Progress in Identifying MLN Resistant Maize Germplasm
George Mahuku, D. Makumbi, D. Bish, A. Wangai,
Y. Beyene, S. Mugo, K. Semagn, and B.M. Prasanna
Workshop to develop a strategic plan for Maize Lethal Necrosis disease for Eastern and Central Africa, Jacaranda Hotel, Nairobi, Kenya, 21-23 August, 2013.
What is Maize Lethal Necrosis
MCMV
PotyvirusSCMVMDMVWSMV
MLN
• Individual infection with each virus can also cause disease• Typically, infection with one virus results in milder symptoms
than MLN but reaction depends on germplasm and viral strain.
MLN Development
Vectors: Presence of aphids and
thrips, other insects
Environment: Conditions
favoring vectors and
disease
Susceptible Germplasm
Virus: Either individual or compound
1. Disease screening tools
2. Diverse germplasm
3. Test locations with consistently high biotic stresses pressure
4. Standardized screening protocols
Requirements to identifying MLN resistant germplasm
Resistance screening tools
Use field, screen house and laboratory-based screening tools
Established using artificial inoculation or infestation Standardized disease establishment and evaluation protocols Ability to handle large populations / germplasm (high throughput for
large scale screening) Diagnosis of viruses
Use hot-spot locations with consistently high disease, and pest pressure for screening
Re-evaluate selected resistant materials in multiple-locations to expose them to different strains or biotypes
SCMV amplificationMCMV amplification
MCMV + SCMV amplification
Screening maize germplasm for MLN: Artificial inoculation
Field inoculation and reaction of germplasm
7Variation for reaction to MLN disease as of 2, January 2012
Centralized MLN screening Facility
• 20 Ha being established in Naivasha for MLN screening– Labs– Greenhouses– Field
Use of Disease Hot Spots
• Use hot-spot locations with consistently high disease, and pest pressure for screening– Naivasha, Kenya– Bomet, Kenya– Babati, Tanzania– Arusha, Tanzania
• Need to identify more sites in other countries
Diverse germplasm Sufficient genetic variation exists for most diseases, pests
and parasitic plants in maizeLocally adapted or introduced maize germplasmLandrace collectionsOld varieties and breeding stocks
Resistance alleles in these genetic resources can occur at low or high frequencies
Resistance genes occurring at low frequencies can be gradually increased
Genes at high frequency are easy to transfer
Distribution of severity scores among 200 inbred lines
12
Reaction of selected hybrids to MLN disease
Entry 1-17: elite experimental hybridsEntry 18: Susceptible testerEntry 20-22: Tolerant SC testersEntry 23-28: Commercial checks
1 2 3 4 5 6 7 8 9 101112131415161718192021222324252627280
10
20
30
40
50
60
70
80
Why develop standardized protocols?
• Harmonize data collection amongst sites• Enable collaborators and partners to interpret and
share data• Support registration, regulation and release of
varieties across borders• Optimization time and resources devoted to
screening• Good phenotyping is key to identifying good markers
and developing MLN resistant germplasm.
Conclusions• There is genetic variation for response to MLN• Extensive screening of germplasm may lead to
identification of more sources of resistance• Need to identify more sites and create a network of
Screening Sites• Develop standardized protocols and transfer to
partners
Challenges• Lack of knowledge on number of strains of different
viruses• Little or no knowledge on epidemiology of MLN in
Africa• Lack of guidelines on management of vectors• Restrictions on movement of seed for research across
countries• Lack of dedicated greenhouse for inoculum
generation and large farms for germplasm screening
Acknowledgement
• KARI• Monsanto• USDA Ohio State University• Sunripe Farm, Naivasha• Olerai Farm, Narok