| dc.description.abstract | The causal agent of northern leaf blight of maize Exserohilum turcicum has a high variability in
terms of cultural, virulence and genetic characteristics. The study was conducted with a view to
assessing this variability by cultural and molecular characterization, race typing, phytotoxin
production, aggressiveness determination and histopathology studies. An evaluation of reaction
, on both susceptible and resistant genotypes was made using different isolates of the pathogen. A
survey was carried out in the main maize growing areas of Kenya to determine the distribution of
the diascasc and collect di ffcrcnt isolates. The: survey covered 132 farms in 9 agro-ecological
zoncx (;\I~/,)and )() isolates were selected. III nddtition, 26 and 7 isolates were isolated frum
diseased maize samples Irom Germany and Austria, respectively. Different temperature, light
and nutritional regimes were tested and their effects on pigmentation, growth rate and
sporulation determined. Amplified Iragmcnt length polymorphism (AFLP), a molecular
technique that is able to reveal differences between closely related individuals and therefore
delineate strains or a pathogen was used to characterize the E. turcicum isolates.
Differential cultivars were uscd to race type the pathogen isolates and aggressiveness on maize
was determined using the detached leaf technique. Phytotoxins were produced from liquid media
and from inoculated maize leaves. Separation and identification was done using sephadex
columns, thin layer chromatography (TLC) and high performance liquid chromatography
(HPLC). Evaluation of the reaction of maize germplasm to the pathogen was conducted under
greenhouse and field conditions and in both cases artificial inoculation was done to initiate
infection and achieve high disease pressure necessary for germpalsm screening. The accessions
screened were obtained from CIMMYT and local seed stockists and included the commonly
grown varieties such as H511, H513, J 1614, Pannar, Pioneer, Katumani, and Pwani hybrids.
Histological studies were conducted on both compatible and incompatible host systems using
fluorescence microscopy.
Disease prevalence was found to be 100(i"() in 5 out of the 7 districts and in 4 out of the 9 AEZs.
'The disease was more in the upper midland zones characterized by warm and humid conditions
compared to highlands and lower midlands. The cultural characteristics of the isolates differed in
terms of pigmentation, growth rate, sporulation, mycelial texture, colony margins, shape of
conidia, growth rate under different temperature and light regimes. There was no distinct
con-elation between cultural characteristics and the origin or geographical locality of the isolates.
The different cultural characteristics were observed across the isolates from the three countries.
The different maize genotypes differed widely in resistance reaction to northern leaf blight with
17% of the genotypes being resistant, 53% being moderately resistant and 30% being
susceptible. Most of the hybrids commonly grown by farmers were found to be moderately
resistant with a few being susceptible.
Race typing showed that, races 0,1,2,3,N,12,13,13N,123,23 and 23N were present in Kenya.
Races O,1,2,12,13,23,23N and races 0,1,2 were detected in isolates from Germany and Austria,
respectively. E. turcicum isolates varied greatly in terms of aggressiveness and phytotoxin
production as indicated by lesion sizes, lesion density, area under disease progress curve and
severity scores. The lesion sizes from aggressiveness studies ranged from 1.81ml112to 57.04 111m2
with mean lesion sizes of 19.18 mrrr'. Thirty eight percent of the isolates were found to be the
most aggressive and majority of these were from Kenya. TLC and J IPLC studies revealed the
presence of 2-3 phytotoxins being produced by the various E. turcicum isolates. Out of all the
isolates assessed for phytotoxin production, 4g J% were found to produce' phytotoxins at various
levels. The phytotoxins induced characteristic symptoms on leaves. which were similar to
symptoms observed after inoculation with the pathogen, Histological studies showed sparse
mycelial growth on detached leaves of resistant varieties and dense mycelial growth in
'susceptible varieties. Germination and penetration was similar in the two host pathogen
reactions. Amplified fragment length polymorphism (APLP) analysis using NTSYSpc, pop gene
and Arlequin programs showed a wide genetic diversity among the E. turcicum isolates with
82% level of polymorphism being revealed by the various primer combinations. Clustering from
the various dendograms revealed 7 main APLI' groups of the isolates with isolates from different
localities grouping together and only two outliers were observed.
The study has shown that, northern leaf blight occurs in all the maize growing areas of Kenya
and the pathogen has a high cultural, virulence and genetic diversity. The presence of various
races of the pathogen and the high variability in terms of aggressiveness implies that, maize
breeding programmes should aim at incorporating polygenic resistance which is more durable
thus guaranteeing fanners a reliable protection of their crop. The study forms a strong base for
resistance breeding of northern leaf blight in maize. | en |
