Print.
  21. PMID 29969057
    .

External links
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