As deadly maize lethal necrosis (MLN) disease continues to decimate crops across eastern Africa, it is crucial to develop new maize varieties with tolerance to the disease. According to a community survey held in 2016, annual maize losses due to MLN are estimated at 0.5 million tons in Kenya alone, 22 percent of the average annual production before MLN, with a value estimated at $180 million. CGIAR Research Program on Maize (MAIZE) scientists have made exciting progress against MLN in 2016 on many fronts, from developing and deploying MLN tolerant varieties to the development of innovative technologies, such as standardized MLN survey and sampling protocols and digital surveillance tools. Information management was enhanced with the launch of the MLN information portal and establishment of a MLN Phytosanitary Community of Practice. Strategic capacity building took place via training of personnel from National Plant Protection Organizations (NPPOs) in eight countries in eastern and southern Africa in 2016.
When MLN first struck in Kenya in 2011, MAIZE and its partners immediately launched intensive efforts to identify and develop MLN-tolerant maize hybrids, while developing the capacity of partners to identify sources of MLN resistance.
Since then, MAIZE researchers and their partners have screened nearly 100,000 germplasm entries from all over the world at the specially constructed MLN screening facility in Naivasha, Kenya in search of maize varieties with natural resistance to the disease. The first phase of these evaluations focused on existing released varieties and materials in the elite breeding pipelines, however, over 95 percent of these materials have proven susceptible to MLN.
A strong pipeline of new MLN-tolerant/resistant MAIZE hybrids has been established, as shown by trials conducted in 2016 that compared 18 first-generation MLN-tolerant hybrids and 19 second-generation MLN-tolerant/resistant hybrids to the 9 most popular commercial hybrid checks in the region. Scientists found that the commercial check hybrids were highly susceptible to MLN under artificial inoculation, giving a mean yield of 0.69 tons per hectare (t/ha), while the mean grain yield of the first-generation and second-generation hybrids under MLN artificial inoculation were 3.41 and 4.49 t/ha. This significant yield increase can help improve the food security and livelihoods of farmers that have suffered crop losses from this disease. While the percentage of tolerant material discovered was low, since 2015 the MAIZE team has succeeded in releasing a number of MLN-tolerant maize hybrids in eastern Africa, including five released through partners in 2016 and 22 currently under development. For example, in 2016, the NASECO seed company commercialized nearly 20 tons of certified seed of “Bazooka” in Uganda, a variety developed by researchers with MAIZE, and harvested around 300 tons of certified seed to be commercialized in 2017. In addition, the MAIZE team produced and distributed 100 kg of seed of MLN-tolerant pre-commercial hybrids to nine partners in East Africa for national performance trials and on-farm demonstrations.
Due to the low percentage of materials with tolerance to MLN, there is a need to further expand the genetic sources of resistance to this complex disease. To this end, MAIZE scientists decided to focus their efforts on maize chlorotic mottle virus (MCMV), the virus that is the major factor in MLN disease and to which most of the material screened was susceptible. Starting in 2015, MAIZE scientists focused on evaluating heirloom maize varieties known as landraces from areas of Latin America and the Caribbean that are known to have high incidences of MCMV and other viruses as well as high levels of maize genetic diversity.
To date, over 1,000 landrace accessions and populations have been evaluated and the 20 most promising landraces for MCMV tolerance have been crossed to high-performing elite lines from the International Maize and Wheat Improvement Center (CIMMYT) and then selfed, or self-pollinated, to create new advanced progenies for advanced evaluation.
To date, over 900 F3, or third generation, maize lines have been evaluated in the greenhouse for MCMV tolerance. These lines have additionally been genotyped in order to evaluate if the genetic source of any observed tolerance is from novel alleles, or gene forms that have not been used before, or is from alleles that already exist in elite breeding materials. The F4 lines derived from the F3 lines with observed MCMV tolerance have been produced and are being sent to the MLN screening facility in Naivasha, Kenya for more in depth evaluation in 2017. Breeders hope to begin releasing semi-inbred lines with novel alleles for MCMV/MLN tolerance to the maize breeding community in late 2017 and in 2018.