CIMMYT Breeding Progress for Tolerance to Maize Lethal Necrosis in Eastern Africa.

Maize lethal necrosis (MLN) has emerged as a serious threat to maize production in eastern Africa, resulting in significant yield reduction up to total crop failure.  MLN in the region is most commonly caused by the co-infection of maize chlorotic mosaic virus (MCMV) and sugarcane mosaic virus (SCMV).   The current east African epidemic was precipitated by the recent introduction and rapid spread of MCMV.  Environmental stresses such as drought and heat also play a major role in disease severity.  Since 2011, MLN has become established in six east African countries, negatively impacting the livelihood of maize smallholder farmers.  Additionally, the vast majority of African maize lines and commercially available varieties are highly susceptible to MLN.  In conjunction with the Kenya Agriculture and Livestock Research Organization (KALRO) and other regional partners, CIMMYT has identified hybrids with moderate tolerance to MLN which demonstrate significant yield advantage over widely grown hybrids under natural MLN infestation.  Promising sources of MLN tolerance have been identified from global resources including CIMMYT elite breeding panels, a USDA virus resistant line collection, and maize landrace accessions from the CIMMYT gene bank.  Among the most tolerant lines identified to date are CML494, a tropical lowland line developed in Mexico, and KS523-6, a tropical line from Katsetsart University.  Landrace accessions from Latin America and the Caribbean with tolerance to MCMV have also been identified.  Major QTL for MLN tolerance have been identified and are being introgressed into elite African lines.  Strategies for accelerating genetic gain for yield under MLN pressure are also being implemented.  A summary of MLN screening, QTL discovery and breeding for MLN tolerance will be presented.