Viral Diseases Pose A Threat
to Pepper Growing
Pepper (Capsium spp) is one of the most diverse vegetable species and
is considered to be a high value crop. Its use has increased by more than 21%
since 1994 and the worldwide planted area now estimated at 1.9 million hectares.
In the region, it is commercially grown in Zambia, Malawi, South Africa and
Kenya, and by smallholder farmers. Pepper can thus be added on the list of major
export crops.
However, viral diseases are a significant constraint wherever the crop is
grown as it causes poor fruit yield and quality. The amount of loss can be up to
60% depending on strain of the virus, the variety of the pepper, the age of the
plant at infection time, temperature during disease development, presence of
other disease, and the extent that virus has spread in the planting.
 In
most cases, old leaves develop mosaic followed by diffuse chlorosis. Affected
leaves frequently drop prematurely.
They may also develop inward rolling and distorted tips. On fruits, solid
circular spots may develop. In many instances, they become rough, deformed and
severely reduced in size. When plants are affected when still young, they are
stunted and express severe symptoms.
In a study that was funded by Deutsche Gesellschaft für Technische
Zusammenarbeit (GTZ), viral diseases were found throughout the surveyed area in
Zambia in both the commercial and smallholder farms. Disease incidence varied
from 20% to 100%. Also, a new virus, which is seed-borne, was discovered for the
first time in Africa; it is called the pepper mild mottle virus. The most
susceptible cultivars are "Zeleminova" and "Long Slim Cayyene",
which are grown in the country.
The host range of the viruses consists of more than 750 plant species in
various families including many vegetables such as tomato, cucumbers and
legumes. The extensive host range serves as a source of infections and
contributes to the spread of the viruses to the crop.
The viruses are introduced to cultivated peppers mainly by aphid species
after they have fed and acquired the viruses from wild reservoir hosts. More
than sixty aphid species, including the common Aphis gossypi and Myzus
persicae, are capable of transmitting viruses in a non-persistent manner.
Attempts to control the aphid vectors have not been effective in reducing the
loss attributed to viral diseases in pepper. Furthermore, due to the wide range
of the viruses and their transmission by numerous aphids, they are difficult to
control in the farmer fields. However, farmers can reduce the spread of diseases
and therefore minimise yield losses by practising various control methods.
The pepper should not be planted near source plants such as Solanum nigrum
and hosts such as tobacco, tomato and cucumber. In addition, farmers can be
trained on the causes, identification and management of the viral diseases that
affect pepper.
Besides using the recommended methods such as planting disease-free seedlings
and uprooting the diseased plants, the fields and nurseries should be protected
from the aphids. Using sticky yellow polythene sheets, which are erected
vertically on the windward side of the fields and nurseries, can help ward off
these vectors. The aphids are attracted to the yellow colour and are caught on
the sticky polythene.
Perhaps, the most effective control method is the use of virus-resistant
varieties, which can be identified by researchers and then distributed to
farmers. Already, transgenic resistance, resulting from the incorporation of
virus coat protein genes through genetic engineering, has been obtained in
certain cultivars for some viruses. This would make an attractive option for
African farmers, particularly in light of the future prospects for the pepper
industry on the continent.
For further information:
Joseph Ndunguru
Dept. of Microbiology and Plant Pathology
University of Pretoria
Pretoria 0002, South Africa
E-mail: ndunguru@brighton.zzn.com
Article courtesy of Agriforum: Quarterly
newsletter of ASARECA the Association for
Strengthening Agricultural Research in Eastern and Central Africa.
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