Cacao swollen-shoot virus
| Cacao swollen-shoot virus (CSSV) | |
|---|---|
| Virus classification | |
| Group: | Group VII (dsDNA-RT) |
| Family: | Caulimoviridae |
| Genus: | Badnavirus |
| Species: | Cacao swollen-shoot virus |
| Synonyms | |
|
Theobroma virus 1 |
|
Theobroma virus 1
Cacao swollen shoot virus
Cacao mottle leaf virus
Cacao swollen-shoot virus (CSSV) is a plant pathogenic virus of the family Caulimoviridae that primarily infects cacao trees. It decreases cacao yield within the first year of infection, and usually kills the tree within a few years. Symptoms vary by strain, but leaf discoloration, stem/root swelling, and die-back generally occur. The virus is transmitted from tree to tree by mealybug vectors. It was first discovered in Ghana in 1936, and is currently endemic in Togo, Ghana and Nigeria. Over 200 million trees have already been claimed by this disease, which has prompted Ghana to launch the most ambitious and costly eradication effort of any country in the world against a viral plant disease.
Cacao swollen-shoot virus (CSSV) principally infects Theobroma cacao (cacao tree) and has a major effect on crop yields. Within one year of infection yields decrease by 25%, and within two years by 50%. The trees are usually killed within 3 to 4 years. Symptoms vary depending on the strain of virus. Main symptoms include: leaf chlorosis (interveinal), root necrosis, red vein banding in young leaves, small mottled pods, and stem/root swelling followed by die-back. CSSV has also been isolated from alternative hosts Cola chlamydanta, Ceiba pentandra, Adansonia digitata, Cola gigantean and Sterculia tragacantha. These alternate hosts display the same symptoms as infected cacao trees, but with less severity. Symptoms also vary with environmental conditions. Neither nutrition nor temperature changes have a perceivable effect on symptoms, but increased light intensity inhibits the development of root/stem swellings in infected plants. Shaded cacao trees exhibit decreased growth and slightly more severe symptoms.
Cacao swollen-shoot virus has a double stranded circular DNA genome that is 7.4 kb in size and bacilliform in structure. It is transmitted primarily through mealybug (Pseudococcidae) vectors. Once within the host, the virus uses the host to replicate. The virus codes for RNA silencing genes, which turn off the host plant's defenses. Its genome also codes for a movement protein, which allows the virus to move throughout the plant using the plasmodesmata. Of the many identified strains, the most studied and severe is the New Juaben strain, which is widespread in eastern Ghana. There is some spread of the virus at all times of year, but seasonally, spread depends on the size and activity of the mealybug population. It is hard to see this correlation within the cacao population because symptoms start to show at times of new growth.
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