New approaches in the fight against bacterial rice disease

An international research team, the ″Healthy Crops″ consortium, has developed rice varieties resistant to a detrimental crop disease in East Africa and Madagascar. The new varieties are resistant to bacterial leaf blight (for short: BB), caused by the bacterium Xanthomonas oryzae pv. oryzae (Xoo). The intention is to transfer these varieties to local breeders in Madagascar and Tanzania, where they will first be subjected to field tests and subsequently distributed to farmers.

Rice is an essential staple food for more than four billion people worldwide and of central importance for food security and economic stability in low- and medium-income countries. In Tanzania and Madagascar, the recent BB outbreaks pose a serious threat to the livelihood of smallholder farmers, impacting both food supplies and the local economy. The disease causes significant yield losses, leading to food shortages and income losses in agricultural communities.

Rice field in Betafo, Madagascar, with FOFIFA staff. The leaves of the rice plants show the typical lesions caused by bacterial blight infection.

IRD/Mathilde Hutin

The BB outbreak in East Africa has now also spread to neighbouring regions. Dr Ibrahim Hashim from the Tanzanian Agricultural Research Institute (TARI) explains: ″The outbreak, which was first noticed in 2019, is now causing yield losses of 20 – 25% in Tanzania and is having a massive impact on the economic stability of smallholder farmers and consequently on regional nutrition.″ Comparable losses are estimated in Madagascar, as Dr Mathilde Hutin from the French National Research Institute for Sustainable Development (IRD) reports.

Dr Hutin and Dr Boris Szurek (also from IRD) are working in close collaboration with Dr Hashim from TARI and Dr Harinjaka Ravelonson from the National Center for Applied Research in Rural Development (FOFIFA) to evaluate the spread of the disease in the two countries. Together they were able to identify the responsible bacterial strains and the infection mechanism. The research revealed that the outbreaks were caused by the unintended introduction of highly virulent Xoo strains from Asia. These Asian strains use a different infection mechanism to the endemic African relatives, to which the African rice varieties are highly susceptible.

In response, the ″Healthy Crops″ team, led by Alexander von Humboldt Professor Dr Wolf B. Frommer from Heinrich Heine University Düsseldorf (HHU), has developed three new rice varieties that are expected to protect East African rice harvests against the aggressive Xoo strains. The so-called marker-assisted backcross breeding (MABB) technique was used, which adds the resistance while retaining the general properties of the original variety. This advanced breeding method uses genetic markers to establish desirable traits in plants on a targeted basis more quickly and precisely.

Reuben Mihayo, scientist at TARI in Tanzania, takes leaf samples to examine them for the presence of BB. Right: Rice plants in Madagascar, which are heavily infected with bacterial blight.

TARI/Ibrahim Hashim; IRD/Mathilde Hutin

Using this special breeding technique, Alexander von Humboldt Fellow Dr Yugander Arra from HHU transferred disease-resistant traits from an Asian rice variety into two widely used African elite rice varieties: NERICA4 and Komboka. The accelerated breeding process has reduced the typical development period from decades to just a few years and offers new hope in the fight against disease in these critically important crop plants in East Africa.

The newly developed BB-resistant rice varieties will now be transferred to plant breeders and researchers working locally in Madagascar and Tanzania for field testing. Sergio Antonio Castro Pacheco, scientist at CIRAD – the French Agricultural Research Centre for International Development, hosted by FOFIFA – is coordinating this work in Madagascar, while Dr Hashim is monitoring the tests in Tanzania. Following additional crossing and through validation under local conditions, these varieties could be released in the national registration processes and could be made available to farmers if they prove beneficial.

Dr Eliza Loo, project leader at the Institute for Molecular Physiology at HHU, states: ″We want to improve the income and livelihoods of smallholder farmers by using knowledge-based approaches to defeating plant diseases. Transferring the rice varieties generated at HHU brings us a big step closer to the goal of our project.″

Professor Frommer adds: ″Healthy Crops is prepared to share the lines with other interested institutions in further East African countries affected by BB, including Malawi, Uganda, Rwanda, Democratic Republic of Congo, Zambia and Mozambique.″

The Healthy Crops project is supported by the Bill & Melinda Gates Foundation (grant ID: INV-008733) and the Alexander von Humboldt Foundation, with further funding from the EU DESIRA project DINAAMICC led by CIRAD and involving FOFIFA and IRD.