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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 8, Num. 3, 2000, pp. 311-316
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African Crop Science Journal, Vol. 8. No. 3, pp. 311-316
African Crop Science Journal, Vol. 8. No. 3, pp. 311-316
AN ASSESSMENT OF CASSAVA MOSAIC DISEASE SEVERITY IN CASSAVA-MAIZE MIXTURE AT DIFFERENT NPK LEVELS
E. O. EKPE and C.C. Chinaka1 Department of Agronomy, University of Uyo, Uyo, Nigeria1NAERLS/ABU South East Zone, Umudike, Nigeria
(Received 17 June, 1999; accepted 2 March, 2000)
Code Number: CS00033
INTRODUCTION
Insect pests and diseases have been identified as the major
limiting factor to cassava (Manihot esculentum) production in Nigeria
(Hahn, 1989). In Africa, cassava mosaic virus disease (CMD) is widespread and
conspicuous in all cassava growing areas (Hahn, 1978), causing reductions in
tuberous root yields of up to 90% in severely affected crops. In Uganda Otim-Nape
(1993) reported a country-wide incidence of 54%, but with 100% infection in
the worst affected areas. Epide-miological information on CMD has been obtained
in some parts of Africa including Nigeria (Thresh et al., 1994). CMD
spreads rapidly in some areas but not in others, and infection is attributable
to the use of infected cuttings and, to lesser extent, whiteflies (Byabakama
et al., 1999). The disease is transmitted by a whitefly vector (Bemisia
tabaci) and is caused by one of the cassava mosaic geminiviruses.
Osiru et al. (1999) have suggested that the use of varietal
or crop mixture could be used to minimise CMD incidence and severity in the
Southeastern ecological zone of Nigeria where intercropping is common, availability
of CMD - resistant cassava varieties still remains a problem to the farmers.
This work therefore aimed at assessing CMD severity in cassava intercropped
with maize which is the most popular crop mixture in the zone.
MATERIALS AND METHODS
The experiments were sited at the Teaching and Research Farm
of the University of Uyo, Nigeria, Uyo (05° 00N and 07° 50E) in 1995/96 and
1996/97. Each trial was planted on land that had been left fallow for two years.
The experimental fields were cleared and ploughed. Disc harrowing
and ridging were done two days after ploughing with a four-wheel tractor. Ridges
were made 1 m apart. Four cassava genotypes - a local selection (Obubit Okpo),
TMS 30572, TMS 89/00077 and NR 7721 were used. Cassava was intercropped with
a maize hybrid obtained from the National Root Crops Research Institute (NRCRI),
Umudike. NPK 15:15 fertiliser was used primarily because of its availability.
The experiment was a Randomised Complete Block Design (RCBD)
replicated four times. The four cassava genotypes formed the main plots and
four levels of NPK 15:15:15 fertiliser, 0, 200, 400 and 600 kg ha-1 were applied
to the sub-plot. The cropping systems (sole cassava and cassava interplanted
with maize) formed the sub-sub plot. Both cassava and maize were each planted
1 x 1 m apart (10,000 plants per hectare). