Skip to main content

Breeding cassava for the next generation

Last week I helped to harvest and score cassava tubers a breeding trial at the National Crops Resources Research Institute (NaCRRI). The trial is part of the NEXTGEN Cassava project which applies genetic techniques to conventional breeding and aims to produce new varieties with Cassava brown streak disease (CBSD) and Cassava mosaic disease (CMD) resistance.

Why cassava and what’s the CBSD problem?

Approximately 300 million people rely on cassava as a staple food crop in Africa. It is resilient to seasonal drought, can be grown on poor soils and harvested when needed. However cassava production is seriously threatened by CBSD, which can reduce the quality of tubers by 100% and is currently threatening the food security of millions of people.
Cassava brown streak symptoms on tubers

Crossing cassava from around the world

Cassava varieties show a huge variation in traits including disease resistance. The NEXTGEN Cassava project has crossed 100 parent plants from Latin America with high quality African plants to produce new improved varieties, with higher levels of CBSD and CMD resistance. Crossing involves rubbing the pollen from one parent variety on to the female flower part (pistil) of the second parent variety to produce seeds.

Cassava flowers used to cross different varieties

Cutting back on time

The process is not easy. The complex heritability of traits in cassava means that many plants have to be screened to identify plants with the best traits. To cut down on this time, researchers from Cornell University sequenced the DNA from 2,100 seedlings and selected plants containing sequences linked to desirable traits.

Screening for resistance

These plants were transferred to field site in Namulonge, where there is a high level of CBSD, making it easier to spot resistant plants. After 12 months the tubers were dug up and cut into sections. Each root was scored for the severity of CBSD. Plants which  show no disease symptoms have now been selected for the next stage of breeding. Eventually varieties will be tested for their performance at sites across Uganda and given to farmers for their feedback.

We harvested and scored tubers for Cassava brown streak symptoms. I then tagged disease free plants for selection!

Time to harvest!

Alfred Ozimati is managing the breeding  programme
I helped to score and tag plants, it was hard work! I was impressed by the stamina of the workers who harvested from 8 am until 3 pm without a rest. I was struck by the mammoth task of breeding cassava for so many traits and by the programme manager Alfred Ozimati’s determination to get the work done as quickly as possible. Alfred is currently a  PhD student at Cornell University; he kindly offered to answer these questions:

What are the challenges of conventional breeding and how does sequencing help to address these?

Typical conventional breeding cycle of cassava is 8-10 years before parents are selected for crossing. The sequencing information allows a breeder to select parents early at the seedling stage, allowing more crossing cycles over time than conventional cassava breeding. With sequencing, the process of releasing varieties with improved CBSD and CMD resistance should take about 5 years.

What are your long term hopes for the project and the future of cassava breeding?

We hope to use genomic selection routinely, to address any other challenges cassava as a crop of second importance to Uganda will face. And also to take the technology to other East African, cassava breeding programs to faster address their major breeding constraints.

This blog has been written by University of Bristol Cabot Institute member Katie Tomlinson from the School of Biological Sciences.  Katie's area of research is to generate and exploit an improved understanding of cassava brown streak disease (CBSD) to ensure sustainable cassava production in Africa.  This blog has been reposted with kind permission from Katie's blog Cassava Virus

Katie Tomlinson
More from this blog series:  

Popular posts from this blog

Converting probabilities between time-intervals

This is the first in an irregular sequence of snippets about some of the slightly more technical aspects of uncertainty and risk assessment.  If you have a slightly more technical question, then please email me and I will try to answer it with a snippet. Suppose that an event has a probability of 0.015 (or 1.5%) of happening at least once in the next five years. Then the probability of the event happening at least once in the next year is 0.015 / 5 = 0.003 (or 0.3%), and the probability of it happening at least once in the next 20 years is 0.015 * 4 = 0.06 (or 6%). Here is the rule for scaling probabilities to different time intervals: if both probabilities (the original one and the new one) are no larger than 0.1 (or 10%), then simply multiply the original probability by the ratio of the new time-interval to the original time-interval, to find the new probability. This rule is an approximation which breaks down if either of the probabilities is greater than 0.1. For example

1-in-200 year events

You often read or hear references to the ‘1-in-200 year event’, or ‘200-year event’, or ‘event with a return period of 200 years’. Other popular horizons are 1-in-30 years and 1-in-10,000 years. This term applies to hazards which can occur over a range of magnitudes, like volcanic eruptions, earthquakes, tsunamis, space weather, and various hydro-meteorological hazards like floods, storms, hot or cold spells, and droughts. ‘1-in-200 years’ refers to a particular magnitude. In floods this might be represented as a contour on a map, showing an area that is inundated. If this contour is labelled as ‘1-in-200 years’ this means that the current rate of floods at least as large as this is 1/200 /yr, or 0.005 /yr. So if your house is inside the contour, there is currently a 0.005 (0.5%) chance of being flooded in the next year, and a 0.025 (2.5%) chance of being flooded in the next five years. The general definition is this: ‘1-in-200 year magnitude is x’ = ‘the current rate for eve

Coconuts and climate change

Before pursuing an MSc in Climate Change Science and Policy at the University of Bristol, I completed my undergraduate studies in Environmental Science at the University of Colombo, Sri Lanka. During my final year I carried out a research project that explored the impact of extreme weather events on coconut productivity across the three climatic zones of Sri Lanka. A few months ago, I managed to get a paper published and I thought it would be a good idea to share my findings on this platform. Climate change and crop productivity  There has been a growing concern about the impact of extreme weather events on crop production across the globe, Sri Lanka being no exception. Coconut is becoming a rare commodity in the country, due to several reasons including the changing climate. The price hike in coconuts over the last few years is a good indication of how climate change is affecting coconut productivity across the country. Most coconut trees are no longer bearing fruits and thos