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Kingdom |
Fungi
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Phylum |
‘mitosporic
fungi’
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Class |
Coelomycetes
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No
reports have been found on the economic importance of the disease but severe
damage on the leaves may reduce the quality of vines used as fodder.
The disease is
considered to be of minor importance, because it is only present in mature
leaves and toward the end of the growing season.
Probably throughout the humid zones worldwide. Has been reported in Africa (Ethiopia, Ghana, Guinea, Nigeria, Sierra Leone, Sudan, Tanzania,
Uganda, Zambia, Zimbabwe), Asia (China, Hong Kong, Japan), Caribbean (Bermuda,
Cuba, Jamaica), Pacific Islands (Papua New Guinea), North America (USA), and
South America (Argentina, Brazil, Venezuela).
The most obvious symptom is observed in older leaves as whitish to tan brown
lesions surrounded by a dark purple to brown margin. These lesions are roundish
with irregular borders and measure 5-10 mm wide. The centre of the mature lesions
have few pinhead-like black to brown structures clearly visible to the naked
eye; they are pycnidia at different stages of development. Many lesions can
coalesce and cover a great portion of the leaf. Usually the lesions are more
prominent on the upper surface of leaves.
The disease is restricted to the leaves and has not been found in any
other part of the plant. However, for some time it has been thought to also
attack the fleshy roots in the field and in storage but damage on
roots has been caused by Phomopsis batatae, a fungus of the same group
that is found to cause root dry rot. It has some resemblance to P. ipomoea-batatae,
but produces B instead of A conidia.
The fungus produces black to dark brown, usually solitary pycnidia
120 - 180 µm wide with septate, occasionally branched conidiophores.
The fungi in this group usually produce two kinds of conidia,
the so called A and B conidia. However this particular species produces only A
conidia that are unicellular, hyaline, oblong or ovoid, and measure 4 - 8 x 2.5
- 3.5 µm. B conidia have not been found.
Very little is known about the biology of the causal agent. It has been
suggested that the fungus overwinters in affected leaves and when the field
receives moisture, the pycnidia swell and liberate conidia that become the
primary infection agents for the new crop.
No environmental factor (eg. temperature and relative humidity) has been
reported to favour the development of the disease. It is noted, however, that
the disease starts late in the season, and develops mainly in the mature leaves.
This phenomenon is very common with fungi that affect the foliage.
No other hosts of P. ipomoea-batatae are known.
A diagnostic feature is the presence of the pathogen's visible fruiting
bodies at the centre of lesions in mature leaves. It is easy to recognize
these fruiting bodies (pycnidia) and conidia, especially under the microscope.
Pycnidia are brown to black structures, with an opening through which the
conidia are discharged. Conidia are oblong with two refringent spots near both
ends.
Cultural control
Although no control measures are mentioned in the literature, sanitation
should be practised, since the fungus overwinters in affected crop debris that
are left in the field.
Host-plant resistance
Resistance to the disease has not been observed.
Chemical control
Chemical control is not normally necessary.
Clark, C.A. and Moyer, J.W.1988. Compendium of sweet potato diseases. APS
Press.74 p.
Holliday, P. 1995. Fungus diseases of tropical crops. Dover Publications
Inc., New York. 607 p.
Punithalingam, E.
1982. Phomopsis ipomoea-batatas. CMI Descriptions of Pathogenic Fungi and
Bacteria No. 739. 2 p.
Contributed
by: Teresa Ames |
