Control of cassava brown streak disease in Zambia

Backgrounder

Introduction 

 

Cassava is the second largest agricultural crop after maize in Zambia. Cassava is predominantly grown in the Northern, Luapula, North-Western, and Western provinces where it is considered a staple crop and has been the main source of livelihoods for many generations. Although an estimated 92% of cassava produced in the country is consumed in households, the crop has potential to improve the rural economy if farmers plant disease-free cuttings, and adopt farming practices that improve soil fertility and contribute to yield gains. With improved yields, surplus cassava can be sold to enhance rural economies. However, the invasive cassava brown streak disease (CBSD), first detected in 2018 in Zambia, poses a major constraint threatening cassava yields in Zambia. If not controlled, CBSD can cause yield losses of up to 100 percent in cassava fields.

Why is this subject important to Zambian listeners?

This subject is important because cassava has been recognized as an important drought insurance crop, is the main staple crop in four out of 10 provinces in Zambia, and has potential to improve the country’s economy. Thus, farmers and other stakeholders in all parts of the country need to be aware of this new threat to cassava production and productivity in order to establish ways to recognize and manage it effectively.

Key messages

Following the information on CBSD presented in this backgrounder, Zambian farmers are urged to:

• be aware of the prevalence of CBSD in the cassava-farming regions of Zambia
• understand what factors contribute to the continued spread of CBSD
• know how to minimize the spread of CBSD to their farms
• learn how to identify the symptoms of CBSD on infected cassava roots, stems, and leaves.
• understand the impact of CBSD on cassava at household, community, and national levels
• engage with extension workers on ways to mitigate the impacts of CBSD
• know where to get disease-free cassava planting materials
• understand that CBSD spread to Zambia from neighbouring countries

What are some key facts on CBSD in Zambia?

• The two viruses that cause CBSD are Cassava Brown Streak Virus (CBSV) and Ugandan Cassava Brown Streak Virus (UCBSD). Currently in Zambia, UCBSV presence has been confirmed but CBSV has not yet been detected. The two viruses can affect a cassava plant individually or together. When both viruses infect cassava plants, symptoms are more severe than when only one infects plants.
• Zambian farmers share cassava cuttings infected with CBSD, which enhances the spread of the disease in the country. To address this issue, extension officers in cassava-growing regions should help farmers understand that they must source planting materials from credible sources in order to reduce the spread of CBSD to their farms and beyond.
• Farmers can destroy infected cassava cuttings by drying them.
• The Zambia Agriculture Research Institute (ZARI) in Mount Makulu has a tissue culture facility that is currently engaged in mass production of CBSD-free cassava cuttings. Contracted and trained cassava farmers multiply the disease-free cuttings to supply their immediate community.
• Sensitization efforts in Zambia are being led by CABI in partnership with ZARI and the Department of Agriculture (DoA) to help farmers learn about CBSD. These efforts have so far covered Kaputa, Chienge, and Kabompo districts.
• CBSD is spreading quickly across the country and ZARI is tracking the spread of the disease.
• Enforcement of phytosanitary* regulations is required to reduce CBSD spread caused by the exchange of infected cuttings, but this is a challenge due to limited sources of disease-free cuttings for farmers.
• Zambian cassava farmers are concerned that if CBSD spread is not controlled, the disease could destroy their cassava and adversely affect their livelihoods.
• The first radio programs with key information for farmers on CBSD were organized by CABI and aired on six radio stations, including Kulungwishi, Lubuto, Liseli, Kabangabanga, ZNBC1, and Mufumbwe, which cover most of the traditional cassava-growing regions of Zambia.

What are the big challenges in controlling CBSD in Zambia?

• Sharing of cassava cuttings as planting materials among farmers increases the spread of CBSD with each planting season. Farmers are not yet fully aware that CBSD spread is perpetuated through sharing cassava cuttings for planting new fields.
• In Zambia, there are no varieties that have validated resistance to CBSD that farmers can plant.
• Farmers have insufficient agronomic knowledge about how to manage their cassava fields to control or prevent CBSD infection.
• Farmers may not be able to differentiate visible CBSD symptoms from the symptoms of other cassava diseases such as cassava mosaic disease (CMD).
• Zambian farmers lack knowledge on how to space cassava cuttings when planting to optimize plant vigour during growth, which is essential for a plant’s natural defense mechanism.

Gender aspects of the impact of CBSD in Zambia

• In Zambia’s typical cassava-growing communities such as Kapepula camp in Kaputa district, women play important roles in the cassava value chain, including the selection of cassava varieties to plant and field management. Consequently, there is potential for women to be actively involved in CBSD management by helping to identify and promote the use of less affected cassava varieties. This has implications for which gender to target most with messages on CBSD identification in the field, which would enable farmers to avoid propagating cassava using cuttings from CBSD-infected fields.
• In Zambia, a CABI survey showed that mothers and their children are more involved in field activities such as weeding and overseeing cassava fields and are thus better-positioned to ensure that CBSD control measures like roguing* and the use of biopesticides are practised.

Predicted impact of climate change on CBSD spread

• Climate change has increased the prevalence of invasive species. In the Zambian context, farmers are gravitating towards using chemical pesticides to control CBSD and those pesticides in turn, are harming the natural and beneficial enemies of various other invasive and non-invasive species.

 

Key information about controlling CBSD in Zambia

 

Origins of CBSD in Zambia

• In Zambia, CBSD was initially detected by ZARI scientists between 2016-2018 in the Kaputa district of Northern Province and in the Chiengi district of Luapula Province. CBSD is prevalent in East Africa and has been a challenge among cassava farmers for a long time.

