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ProMED-mail post
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International Society for Infectious Diseases
Date: April 27, 2007
From: The American Phytopathological Society (APS), Plant
Disease [edited] <http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-91-4-0352?prevSearch=allfield%3A%28common+scab+new+strain%29>
Common scab is a serious disease of potatoes (Solanum
tuberosum) and other root and tuber crops, affecting the
quality and market value of these crops. The disease is caused
by gram-positive soil bacteria in the genus Streptomyces.
A new common scab-causing streptomycete was isolated from scabby
potatoes originating in southeastern Idaho.
Research has supported a model of horizontal transfer of
pathogenicity determinants among streptomycetes, and the new
strain has hallmarks of the recently characterized
_Streptomyces_ pathogenicity island (PAI); it has genes encoding
the synthetase for the pathogenicity determinant thaxtomin and
for a 2nd pathogenicity factor, tomatinase, although it lacks a
third gene characteristic of the _Streptomyces_ PAI, the nec1
gene. The new strain has a unique 16s rDNA gene sequence closely
related to those of other pathogenic _Streptomyces_ species.
This 16s rDNA sequence was also found in isolates lacking a PAI,
suggesting that the new pathogenic strain arose by horizontal
transfer of a PAI into a saprophytic streptomycete.
Isolates of the new strain are pathogenic on radish and potato,
and are more virulent than the _S. scabies_ type strain. In
addition to scab lesions on potato tubers, lesions were also
seen on underground stems and stolons. This new strain
represents additional complexity in the pathogenic strains
causing plant disease in the United States.
[Byline: Leslie A. Wanner]
--
Communicated by:
Dick Hamilton
Former ProMED-mail plant disease moderator
471 Foxglove Crescent
Richmond, BC
Canada, V7C 2K4
<rihamilto@shaw.ca>
******
[2] Additional information on potato common scab
Date: Tue 24 Apr 2007
From Dick Hamilton <rihamilto@shaw.ca>
Common scab is prevalent in most potato growing areas worldwide.
This disease, characterized by deep- or shallow-pitted lesions
on potato tubers, reduces the quality of severely infected
crops. Although cultivars differ in their level of resistance,
very few are highly resistant and none is immune (Scholte &
Labruyere, 1985). Potato cultivars known to be particularly
susceptible to common scab include 'Desiree', 'Foremost',
'Majestic', 'Maris Piper', and 'Red Craigs Royal'; more
resistant cultivars include 'Arran Comet', 'Arran Pilot',
'Golden Wonder', 'King Edward', 'Maris Peer', 'Pentland Crown',
and 'Pentland Javelin'.
The pathogen is also found on a number of root crops, including
carrot, radish, beet, and turnip. If grown in rotation with
potato, these susceptible crops may increase soil populations of
pathogenic _Streptomyces_ and therefore scab incidence. The
disease is more prevalent on alkaline soils which have a light
sandy texture and which are deficient in organic matter. Some
less common strains cause scab in soils with pH below 5.0.
Infection is usually only superficial and the potatoes are fit
to eat. Infection occurs when the tubers are just beginning to
form, and if this stage of growth coincides with dry soil
conditions then levels of the disease can be high.
For disease management, use certified seed potatoes. Planting
infested tubers not only results in disease at harvest, but also
can introduce new more aggressive strains of the pathogen.
Increase the length of time between successive potato crops.
Avoid soil treatments that raise pH such as application of lime.
Apply plenty of organic matter to light soils to increase their
moisture-holding capacity, and ensure that the soil is kept
moist during the crucial stage of early tuber formation.
Pictures of common scab symptoms on potato:
<http://vegetablemdonline.ppath.cornell.edu/PhotoPages/Impt_Diseases/Potato/Pot_Scab.htm>
Common potato cultivars in Canada:
<http://www.inspection.gc.ca/english/plaveg/protect/dir/potgare.shtml>
Common potato cultivars in the UK:
<http://www.rhs.org.uk/advice/profiles0902/potato_skin.asp>
--
Dick Hamilton
Former ProMED-mail plant disease moderator
471 Foxglove Crescent
Richmond, BC
Canada, V7C 2K4
<rihamilto@shaw.ca>
[_Streptomyces scabies_, the organism causing common potato
scab, is a soil-borne bacterium. Therefore fungicides are not
effective for disease control. Research is being carried out
into biological control of _S. scabies_ by antibiotic-producing
strains of _Streptomyces_ isolated from suppressive soils (see
link below).
There is a large variety of _Streptomyces_ strains not all of
which cause disease (Doering-Saad et al. 1992).
Another potato disease called powdery potato scab is caused by
the fungus-like organism _Spongospora subterranea_. Although
symptoms on potato tubers look somewhat similar, the 2 pathogens
are not related at all. _S. subterranea_ is also the vector for
_Potato mop-top pomovirus_.
As in this case of the 2 potato scabs, it is not unusual that
similar symptoms are caused by entirely different pathogens.
Therefore, most often symptoms alone cannot be considered a
reliable diagnostic tool in plant pathology or biosecurity
applications and suitable molecular diagnostic tests need to be
used.
Links:
Interpretive summary of title [1]:
<http://www.apsnet.org/pd/summaries/dap07sum.asp#Dalmacio>
Common scab information with pictures:
<http://plantclinic.cornell.edu/FactSheets/potatoscab/potatoscab.htm>
Doering-Saad et al., 1992:
<http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=183207>
Biological control of common scab:
<http://www.entomology.wisc.edu/mbcn/veg411.html>
Information on powdery scab of potato:
<http://www.agri.gov.il/gilat-molcho/spongospora.html>.
- Mod.DHA] |
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