| dc.description.abstract | Over the last decade, root rot diseases have increased in importance and in some cases resulting
in up to 100% loss in the yield of beans. In Kenya, the incidence of bean root rots is well
documented in Western province. However, it has not been the case for other provinces where
bean is important for example Eastern province, which produce more than 40% of the total bean
yield in the country. Participatory rural appraisal carried out in Embu district in Eastern province
showed angular leaf spot caused by Phaeoisariopsis griseola, root rots caused by Rhizoctonia
solani, Macrophomina phaseolina and Fusarium solani to be the major bottlenecks to realizing
optir+-n bean yields.
This study aimed at documenting the incidences of root rot diseases and bean stem maggot in
Embu district. Two surveys were carried out during the short rain season in 200 I and long rains
seasons in 2002. Samples were taken from five divisions namely Manyatta, Nembure,
Runyenjes, Kyeni, and Central in Embu district. Pathogens were isolation from the sampled bean
roots followed by pathogenicity tests. In addition, bean stem maggot incidence was determined
by noting the presence of the insect in the bean root/stem, and type of insect was noted by
determining its coat coloration Fifty fungal samples were taken for molecular characterization
but only 41 samples yielded the required amount of DNA for further analysis.
All the fields sampled during the short rains were infected and in tested by the root rot pathogens
and bean stem maggot respectively. On the contrary, only few farms had the same problems
during the long rains. The average root rot diseases/bean stem maggot complex during the short
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rains was 52% with individual farms ranging from 5-99%. These root rot diseaseslbean stem
maggot complex occurrences were significantly different among the five divisions (PS 0.05),
whereas disease incidence during the long rains ranged from 5-95% with divisional average
being 23%. Pathogens isolated from the bean roots were Fusarium solani fsp. phaseoli the causal
agent of fusarium root tot, rhizoctonia root rot caused by Rhizoctonia solani, ashy stem
rot/charcoal rot caused by Macrophomina phaseolina. These pathogens either occurred
individually or in combination of two or more. In both the short and long rains, Fusarium root
rot was found to be the most predominant.
The divisional average for the bean stem maggot was 47% during the short rains. All the farms
sampled were noted to have the insect and the occurrence varied significantly among the five
divisions (P:::O.OS). However, the insect did not infest most of the farms during the long rains and
the divisional mean was 18%. Ophiomyia spencerella Greathead (black species) (20%) so far
had the highest incidence compared to that of Ophiomyia centrosematis/phaseoli (4%). The AEZ
with the highest root rot incidence during the short rains was UMI (73%) while LH2 was 70%
during the long rains. Root rot incidence in beans intercropped with maize was 24%, which was
not significantly different from that recorded in monocrop (19%) (PSO.OS). Bean stem maggot
incidence did not differ significantly among different AEZs and cropping systems during the
short rains. However LH2 23% differed significantly from UM4 (3%) (PsO.05) during the long
rains while the cropping systems did not vary significantly (P<0.05). The phylogenetic analysis
of Rhtzoctonia solani showed relationships of the pathogen in 3 clusters. All the isolates from
Embu district with the exception of those from Runyenjes division occurred in at least two
clusters. Tigoni Rhizoctonia solani isolates also occurred in more than one cluster | en |
