Cassava brown streak disease (CBSD) is a major constraint for smallholders in East and Central Africa, where new outbreaks have been reported in the past two decades. Caused by viruses that are spread by infected planting material and sap-sucking whiteflies, CBSD can destroy a significant portion of a farmer’s cassava production. SSA government institutions have focused on breeding high-yielding cassava varieties—some of them CBSD tolerant—and supervising the production and dissemination of disease-free planting material for them. However, plants are rapidly infected with the virus in areas where CBSD is prevalent, due to the abundance of whiteflies. In response, researchers at Tanzania’s Ministry of Agriculture, Livestock and Fisheries and IITA tested community phytosanitation – coordinated monitoring and on-farm practices – for controlling CBSD in areas where clean planting material is introduced.
The three-year (2013–2016) community phytosanitation pilot study engaged smallholders in two Tanzanian districts – Mkuranga, near the eastern coast, and Chato, in the northwest where CBSD incidence was higher than 90%. It began with a year of sensitization of men and women farmers and monitoring by locally recruited taskforces, followed by removal of all infected plants and replanting with CBSD-free planting material of improved varieties (‘Mkombozi’ in Chato and ‘Kiroba’ in Mkuranga) on a portion of the community’s farms. Community members monitored the fields and controlled CBSD by roguing plants that showed symptoms. Disease-free planting material was also distributed to farmers in another community in the same area where farmers did not employ such phytosanitation measures, as a control. To ensure that families had enough to eat and sell while the new cassava crop matured, participating farmers were given seed for more quickly maturing crops such as maize, sweetpotato, beans and cowpeas.
In the second year, the area in each community planted with the new variety was expanded, with disease-free planting material distributed to the original participants and a second group of farmers. In the third year, clean planting material distribution was expanded to include a third group. Whereas CBSD incidence was greater than 90% in the communities when the project began, maximum incidence was less than 40% in the third season in Chato.
Meanwhile, in the control community where farmers planted disease-free cassava but did not practice phytosanitation, incidence was greater than 60%. Kriging and geostatistics showed that community phytosanitation had an area-wide impact in reducing the levels of CBSD inoculum pressure.
“This research demonstrated that community actions to control CBSD produced strong reductions in disease inoculum levels.”IITA plant health specialist James Legg
For Khalifa Omar Nkurumah, a cassava farmer in Mkuranga and father of six, the project has opened up new income opportunities, allowing him to sell clean seed from his farm to neighboring farmers and communities.
“It has helped me—as you can see I have bought solar panels, I have dug a borehole, I have made my own cupboard for storing my clothes and food … and my child is at boarding school and I send to her bus fare,” he said, speaking of the profits made from selling cassava seed.
Khalifa now cultivates two plots, one to produce roots for household consumption and the other dedicated solely to the production of seed. “I can wake up in the morning and uproot a cassava plant and boil it for breakfast. Or even for ugali, my wife cooks with my kids and we eat,” he added.
In the first year of growing the improved varieties, farmers also achieved yield increases of 94% relative to the local variety baseline in Chato and 124% in Mkuranga. However, the most important indicator for the pilot project was that the improved variety ‘Mkombozi’ yielded 86% more in the areas of Chato where farmers practiced community phytosanitation than in the control area.
In Mkuranga, while there was an 81% reduction in CBSD incidence compared to the control area, this did not result in higher yields because the improved variety planted there, ‘Kiroba’, is CBSD tolerant. These results indicate that while farmers who plant CBSD-susceptible varieties can significantly improve yields by practicing community phytosanitation, those who have access to varieties that are CBSD tolerant or resistant may be less motivated to adopt the practice.
Legg explained that resistant varieties are currently unavailable in most areas affected by CBSD, so the research team recommends that community phytosanitation be considered as a component of integrated cassava virus management programs, particularly in areas that are severely affected by CBSD and where new cassava plantations are being established with varieties that lack tolerance or resistance to the disease.
Photo: Cassava farmer, Khalifa Omari Nkrumah, inspects the health of his crop. H.Holmes/RTB