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Kingdom
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Fungi
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Phylum
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‘mitosporic
fungi’
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Class
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Hyphomycetes
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The disease is a minor one but market losses can be important because of the
disfiguring effects of infection. The superficial damage to outer fleshy root
tissue layers can lead to moisture loss and shrinking in storage.
The disease has been reported from Africa, Asia, the Pacific Islands,
Caribbean, North America and South America.
On the host, definite vegetative hyphae
are lacking. The conidiophores are
septate, erect, unbranched, dark, measure 40-300 µm long and 4-6 µm wide,
and attached to the host by a bulb-like enlargement. The conidiophores
usually emerge from the host either singly or in pairs. The conidia
are formed successively into chains that soon bend. They are aseptate, oblong to
ovoid, unicellular, 12-20 x 4-7 µm in size, and are initially hyaline,
then turn light brown.
Symptoms begin as small brown spots on the fleshy root surface. These enlarge
and coalesce to give discoloured, superficial, necrotic
areas of varying sizes and shapes, without a definite outline. The colour of the
spot or lesion depends on the skin colour with copper-skinned cultivars having
brown lesions and red ones having almost black lesions. There is no general
rupture of the epidermis. Only the periderm is affected, but this may cause
increased water loss from roots in storage. Infection may cover most of the
storage root surface in storage. Above-ground parts of the plant are not
affected unless in contact with infected soil.
When slightly infected slips are planted, the fungus colonizes the periderm
in the root system and spreads on the outer tissue of fleshy roots.
In storage the diseased areas increase and can cover the entire root giving a
very unpleasant appearance. Affected areas become leathery and dry, there is a
loss of moisture and consequently shrinkage of the root.
The fungus is soil borne for a short time but may survive longer in soil high
in organic matter, such as animal manure. The disease is more severe in heavy
soils.
Scurf is mostly restricted to more temperate sweetpotato growing areas,
presumably due to a temperature effect. It may also be due to the different
methods of propagation. In the tropics this is almost always by direct field
planting of stem cuttings. In temperate regions propagation is by sprouts or
slips obtained by planting smaller storage roots in nursery beds (or hot beds);
when the sprouts are some 25 cm long they are transplanted to the field. The
fungus is thus spread from infested nursery beds to the field where the
developing storage roots are infected.
In storage the optimum temperature for disease development is around 24°C,
but temperature is not a limiting factor since the fungus can develop up to a
certain extent within a wide range of temperatures.
Moisture does not limit the growth of the fungus in the soil although the
disease is most severe during the rainy season and in low, wet soils.
The disease further develops during storage but no new infections are
observed.
The disease affects only sweetpotato naturally but the pathogen can infect
other convolvulaceous plants. No other host has been found.
Cultural control
Quarantine, avoid mobilising unclean planting material.
Rotation (2-3 years in lighter soils and 3-4 years on heavier soils).
Cutting plants at least 2-3 cm above the soil line.
Dipping of planting material in hot water 0.5 minutes at 55°, or 5 minutes
at 49°C.
Host-plant resistance
Although cultivars differ in reaction to infection there is apparently no
high host resistance.
Chemical control
Treating storage roots with thiabendazole or dichloran before bedding them
for plant production.
Daines, R.H. 1955. Development of sweet potato scurf in storage. Plant Dis.
Rep. 39: 617.
Holliday, P. 1995. Fungus diseases of tropical crops. Dover Publications,
Inc. New York. 607 p.
Contributed
by: Teresa Ames |
