PlantVillage University Seminar - Cassava Viruses

Cassava is the highest-produced fresh staple crop in Africa. In 2019 alone about 192 million tons of cassava were produced in Africa. After cassava, the most produced staple crops in Africa are maize, yam, banana, rice, sorghum, sweet potatoes, potatoes, and millet in descending order. Among all the major staple crops in Africa, cassava is the most adapted crop for climate change. Cassava is the only staple crop that is expected to have the least negative impacts from climate change. Drought tolerance of cassava enables cassava to be grown even in areas with harsh and warmer climates. Many believe that cassava is the answer to climate adaptation in Africa, unfortunately, cassava is under serious threat from diseases [1].  

                                                  

                                                        Total Fresh Staple Crop Production - Africa 2019 (FAOSTAT, 2021) 

Most of the cassava plants we see today in Africa are infected with diseases. Cassava mosaic disease (CMD), and cassava brown streak disease are the two major cassava diseases in Africa. CMD causes severe stunting of the cassava plants and distortion of the cassava leaves [2]. CBSD causes necrosis on cassava leaves and destroys the cassava root tuber. CBSD has been known in Africa for a long time. It was first recorded in 1920 and 1930 in coastal Eastern Africa. Recently there was an outbreak of CBSD in Uganda in 2004, and since then it has been spreading all over East and Central Africa. There is a major fear of the spread of CBSD to West Africa, where cassava is a major staple crop and supports millions of smallholder farmers. Whereas CBSD is currently in Eastern and Central Africa, CMD is currently in all countries where cassava is grown in Africa. CMD spread throughout Africa in the 1920s.  

CMD and CBSD are caused by viruses that are spread by an insect vector called whitefly. The cassava whitefly lives, feeds, and breeds under the cassava leaves. The larvae of the whitefly feeds on the phloem tissue in the plant and the sugar of the plant. The larvae also excrete a sugary liquid that drops down onto bottom leaves, which can cause fungus infection for the cassava plant. Since the 1990s the populations of the whitefly insects have been increasing at alarming rates. This increase in the whitefly population has increased the spread of cassava diseases. The increase of the whitefly population in Africa and the spread of cassava disease is a serious threat to food security for about 500 million people in Africa [2]. 

The role of the whitefly in spreading cassava diseases is complex. Crop varieties, weather, and other environmental factors all affect the distribution of the whitefly and the spread of cassava diseases. Scientists today still do not understand why the whitefly population in East and Central Africa has been spreading and increasing at alarming rates since the 1990s. There is an urgent need for scientific research to understand the dynamics of the cassava disease movement. This knowledge of the disease movement is needed to develop management strategies to maintain cassava productivity and to ensure food security for millions of people in Africa. 

Dr. James Legg is a plant virologist from IITA, with more than 20 years working on viruses and their insect vectors in Africa. Dr. James' research focuses on understanding cassava viruses and using his understanding of viruses to improve control measures against cassava diseases. Dr. James explains that there are three things that need to be done to secure the future of cassava production in Africa. (1) Knowing the enemy, (2) Identifying and tracking the enemy, and (3) Preventing spread and controlling damage.  

Although most whiteflies in Africa look similar there are multiple distinct species of whiteflies with different behavior and characteristics. Some species of whitefly spreads CMD, and CBSD more than others. Like whitefly, there are multiple species of virus that cause CMD, and CBSD. There are about 9 distinct species of viruses that can cause CMD and about 2 species that can cause CBSD. A cassava variety that is resistant to one CMD virus is generally resistant to all the other CMD viruses and the same goes for CBSD. On the other hand, a cassava variety that is resistant to CMD is not necessarily resistant to CBSD and vice versa. The interaction between whitefly and the distinct species of viruses is important for management planning and that is why proper identification of diseases in the field is critical.  

There are multiple ways to identify cassava viruses. Some of the methods of identification include the qPCR, ELISA, LAMP, Digital PCR, and others. All these methods are primarily lab-based and take a lot of time and resources. Some methods of identification like the LAMP can be used in the field but require transporting expensive equipment to the field. The LAMP method can identify if you have CMD or CBDS and provide additional information about what species of virus might be causing the disease. Another method of identifying cassava diseases in the field is by using PlantVillage Nuru. This method only requires a mobile phone. PlantVillage Nuru uses Artificial Intelligence to detect CMD and CBSD infection in cassava plants. Despite being imperfect, the PlantVillage Nuru is extremely good. In research done in Tanzania to compare PlantVillage Nuru predictions to farmers, extension officers, and researchers [3]. PlantVillage Nuru's performance was second to trained researchers and better than trained extension officers. In addition to in-field identification, the PlantVillage Nuru provides scientists with a unique opportunity for remote monitoring of cassava diseases in the field. 

