|
|
Class |
Actinobacteria |
|
Sub Class |
Actinobacteridae |
|
Order |
Actinomycetales |
|
Sub order |
Streptomycineae |
|
Family |
Streptomycetaceae |
Soil rot caused by Streptomyces ipomoeae is considered a minor disease
but under certain circumstances, can produce a marked reduction in yield. In the
past this was a serious disease of sweetpotato in the USA. Besides reducing
yield, the disease also affects the quality of the storage root in size
appearance and flavour. Affected storage roots are unmarketable in places demanding high
quality.
The disease has been reported only from Japan and USA.
The pathogen causes lesions
on all underground parts of the plant, including fibrous roots, storage roots,
and the base of stems. The disease can be initiated at any stage of crop
growth, when appropriate conditions occur (particularly dry weather).
Infection of the fibrous root system is often referred to as "rootlet rot".
Affected plants become stunted, the leaves are smaller and pale and early
flowering results. They often occur as large patches in the field.
Diseased fibrous roots present dark lesions
and break easily as they are dug out. The thicker roots show deep, dark lesions of different sizes and shapes.
Stems at the base of the plant may develop circular black, sunken or corky
lesions. Generally these are of minor importance compared with the fibrous
root damage. In some cases, especially if the disease initiates early in the
season the plant can be girdled.
Lesions on storage roots are often referred to as "pox". When infection
occurs late in the season, they may be the only symptom apparent, as late
rootlet damage may have no conspicuous impact on crop growth. The lesions
are round to irregular shape, generally less than 3 cm across, and less than 5
mm deep. They are dark, dry and frequently cracked from the centre or have
a rough scaby surface. They do not spread after harvest, and sometimes the
tissue under the lesion becomes partially healed. In some genotypes, the
disease can cause
roughening of the skin rather than discrete lesions.
When storage roots are infected early,
the affected portion of the root stops
growing and a misshapen root is formed. It may be indented at the
lesions, or have narrow constricted bands or "dumbell" shape.
S. ipomoeae is soilborne, aerobic, strongly oxidative, gram-positive,
forming chains of 3-10 oval spores (mycelium), 0.8 -0.9 x 0.9 -1.8µm, with
smooth walls . The spirals are about 3-5µm radius that might be incomplete,
giving hooks and loops, or 1-2 turns.
In the culture medium, colonies have a bluish green sporulation, but do not
have the peculiar earthy smell of other Streptomyces species.
The pathogen is soil borne and dissemination occurs by water movement,
transit by humans and animals, infected tools, infected planting material, and
cow manure.
Once the soil is infested, the bacteria remain inactive until the host is
present and conditions are appropriate for infection. Once soil is infested,
there is very little chance to eliminate the infection.
Disease is favoured by dry weather and a soil pH above 5.2 and observed to be
favoured by warm temperatures.
Ipomoea batatas (sweetpotato) is the only reported host. However the
disease can also infect other members of the Convolvulaceae, including
morning glory.
There is an ELISA (enzyme-linked immunosorvent assay) test, for rapid
detection of the disease. Special techniques have been developed to isolate the
pathogen in the laboratory because isolation from affected tissue has been
difficult.
Host-plant resistance
Some cultivars, like Beauregard and Jasper, showed high levels of resistance while
some like Jewel and Centennial, are susceptible.
Cultural control
Crop rotation with 3-4 years between sweetpotato crops.
Avoid dryness of soil with timely irrigation.
Careful selection of planting material.
Avoid introduction from affected to unaffected areas, by restricting movement
of equipment, animals, manures or planting material.
If soil is mildly acidic, do not apply lime. Sulfur can be used to
decrease soil pH, but may not be cost-effective.
Chemical control
Soil fumigation with chloropicrin.
Bradbury, J.F. 1981. Streptomyces ipomoeae. CMI Descriptions of
Pathogenic Fungi and bacteria No. 697 2 p.
Bradbury, J.F. 1986. C.A.B. International Mycological Institute. 332 p.
Schaad, N.W.,
Jones, J.B., and Chun, W. Laboratory Guide for Identification of Plant
Pathogenic Bacteria. Third Edition. APS Press, St. Paul Minnesota 373 p.
Contributed
by: Teresa Ames |
