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Source: SpudSmart [summ. Mod.DHA, edited]

Potato crops across Canada have fallen victim to common scab lurking in the soil for years. If more than 5% of the surface of tubers are covered with scab, they are unable to be sold for the table market. The disease is also an issue for processing and, of course, even worse for seed production. But not much has been done on a national level to address the problem.

In 2018, Claudia Goyer, Agriculture and Agri-Food Canada, started a common scab research project. Different species are causing common scab, but the one that is found pretty much everywhere in the world is _Streptomyces scabies_. Other species found regionally include _S. acidiscabies_ and _S. turgidiscabies_. They all produce a group of plant toxins called thaxtomins, which is how common scab causes the brown lesions on potatoes.

Samples of potatoes with common scab symptoms were collected in several states for testing. Scab pathogens from infected tubers were characterised using molecular testing. So far, the scientists have a collection of 300 isolates with at least 20 genetic groups. This shows that there is a lot of diversity in the pathogens, which might explain why finding solutions to control the disease has been so difficult.

After determining the genetic groups, the team started to look at how they are distributed across Canada. They are also trying to find ways to control common scab in fields, such as growing certain crops before potatoes to hold soil moisture, which reduces scab incidence, or biopesticides. Full project results are to be released in 2023.

[Project details:]

[Byline: Ashley Robinson]

Communicated by:

[Common scab of potato caused by the bacterium _Streptomyces scabiei_ has been reported from most potato-growing areas worldwide. Symptoms include cracks and lesions on tubers that coalesce into large rough or corky areas, but above-ground plant parts appear normal. Scab reduces crop quality, although scabby potatoes are generally fit to eat and store well. Infection occurs during early tuber formation; if soils are dry at that stage, disease levels can be high. The pathogen can also affect other root crops, such as carrot, parsnip, radish, beet, and turnip. In South Africa, _S. scabiei_ was also found to be associated with pod wart disease on peanut crops (see ProMED post 20190722.6581049). Each of the crops can serve as reservoir hosts in case of successive rotations with the others and so increase overall scab incidence. The disease is favoured by alkaline soils.

The bacterium is soil-borne and can survive on infected tubers and plant debris. Disease management requires an integrated approach and may include cultural techniques such as 3-5-year rotations with non-host crops, increase of organic matter in fields to increase soil moisture, and adjusting soil pH. Use of certified scab-free seed tubers is essential. Commercial potato cultivars differ in their levels of susceptibility, but none with complete resistance to scab are currently available. Research is being carried out into biological control of scab by antibiotic-producing strains of _Streptomyces_.

Similar symptoms on potato tubers can also be caused by powdery scab due to the fungus-like organism _Spongospora subterranea_, which is also the vector for _Potato mop-top virus_ (see, for example, ProMED-mail post 20100616.2020).

Canada (with provinces):

Common scab on potato:,, and
Close-up of initial scab lesions:
Scab on beet:

Information on potato scab and _S. scabiei_:,,,, and
Potato common scab life cycle:
_S. scabiei_ taxonomy and synonyms:
Taxonomy of other _Streptomyces_ species via:
- Mod.DHA]


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