1. 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, August, 2013.

2. What is Maize Lethal Necrosis
MLN
Potyvirus
SCMV
MDMV
WSMV
MCMV
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.

3. Susceptible Germplasm
Virus: Either individual or compound
Susceptible Germplasm
MLN Development
Vectors: Presence of aphids and thrips, other insects
Environment: Conditions favoring vectors and disease

4. Requirements to identifying MLN resistant germplasm
Disease screening tools
Diverse germplasm
Test locations with consistently high biotic stresses pressure
Standardized screening protocols

5. 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

6. Screening maize germplasm for MLN: Artificial inoculation
SCMV amplification
MCMV amplification
MCMV + SCMV amplification

7. Field inoculation and reaction of germplasm
Variation for reaction to MLN disease as of 2, January 2012

8. Centralized MLN screening Facility
20 Ha being established in Naivasha for MLN screening
Labs
Greenhouses
Field

9. 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

10. Diverse germplasm
Sufficient genetic variation exists for most diseases, pests and parasitic plants in maize
Locally adapted or introduced maize germplasm
Landrace collections
Old 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

11. Distribution of severity scores among 200 inbred lines

12. Reaction of selected hybrids to MLN disease
Entry 1-17: elite experimental hybrids
Entry 18: Susceptible tester
Entry 20-22: Tolerant SC testers
Entry 23-28: Commercial checks

13. 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.

14. 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

15. 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

16. Acknowledgement
KARI
Monsanto
USDA Ohio State University
Sunripe Farm, Naivasha
Olerai Farm, Narok