ABSTRACT
The incidence and severity of Alternaria leaf spot of
cotton induced by Alternaria macrospora Zimm. in the northern
growing zone were investigated. Field trials were conducted
at Zaria, Nigeria in 1993 and 1994.
Field survey carried out in 1993 revealed that there was
high incidence of the disease in all the four areas visited
with an average of 75.13%. Although the disease was presented
in all the surveyed areas, the incidence was highest in
Daudawa (80.5%). However, the incidence and severity varied
considerably between the cotton plants grown at the on-set of
the rain and those planted at various times in the rainy
season.
Symptomatological studies and pathogenicity test
conducted in the screen-house in the Institute for
Agricultural Research, Zaria confirmed that the Alternaria
leaf spot of cotton observed was incited by A. macrospora
Zimm. The leaf spot was amphigenous with irregular margin,
brown at the centre with dark brown advancing border. The
conidiophore was single, simple, straight and sometimes
flexous with septation while the conidium was solitary,
frequently catenate, mostly obclavate and rostrate with size
of 127.5/xm (length), 21/μm (width) and beak which was more or
less the same size with the body.
The varietal resistance screening trial conducted in the
field and screenhouse in 1994 followed the same pattern in
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terms of susceptibility to the disease. The ten varieties
(comprising local and exotic lines) were susceptible to the
disease, but the degree of susceptibility varied. In terms of
breeding for resistance, Samcot-9, TXCDP37-HH-1-83, S-295 and
GCA-NH-1-83 were found to be advisable and promising while CAM
D-E and Tamcot Sp 215 were observed to be tolerant to the
disease.
Four fungicides (Benlate, Delsene M, Tecto and Dithane
M4 5) were evaluated both in the laboratory {in vitro) and in
the field. Delsene M, Tecto and Dithane M-45 were found to
completely inhibit mycelial growth (100%) of the pathogen in
all the four concentrations in the laboratory, but Benlate had
a 100% inhibition at 1.2 kg a.i/ha first and 0.6kg a.i/ha with
86.3% and 67% in the lower concentrations respectively. In
the field, four sprays each of the four fungicide at 7-days
intervals starting with 5/7 leaf stage were found to reduce
incidence and severity of the disease. However, the best
result was obtained from Dithane M-45 which reduced incidence
from 25.4% to 12.5% and severity from 2.67 to 1.83 at 2.5kg
a.i/ha in 1994. This was followed by Delsene M, Tecto and
finally Benlate. Their effect resulted in the reduction of
incidence and severity of the disease with significant
(P = 0.05) increased grain yield when compared with control.
Dithane M-45 sprays gave 2572.2kg/ha grain cotton while
control was 1388.9kg/ha. Delsene M sprays at 1.2kg a.i/ha
were found to control the disease but was highly phytotoxic to
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the cotton under field condition. However, Delsene M at 0.6 kg
a.i/ha were found to reduce disease incidence and severity
without phytotoxicity.
Sowing date trials conducted in 1993 and 1994 showed that
July sown crop were not heavily infected. The incidence and
severity of the disease was lower and the yield high when
compared with the later date of planting (August), although
there were no significant (P = 0.05) differences within the
treatments in yield for both years. The later planted crops
suffered precipitation and this resulted to premature
flowering in response to favourable photoperiodism in 1993.
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