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Outbreak of virus-like disease on Sunflower in UgandaDiscovery
of Bruchid
Dr. Valentine Aritua
National Agricultural Biotechnology Center,
Kawanda Agricultural Research Institute, Kampala, Uganda (e-mail:
valentine@kari.go.ug) and Central Science Laboratory, Sand Hutton,
York YO41 1LZ, UK (e-mail:
v.aritua@csl.gov.uk).
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Figure 1. Yellow blotching, chlorosis and distortion of
leaf |
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Figure 2. Yellow blotching and chlorosis of leaves
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Background. Sunflower (Helianthus annuus L.) is
one of the cultivated species in the genus Helianthus. Its center of
origin has been identified as the western plains of North America. Spanish,
English and French explorers, finding sunflower in common use by the
American Indians, introduced the plant to Europe as early as the 16th
Century. From Europe, the crop spread along trade routes to other parts of
the world. Presently, sunflower is grown worldwide for oilseed production
and as non-oilseed for the home and bird food markets. The development of
high-oil varieties hybrids has been linked to the expanded world production
of the crop.
Sunflower viruses. Sunflower is susceptible to several viruses
including sunflower crinkle virus, sunflower yellow blotch virus, sunflower
chlorotic mottle virus, sunflower mosaic virus, cucumber mosaic virus,
tobacco ring spot virus, tobacco streak virus and tomato spotted wilt virus.
Suspected cases of viruses and virus-like diseases have been reported on
sunflower from African countries including South Africa, Zambia, Malawi,
Tanzania and Kenya, but there has been no confirmed reports from Uganda.
However, at the beginning of 2005,
Lira District Agricultural extension staff reported an outbreak of an
unusual disease with symptoms characteristic of viral infection in the
districts of Lira, Apac and Masindi. Subsequently, the
Ministry of Agriculture, Animal Industries and Fisheries
in Uganda directed the
National Agricultural Research Organisation (NARO)
to investigate the problem. NARO scientists visited the affected areas in
June 2006 and confirmed the existence of virus-like disease on sunflower.
Most of the
affected sunflower plants showed symptoms of yellow blotching, vein
clearing, leaf chlorosis, mosaic to ring spot patterns, leaf distortion and
stunting of plants, especially in the early stages of development (Figure 1
and 2). In some farmers’ fields, over 40% of the crops were diseased.
Imported PANNAR sunflower hybrids from South Africa appeared to be worst
affected. No previous work has been undertaken on viral diseases of
sunflower in Uganda, so farmers uproot and destroy diseased plants if and
when they occur.
The virus outbreak in Uganda. The identity of the virus currently
occurring in Uganda is still unknown. The first description of an out break
of a virus-like disease in East Africa was that reported in Uganda in 1954
(Wiltshire, 1954). Later, a disorder named rugose mosaic disease, that
caused vein clearing and leaf-crinkling symptoms was seen in Kenya (Martens
et al., 1970). Two similar viruses were associated with this and
another disease that caused yellow blotching of leaves (Theuri et al.,
1987). This led to existence of two possible sunflower viruses in Africa;
sunflower crinkle virus, syn. sunflower rugose mosaic virus and sunflower
yellow blotch virus, syn. sunflower yellow ringspot virus.
Based on field level observations, the virus currently infecting sunflower
in Uganda appears to be the sunflower yellow blotch.
In addition to being mechanically transmissible,
sunflower yellow blotch
virus reported in Kenya was transmissible from an alternative field host,
Tridax procumbens, by Aphis gossypii to 23 species in three families.
Due to the wide host range, it is possible that the viruses infecting
sunflower in Kenya, and probably the one in Uganda, have a number of
reservoir hosts. If this scenario is true, it can be speculated that, the
virus in Uganda could have come from weeds growing in close proximity to
crops. The newly introduced sunflower hybrids appear to be more prone to the
virus than the locally bred varieties, indicating increased susceptibility
of the crop.
Since, the Compositae family contains many genera, some of which do
occur in Africa, any build up of a sunflower virus(es) in East Africa may
present a risk to a wide range of crops. The most affected are likely to be
the domesticated sunflower and beneficial related species that harbour
pollinating bees and sometimes have local medicinal value.
The current epidemic of the disease in Uganda, presents an opportunity for
conducting epidemiological and diagnostic research since the outbreak is
still active. In a preliminary move,
samples from diseased plants have been sent to Global Plant Clinic (UK)
where analysis is currently being carried out at the Central Science
Laboratory. Results of the analysis will be presented in the future issues
of the African Crops Newsletter.
References
Martens JW, Ravagan G and Macdonald WC, 1970. Diseases of sunflower in
Kenya. East African Agricultural and Forestry Journal 35: 103-106.
Theuri JM, Bock KR, and Woods RD, 1987. Distribution, host range and some
properties of a virus disease of sunflower in Kenya. Tropical Pest
Management 33: 202-207.
Wiltshire S P, 1954. New record of sunflower disease. FAO. Plant Protection
Bulletin 2: 188.
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