Cassava production has been ravaged by a number of endemic cassava pests and diseases in several sub-Saharan African. Diseases caused by viruses have assumed prominence because of their high destructive effects on the yield and quality. Cassava mosaic disease (CMD), caused by at least 9 whitefly (Bemisia tabaci)-transmitted begmovirus species (family Geminiviridae), is one of the most important virus diseases of cassava which can cause as much as 100% yield loss in susceptible varieties.
A new threat to cassava – cassava brown streak disease (CBSD) – has emerged in early 21st century in the Great Lakes region of the East African nations of Uganda, Kenya and Tanzania, which has demonstrated its sinister potential to decimate cassava. Two distinct ipomoviruses (family Potyviridae), viz., Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), also transmitted by whitefly, have been shown to be associated with the etiology of CBSD, and both single and mixed infections of these two viruses cause symptoms characteristics of CBSD.
CMD is known to occur in all the cassava production zones in the continent and its off-shore islands. CBSD was recorded in the early 20th century from the coastal zones of Mozambique, Tanzania, Kenya and shores of Lake Malawi. CMD infection results in mosaic, deformed shoot growth, and severely reduced tuberous roots. CBSD produces feathery vein-associated chlorosis and/or chlorotic blotching on the lower leaves of affected plants, but also a dry corky necrosis and extensive rotting of the tuberous roots making them inedible and unmarketable, leading to total loss of the useful part of the cassava.
In the early 2000s, CBSD was reported in Uganda where it caused a severe outbreak which quickly spread to neighbouring areas of western Kenya and north-western Tanzania. In 2011, disease outbreaks were recorded in Rwanda, Burundi and the eastern Democratic Republic of Congo (DRC), suggesting the westward spread of the disease assisted by whitefly and also through accidental use of infected stems for cassava propagation.
New outbreaks of CBSD in East Africa caused enormous human suffering due to the severe shortage of edible roots and disease-free planting material necessary for replanting, affecting the livelihoods of cassava growers and the cassava value chain. Emergency responses deployed to understand the nature of the disease and control measures since 2004 have resulted in a suite of technologies and information, including the identification of tolerant varieties to control CBSD. However, the disease has become established in the affected areas and is impervious to many of the currently available control measures.
The current experience from East Africa suggests prevention as the best approach to control CBSD, which is mainly implemented through phytosanitation to eradicate infected sources and supply farmers with clean planting material of farmer-preferred CMD-resistant cultivars. The cultivation of CMD-resistant cultivars has proven to be the most suitable approach to manage CMD across the continent.
There is an urgent need to expand these activities into Central and West Africa to slow the spread of the expanding CBSD pandemic, and to alleviate the impacts of both CBSD and CMD in areas already affected by the two diseases. There is grave concern about the impact that CBSD might have on the food security of Nigeria, if affected, since this is Africa’s largest cassava producer.
Pre-emptive measures against CBSD spread in Nigeria
Cassava is a major food staple and income earner for most of the 162.5 million Nigerians. Considering the crop’s importance for food security, industrial use and as a foreign exchange earner, the Federal Government of Nigeria under the Agricultural Transformation Agenda (ATA) planned to double cassava production, with a special emphasis on improving yields and strengthening cassava value chains. The success of the cassava transformation agenda (CTA) depends on sustaining and further enhancing cassava production and productivity. Various measures are being put in place including policies to support: the wider utilization of cassava flour, new processing industries, and the release of high yielding varieties. These on-going measures to improve the cassava-based economy are wholly dependent on managing the various threats known to hamper cassava production and productivity. The consequences of an outbreak of CBSD in Nigeria would be catastrophic, as it would negate all of the positive gains achieved through the ATA and negatively impact the food security and economy of millions of cassava farmers.
To counter this threat, it is necessary to develop/strengthen early warning systems to facilitate the rapid detection, diagnosis and communication of cases of disease incursion, and to develop an emergency response plan for CBSD. This will complement activities of the WAVE initiative operating in Nigeria, to enhance the country’s resilience to invasive threats.
Pest risk assessment and surveillance for CBSD: A recently developed pest risk assessment (PRA) report on cassava will be reviewed to assess its suitability as the basis for the development of a CBSD control strategy for Nigeria. A key component of this will be the establishment of an emergency response plan for the detection and eradication of infected plants as soon as CBSD is sighted in the country. The PRA will be conducted as per the International Standards for Phytosanitary Measures (ISPM) guidelines of IPPC-FAO to evaluate the probability of the introduction and spread of CBSD and associated potential economic damages and threat to food security in Nigeria.
Early warning network for cassava viruses: Early detection is critical to prevent the emergence and establishment of new pathogens. Traditional methods involving regular surveys by experts are expensive and time-consuming. The recent revolution in digital communications technology has enabled the development of virtual networks for rapid communication and diagnosis of local problems from distance. In this project we envisage to link the Nigerian extension services, including NAQS and the Agricultural Development Programs (ADPs), with local government areas (LGAs) in major cassava-producing States, for real-time monitoring and rapid communication on the disease situation and the implementation of mitigation plans. This involves training on disease recognition, the use of mobile phones for communication with NAQS Post-entry Quarantine Station in Ibadan (which will act as the coordinator for this early warning network) and the development of an action plan for emergency response.
For more information, contact:
|Dr. Lava Kumar
Head, Germplasm Health Unit/Virology
IITA, Ibadan, Nigeria
Mr. D. Ogunfunmilayo
Head, Postentry Quarantine
Nigerian Agricultural Quarantine Services