Development of late blight resistant potato biotech varieties for Sub-Saharan Africa.

Phytophthora infestans remains the most devastating potato pathogen worldwide. In developing countries it causes annual losses of up to 2.75 billion USD, excluding the cost of pesticides. Host plant resistance mediated by single R genes has been rapidly overcome by P. infestans on many occasions. However, durable resistance might be achieved through genetic transformation of a potato variety with different combinations of R genes from Solanum wild species. The RB, Rpi-blb2 (isolated from Solanum bulbocastanum) and the Rpi-vnt1.1 (isolated from S. venturii) genes are being transferred into the susceptible variety Desiree and other varieties grown in sub-Saharan Africa (SSA). Close to 600 transgenic events (TE) were produced from Desiree with either a single R gene (251) or the 3R gene stack (331). TE with high levels of resistance to P. infestans were identified by in-vitro and whole-plant assays. Fewer than 10% of the TE with either a single R gene or the 3R gene stack were found to be highly resistant. A hypersensitive reaction to specific isolates and extreme resistance was observed for 7 out of 64 RB-TE, 4 out of 117 Rpi-blb2-TE, 1 out of 25 Rpi-vnt1.1-TE, and 16 out of 162 3R-TE. Varieties from SSA have been identified as candidates for transformation, with priority given to three: Shangi (rapidly increasing adoption in Kenya), Tigoni (well established variety in Kenya), and Cruza 148 (grown in the Lake Kivu region, especially in Burundi and Rwanda). After greenhouse screening of all existing TE, the top 10 highly resistant 3R-TE will be evaluated in confined field trials. This will be performed during several cropping seasons to monitor resistance stability and dynamics of the pathogen population.