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R448 Common Scab Control

Publication Date: 
7 April 2015
Author/Contact :
Mark Stalham

Contractor :

Common scab control

Common scab symptoms are caused by streptomycetes. These are members of a large grouping of bacteria known as actinobacteria (sometimes also referred to as actinomycetes). The streptomycetes are probably some of the most well-known and well-studied actinobacteria, because they produce compounds that are used in human medicine.

Streptomycetes are ubiquitous in soil, sediments and seawater and there is increasing evidence to show that they form associations with other organisms such as insects (Seipke et al., 2012). A limited number of species of Streptomyces cause disease in plants. Those that are plant pathogenic colonize root structures and grow both inter- and intra-cellularly within the host (Loria et al., 2003 ) and produce phytotoxins and secondary compounds that affect the plant. The phytotoxin thaxtomin A is considered to be essential for scab symptoms to appear. An assay for the thaxtomin pathogenicity gene (txtA) has been developed and used to assess the numbers of pathogenic streptomycetes present in tuber peel. This has allowed comparison of their numbers on different potato varieties grown under the same conditions and studies of how numbers of pathogenic streptomycetes change in response to factors such as irrigation.

 AHDB Potatoes supported the production of a review of common scab in 2004 and was the sole or joint funder for three research projects. Further details of R448 are provided below:

R448 Common scab control - Reducing the irrigation water requirements and the effect of beneficial soil microorganisms and biofumigation (April 2011 - March 2014)

The project has demonstrated varietal differences in common scab in response to irrigation regimes. Maris Piper was clearly much more susceptible to scab than the other varieties examined and there was considerable evidence that other, less-susceptible varieties can be irrigated for shorter periods or at greater soil moisture deficitss than Maris Piper. A tentative grouping of varieties for common scab control using different soils and irrigation schedules has been produced. The potential to delay the start of irrigation until one week after tuber initiation is discussed.

Results from experiments examining the correct length of control period for salad potatoes, gave no indication that irrigation for 8 weeks was more successful in preventing scab than irrigating for 6 weeks.

Trials to assess the effects of the cloddiness of the ridge structure at planting demonstrated little effect on common scab incidence and severity and that irrigation regime was the over-riding factor in determining the level of control of scab.  Aggregate size distribution may have less effect on scab development than previously thought.

A substantial amount of new information on the microbial communities on tubers from different sites and seasons has been obtained. Controlled environment studies have shown that in the absence of microorganisms other than the scab pathogen, irrigation level alone was shown to have no effect on populations of pathogenic Streptomyces or on development of disease, indicating that an intact soil microflora is required to mediate control by irrigation.  However, across the tuber samples there was no clear correlation between particular microbial groups and a reduction in scab severity. As such, it has not been possible to identify any potential antagonists of pathogenic Streptomyces that warrant further study.

Information on the detrimental effects of overwatering (during tuber initiation and the scab control phase) in relation to tuber cracking and nitrogen uptake is provided.

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