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Distribution of
angular leaf spot of common bean in Kenya and virulence diversity of the
causal agent, Phaeoisariopsis
griseola (Sacc.)
Ferr.
I.N. Wagara,
A.W. Mwangómbe, J.W. Kimenju & R.A. Buruchara
Angular
leaf spot, caused by Phaeoisariopsis
griseola (Sacc.) Ferr., is among the most important diseases of
common bean (Phaseolus vulgaris)
in Kenya, causing yield losses of up to 80%. When weather conditions are
favourable, the disease can be very destructive with most crop losses
resulting from premature defoliation. Breeding for resistance to this
disease is complicated by wide pathogenic variation within the causal
agent. A survey
conducted to determine prevalence, incidence and severity of angular
leaf spot revealed that the disease is widely distributed in Kenya. The
disease was prevalent in all the five districts surveyed and disease
incidence and severity were generally high. Fifty-three percent of the
farms visited had mean disease incidence of more than 50%, and 29% of
the farms had average severity scores of over 25%. P.
griseola was isolated from infected bean leaves collected from
randomly selected fields and the extent of virulence diversity in the
pathogen population determined by inoculating 12 bean varieties. Based
on their virulence reactions on the differential varieties, 100 isolates
of the pathogen were grouped into 44 races. Eight of the races
exclusively infected large-seeded bean varieties and were therefore
categorised as Andean. Twenty-nine races were of the Mesoamerican group
that infects both the large- and small-seeded varieties, and 7 races
belonged to the Afro-Andean group. These results indicate a wide
pathogenic variability in the angular leaf spot pathogen that must be
taken into consideration when developing and selecting bean varieties
with resistance to P. griseola.
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