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Source: Sierraloaded [summ. Mod.DHA, edited]
A surveillance and awareness campaign against cassava viral diseases in Sierra Leone and their impacts on food security has been launched for farmers, seed multipliers, and extension agents.

The threat is Cassava mosaic disease (CDM). The other virus disease [presumably brown streak, see comment below] has already been detected in the country, but the current analysis shows that new variants are coming.

The national strategy toward the cassava diseases is a one-health approach, to be implemented over the next 5 years. It is anticipated that this system will then enable the national crop protection resources to respond also to any other major threat emerging from other pathogens.

[Byline: Chernor Alimamy Kamara]

Communicated by:

[Cassava mosaic
Cassava mosaic disease (CMD) has triggered food emergencies in the past by destroying cassava crops throughout whole regions. It is caused by a number of species and strains in the large genus _Begomovirus_ which seem to be associated with it in different areas (for example, see ProMED post 20160623.4304471). Co-infections, as well as genetic recombination between strains, have also been reported and recombinant strains can be particularly virulent.

CMD symptoms include leaf mosaic, distortion, and stunting of plants, leading to severe reduction of tuber yields. The CMD viruses are spread by whitefly vectors and with infected plant material (including seed tubers). At least one of the viruses (_African cassava mosaic virus_; ACMV) has also been found in an unrelated alternative host (soybean) suggesting that these viruses may have a wider host reservoir and may pose a threat to other crops as well. Disease management for begomoviruses usually relies on use of resistant crop cultivars, control of their vectors, and control of pathogen and/or vector reservoir plants.

Because most traditional cassava cultivars are susceptible to CMD, varieties resistant to different viral strains are being developed for local use. In Africa, this has alleviated the problem in some areas, but in others, the new varieties are being wiped out by cassava brown streak disease (CSBD).

Cassava brown streak
Cassava brown streak disease (CBSD; also called root rot) can be caused by _Cassava brown streak virus_ (CBSV) and the more recently identified _Ugandan cassava brown streak virus_ (UCBSV; both genus _Ipomovirus_). Coinfections of both viruses in the same plant also exist (ProMED post 20190214.6315604), but possible synergism or other interactions in the host still need to be clarified. Symptoms may include leaf chlorosis, stem necrosis, and yellow/brown corky necrosis in the starch-bearing tissue of roots. Leaves may appear healthy even when the roots have rotted away. Total crop losses of 20 to 80% have been reported; losses in individual root weight of up to 70% have been observed in susceptible cultivars.

The viruses are transmitted by the whitefly vector _Bemisia tabaci_ (a serious crop pest in its own right) and are spread to new areas by infected cassava cuttings. Disease management includes pathogen exclusion, removal of infected plants, use of virus-free seed tubers for planting, as well as vector control. Resistant varieties are being developed for specific local requirements. Identification of alternative hosts from unrelated plant families for CBSV and UCBSV (ProMED post 20181223.6221346 and see link below) indicate that reservoir hosts may have an impact on the epidemiology of CBSD.

CBSD is considered a threat to the whole of sub-Saharan Africa where cassava is a major food crop. Recent outbreaks appear to indicate a westward spread of the viruses (ProMED post 20130508.1698543). CBSD poses a particular risk to new crop varieties developed with resistance to CMD. A new CBSV strain adapted to higher altitudes has emerged in eastern Africa and is affecting additional CMD-resistant cultivars (ProMED posts 20100525.1735 and 20120706.1191742). Joint management of both diseases has become a major challenge in the region and globally (ProMED posts 20110829.2643 and 20090709.2463).

Sierra Leone:,46 and (with provinces)
Africa (overview):

CMD symptoms:,, and (compared with healthy)
CBS symptoms: (tubers) and (leaf)
Comparison CBS and CMD symptoms:
Whiteflies: and

Information on cassava mosaic disease:,,, and
Information on cassava brown streak:,, (diagnosis),, (vector populations), and (control)
CBS viral variants and strains: and
CBS viruses reservoir hosts:
CBS reviews: and
Virus taxonomy via:
_B. tabaci_ whiteflies, taxonomy and biotypes: (with pictures)
- Mod.DHA]

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