| dc.description.abstract | The study was initiated to determine the etiology of
Septaria leaf spot disease of cowpea that caused severe leaf
defoliation in cowpea plots at National Dryland Farming
Research Centre (N.D.F.R.C. )-Katumani (Machakos District.
Kenya) , the reaction of some cowpea genotypes of Kenyan origin
to the disease, the host range of the pathogen, and the
efficacy of three foliar fungicides for its control.
Laboratory and glasshouse experiments were carried out at
Kabete Campus, University of Nairobi. Field trials were
conducted at two sites: Kabete Field Station (Faculty of
Agriculture, Kabete Campus) and N.D.F.R.C.-Katumani (Kenya
Agrcultural Research Institute <K.A.R.I.» during the long
rains (April-August) of 1992.
The weather conditions at Kabete during the planting
season were: maximum and minimum temperatures 21.50C and
respectively, total rainfall 671. 3 mm, and mean
relative humidity 73%. A t Kat uma n.i , the con d it ion s were: a
maximum and minimum temperatures of 23.5 C\:.:; and
respectively, total rainfall 233.1 mm , and mean relative
humidity 67%.
Studies on symptoms of Septoria leaf spot and
morphological characters of the fungus causing the disease
indicated that Septaria vignicoia V.G. Rao was the causal
organism. Growth of the fungu:s on artificial media was
restricted, the colony diameter of an eight-day-old culture
being 4.9 mm and 4.5 mm on cowpea dextrose agar and potato
dextrose agar, respectively. The hyphae on the colonies were
also short. S. vignicola could be grown best on cowpea
dextrose agar, potato dextrose agar, yeast malt agar, malt
extract agar and potato carrot agar. The fungus sporulated
better on cowpea dextrose agar, potato dextrose agar and yeast
malt agar than on the other media used in the study. The
optimum temperature for its growth in culture was 22-24oC. and
the maximum and minimum temperatures were above 30 ~ and below
l8oC, respectively.
Koch's postulates were verified to the effect that S.
vignicoia was the causal agent of Septoria leaf spot of
cowpea. Conidium of S. vignicoia germinated by eel I elongation
and budding. The fungus produced one or two germ tubes that
emerged from one conidium, and the budded eel Is germinated
independently. It penetrated the host tissue passively through
the stomata and directly through the cell wall of the
epidermis. The fungus attacked all parts of the plant above
the ground except the flowers and pods.
Disease severity on cowpea increased with increase in
inoculum concentration of the fungus. The conidial
concentrations of the fungus
disease severity (in terms of
were 104 to la conidia/mi. S.
which caused above average
number of leafspots per leaf)
vignicola was found to be host
specific to Vigna unguiculata (cowpea).
The fungus caused severe leaf spotting and defoliation of
most of the cowpea genotypes in the field. None of the
genotypes was found to be immune or resistant to the disease.
Genotypes of intermediate resistance were Kvu-419, Kvu-HB48E-
10, ICV2 and ICV5, and the remaining, including the high
yielding and widely grown varieties like Machakos 66 and
Katumani 80, were susceptfble to the fungus. Septoria leaf
spot was more severe at Kabete than at Katumani. The disease
caused up to 32.82% and 20.85% seed yield loss at Kabete and
Katumani, respectively.
Kocide 101 and Antracol (non-systemics), and Folicur
(systemic), all foliar fungicides, controlled Septoria
vignicoia of cowpea at both locations, but only Kocide 101 and
Antracol fungicides gave economic grain yields at Katumani.
though not at Kabete. This was explained by the fact that high
seed yield was realised at Katumani due to favourable weather
and soil conditions for cowpea growth, and that Folicur
fungicide was phytotoxic to cowpeas leading to considerable
yield loss at both sites. | en |
