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

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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

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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