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Unearthing a solution to cassava root rot

During the past decade, Cassava mosaic disease (CMD) has been the focus of extensive collaborative research to understand the nature of the disease and to develop resistant varieties of cassava. However, another disease, although currently confined to the East African coast, can render susceptible varieties unusable if cassava roots are left in the ground for over nine months. Like CMD, Cassava Brown Streak Disease (CBSD) is a viral disease. But, unlike the mosaic symptoms of CMD, the foliar symptoms of CBSD are less conspicuous and farmers are often unaware of the problem until the crop is harvested and the corky, yellow-brown necrosis affecting the roots becomes evident. As a two-way approach to controlling the disease, scientists at NRI and the University of Bristol in the UK, through the DFID-funded Crop Protection Programme, are working to identify the virus and its vector and to produce virus-free material for distribution to farmers.

Disease prevalence

Cassava infected with Cassava Brown Streak Disease
credit: Rory Hillocks, NRi

Estimates of mean losses attributed to Cassava Brown Streak Disease on a national or regional basis are hard to make as the extent of loss depends not only on the susceptibility (or sensitivity) of the cultivar but also on when the crop is harvested. However, results from recent research have shown reductions in root weight of up to 20% and a further 30-40% loss due to roots being rendered unsuitable for consumption. Through work conducted in Tanzania, disease incidence for the coastal region is known to be about 20% and, in Mozambique, a recent survey has revealed CBSD to be particularly prevalent: sometimes as high as 80-90% in parts of the coastal area of two of the northern provinces. However in individual fields, 90-100% incidence of CBSD is not uncommon in coastal areas of both Tanzania and Mozambique. The disease also occurs in Kenya and may well be present elsewhere.

Disease control

Although farmers may not recognize Cassava Brown Streak as a viral disease, they are aware of the resulting root rot symptoms. Farmers need to be made aware of what causes these symptoms and be given advice on control. One management option is to harvest earlier as necrosis tends to affect roots of sensitive varieties from 5-6 months after planting. But early harvesting to avoid the effects of the disease still results in reduced yields because the crop is not in the field long enough to reach its full yield potential.

Farmers are the key to controlling CBSD
credit: R. Hillocks, NRi

In Tanzania, a number of local cultivars, currently grown by farmers, appear to show some resistance or tolerance, to CBSD. These have now been taken into trials to be evaluated more closely and those that demonstrate potential to resist the disease are being multiplied. Farmers will then be able to make their own choices about which of the varieties they prefer to grow from cuttings in order to multiply and subsequently pass material on to neighbours and neighbouring villages. And, in the long-term, Rory Hillocks of NRI believes that farmers are the key to managing this disease. "Although virus-free planting material is being released to farmers, that isn't the end of the story as every year farmers are selecting their own planting material and every year they are the ones that must be responsible for continuing to ensure that their planting material stays free of viral diseases." Dr Hillocks feels very strongly that raising awareness amongst farmers is vital - not just in areas where the disease is known to be prevalent - but also in other regions where the disease could become evident. However, he stresses that organizations involved in distribution of cassava planting material to farmers have a responsibility to ensure that the cuttings are free of mosaic and CBSD.

Understanding more about CBSD will also contribute to the further control of the disease. Through work conducted by the University of Bristol, the virus that causes Cassava Brown Streak has recently been identified. The identification of the virus indicates that, as with the CMD virus, the CBSD virus may also be transmitted by whiteflies. Closer attention is therefore being focused on the two whitefly species that occur in East Africa as possible vectors of the CBSD virus.

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