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
Potyvirus
SCMV
MDMV
WSMV
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.
Virus: Either
individual or
compound
Susceptible
Germplasm
MLN Development
Vectors:
Presence of
aphids and
thrips, other
insects
Environment:
Conditions
favoring
vectors and
disease
Requirements to identifying MLN
resistant germplasm
1. Disease screening tools
2. Diverse germplasm
3. Test locations with consistently high biotic stresses
pressure
4. Standardized screening protocols
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
Screening maize germplasm for MLN:
Artificial inoculation
SCMV amplification
MCMV amplification
MCMV + SCMV amplification
Field inoculation and reaction of germplasm
Variation for reaction to MLN disease as of 2, January
7
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 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
Distribution of severity scores among 200 inbred lines
Reaction of selected hybrids to MLN disease
80
70
60
50
40
30
20
10
0
1
3
5
7
9
11
13
15
17
19
21
23
25
Entry 1-17: elite experimental hybrids
Entry 18: Susceptible tester
Entry 20-22: Tolerant SC testers
Entry 23-28: Commercial checks
12
27
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