 

                                                                                      PlantVillage Nuru Performed better than Extension Officers 

Traditionally disease monitoring has been done by trained extension workers. The extension workers drive around the community, take samples from the field and send them back to the laboratory for analysis. The result from the analyzed samples enables the extension officers and interested parties to understand the distributions of disease in the community. Although this method is highly effective, it takes a lot of time, and it is expensive. The PlantVillage Nuru provides a unique opportunity for remote, fast, and affordable monitoring of cassava diseases. Cassava disease detections from PlantVillage Nuru are geo-tagged and saved in the cloud. This geolocation information of cassava detections can be used to build models and help scientists understand the spatial distribution of the cassava diseases, and how it changes over time. Understanding the spatial distribution of cassava diseases and their environmental drivers can help scientists develop effective management systems to secure cassava production in the future of Africa. Just as proper identification and proper monitoring is essential for the control of the covid-19 virus, the same is essential for the control of cassava virus, and the PlantVillage Nuru provides a platform to help farmers identify diseases in their farm and for scientists to monitor and track the movement of the cassava diseases remotely.  

                                                                                            PlantVillage Nuru Survey Panel  

There is no effective management strategy to deal with a cassava plant that is already infected with CMD or CBSD, therefore the most effective strategy against the virus is to prevent and avoid the virus in your farm. The use of virus-free cassava seedling and resistant varieties are the most effective strategy to control and manage CMD and CBSD. There are good cassava-resistant varieties against CMD and CBSD. These cassava-resistant varieties are not totally immune, but they are extremely hard to infect and highly effective. There is a need for more improved varieties of cassava to be developed. The development of cassava varieties should not only consider the resistance and yield potential of the improved crop, but also the preference of the local communities. Improved varieties not only have to be resistant to viruses but should also meet the taste and market needs of the smallholder farming communities. 

Dr. James explains that there is a need to integrate cassava disease monitoring platforms with cassava clean seed grower network platforms. This integration will enable farmers to acquire the current cassava-resistant varieties directly through the cassava disease monitoring platforms. Farmers that detect CMD or CBSD in their farms will be connected to cassava seed grower networks in their communities. Dr. James calls this strategy the link for impact. The strategy has four steps (1) Train farmers to use PlantVillage Nuru to identify cassava disease. (2) PlantVillage Nuru advises farmers to access clean seed or disease-resistant varieties. (3) PlantVillage Nuru connects farmers with disease detections in their farms to Seed Tracker (a platform for certified cassava varieties network). (4) Seed Tracker provides contacts of certified cassava entrepreneurs in the community to the farmer.  

In conclusion, the fight against the cassava virus is an important one. It is a fight that will determine the status of food security for millions of smallholder farmers. The development of ICT tools like PlantVillage Nuru for disease monitoring, Seed Tracker for cassava variety distribution, and others provides a unique opportunity for affordable and transformative cassava disease management. In a continent where the ratio of extension officers to farmers can be as low as 1:3000, the development of affordable and innovative extension tools is essential for the education of farmers, and for the management of diseases. 

 

Reference 

[1] Jarvis, Andy, et al. "Is cassava the answer to African climate change adaptation?." Tropical Plant Biology 5.1 (2012): 9-29. 

[2] https://news.cornell.edu/stories/2021/02/gift-supports-cornells-tanzanian-cassava-efforts 

[3] Mrisho, Latifa M., et al. "Accuracy of a smartphone-based object detection model, PlantVillage Nuru, in identifying the foliar symptoms of the viral diseases of cassava–CMD and CBSD." Frontiers in plant science 11 (2020): 1964. 

[4] Ogbe, Francis O., et al. "Symptom severity of cassava mosaic disease in relation to concentration of African cassava mosaic virus in different cassava genotypes." Plant pathology 52.1 (2003): 84-91. 

 

Written by: Edward Amoah