The Cassava Virus

Colin Orians


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Interview Participants
For twenty years Monica Endetii, a farmer in Kenya, has followed sound agricultural practices. Planting a variety of crops, including cassava, maze, cow peas, and other vegetables on her three acre plot. For twenty years she has supported herself and earned enough money from selling her produce to put seven children through school.
Last year, however, everything changed when cassava mosaic disease struck Monica’s farm and infected more than 80% of her cassava crop, costing her 50% of her income. Without this income Monica will have trouble paying for her children’s education, providing food and shelter for her family, and investing in new seeds to replace those lost to the cassava mosaic disease.
Sadly, Monica’s case is not an exception. Rather, her story has been repeated countless times throughout Africa’s Great Lakes region. The destruction of this vital crop has major ramifications. Due to the widespread popularity of the cassava plant as a staple crop for hundreds of millions of people throughout Africa, Asia, and South America.
Even worse, there are two major viruses contributing to cassava crop loss in eastern and southern Africa. The first recognized and the one to blame for Monica’s crop loss is cassava mosaic disease which is comprised of three distinct species of virus. The leaves have a distinct mosaic pattern discoloration which leads to fragile leaves that cannot absorb enough sunlight for the plant to grow.
Although CMD has no effect on the roots or the tuber portion of the plant, the deterioration of the leaves stunts the growth which is problematic to crop yield. Cassava crop loss due to CMD has been estimated between 12 and 13 million tons a year or between 15 and 24% of total African cassava production. CMD’s vector is the white fly, a common insect in southern and eastern Africa.
But it is also spread from infected stem cuttings. Fortunately, scientists have developed resistant varieties by crossing the host plant with wild varieties. The brown streak virus, however, is more problematic for farmers and scientists. First recognized in Tanzania in 1936 it was overshadowed by CMD outbreaks until the 1990’s when its resurgence put it back in the spotlight as a primary threat.
Unlike CMD, brown streak attacks the entire plant. There can be leaf chlorosis but the major concern is the root necrosis which renders the entire plant inedible and produces the color markings characteristic of brown streak. Infected stem cuttings are also responsible for brown streak transmission.
Numerous studies have been inconclusive in determining if the white fly is also a vector for brown steak. Presently, resistant varieties are being tested, though only with moderate success. In the most promising developments plants are susceptible to CMD and a milder strain of brown streak, with less severe root necrosis.
Efforts are thus focused on educating farmers to recognize the disease and carefully remove crop from the field before it can spread. Unfortunately, this requires properly sanitized tools and burning crops, both of which put a financial burden on rural farmers, while taking away from their time to grow other food sources.
The consequences of the proliferation of this disease are immense. Sadly, if nothing is done the virus will continue to spread endangering the livelihood of millions while presenting a global threat to worldwide cassava production.
Just as the potato blight triggered a devastating famine in Ireland in the 1840’s, brown streak and mosaic disease have the potential to cause food shortages throughout the continent of Africa. Ethnic and regional fighting over the distribution of aid and lack of resources could result.
Research examining genetic variability in order to find a resistant strain must be continued. Currently, educational programs for farmers are the best hope, but will not suffice in the long run if the disease spreads faster than farmers can work to prevent it.
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