Transmission and prevalence

In Zambia, CBSD is spread:

• largely by the sharing of infected cassava cuttings by farmers when planting new fields of the crop
• to a lesser extent, by the white fly (Bemisia tabaci), a known vector of the two CBSD viruses, which transmits CBSD by sucking the sap from infected and uninfected cassava plants
• by cross-border movement of uncertified and infected cassava cuttings
• through the common practice of using the same knife to cut infected and non-infected cassava stems when farmers are propagating new fields of the crop
• by the use of susceptible cassava varieties currently available to Zambian farmers

The impact of CBSD on cassava-growing households in Zambia

CBSD impacts Zambia’s cassava-farming households through:

• increased post-harvest losses
• reduced overall yields
• reduced marketable cassava yield that reduces household income
• compromised livelihoods manifested in household and community-level poverty

Visible signs of CBSD (See also the photos below)

Farmers should monitor their cassava plants for the following signs of CBSD infection:

• Both small and major veins on cassava leaves turn yellowish in colour. This makes them look like yellowed feathers.
• Cassava leaves develop yellowish-greenish patches known as chlorotic* blotches.
• Plant stems develop dark brown lesions known as streaks.
• Cassava roots have a necrotic* brownish colour that is visible when they are cut.
• Cassava roots become constricted and are non-uniform, appearing segmented, with sugar cane-like buds.
• The main visible difference between CMD (cassava mosaic disease) and CBSD is that leaves infected with CMD are twisted, resulting in smaller roots, while those infected with CBSD are not distorted.

How to select cassava cuttings for planting

Farmers should consider the following factors when selecting cassava cuttings to plant:

• Farmers should learn how to confidently identify CBSD symptoms. This will allow them to reject diseased cuttings.
• Farmers should choose planting materials from fields (either their own or others’) with no visible symptoms of CBSD. If unsure, farmers can consult an extension officer to help them determine if a field can be used for propagation of new fields or not.
• Farmers should obtain disease-free cassava cuttings from a certified disease-free distribution centre where possible.

Please see below, on page 5, photos which describe some key symptoms of CBSD and how to identify them on cassava plants.

Practical measures for preventing CBSD

The measures below can help mitigate CBSD spread and impacts.

For farmers:

• When planting, farmers should select cassava cuttings with no visible symptoms of CBSD.
• Maintain weed-free cassava fields because some weeds are possible hosts for the CBSD causal viruses. Weeds can also host white flies that transmit CBSD to uninfected cassava plants.
• Planting early at the onset of the rains enables cassava to grow vigorously and develop a stronger defense mechanism towards pests generally. Planting late may result in cassava plants with weak stems that are prone to attack by pests, including CBSD.
• A month after planting cassava, apply 100 kilograms of D compound phosphorus (also called di-ammonium phosphate or DAP) fertilizer per hectare. Measure fertilizer using a soda cap and dig a hole by the planted stem and apply the fertilizer in the hole. Fertilizer helps increase the vigour of cassava plants, increasing their ability to defend against CBSD.
• Plant cassava cuttings to face upright rather than at an angle.
• When planting, space your cuttings well by planting rows a metre apart to enable cassava plants to thrive and increase vigour as they establish in the soil, thereby conferring a strong defence against stresses, including CBSD.
• Actively engage with extension officers to learn early detection measures and appropriate responses to CBSD outbreaks.
• Farmers should restrict movement of cassava cuttings to minimize the spread of the disease.
• Farmers should limit the exchange of cassava cuttings.

For seed multipliers:

• Farmers contracted as cassava seed multipliers should carefully inspect and uproot any cassava plants infected with CBSD. Infected stems can be destroyed.
• Farmers contracted as cassava seed multipliers should ensure that they situate new cassava plantations 100 metres from neighbouring cassava fields to minimize the possibility of spreading the CBSD causal virus.

For other audiences:

• Extension officers should use local media and various campaigns to raise awareness about the factors that lead to the spread of CBSD and how to reduce spread.
• The phytosanitary department should pay particular attention to CBSD invasion and strengthen surveillance and enforcement of regulations on movement of cassava cuttings across districts.
• CABI is advocating for the establishment of more centres in Zambia’s cassava-growing regions where disease-free cuttings can be routinely generated and propagated by organized and trained farmer groups.

Sub-Saharan research institutions with CBSD resistant/ tolerant cassava varieties

Although Zambia has not yet released CBSD-resistant or tolerant cassava varieties, some research institutions around sub-Saharan Africa have developed them.

• In 2021, the Kenya Agricultural and Livestock Research Organization (KALRO) released the genetically modified “event 4046” cassava variety that is resistant to CBSD.
• KALRO has also released the KME-08-02 (Tajirika) cassava variety that is tolerant to CBSD.
• Uganda’s National Agricultural Research Organisation (NARO) has developed the CBSD-tolerant NASE-14, NAROCASS 1, and NAROCASS 2 varieties grown by cassava farmers in Uganda.
• In Tanzania, the Namikonga cassava variety is CBSD-resistant.

Definitions

Chlorotic: Loss of chlorophyll, the green pigment in plants, usually marked by yellow or brown discoloration

Lesion: Damaged tissue of an organism as a result of injury or disease

Necrotic: Condition marked by death of plant tissues because of disease or other causes

Phytosanitary actions: Actions taken to ensure that plants are free from specified pests

Resistance: A plant’s ability to develop defense mechanisms and immunity against harmful pests and disease pathogens

Roguing: Removal of diseased plants from a field

Tolerant: A plant that can endure various types of stresses without serious consequences for growth, appearance, and yield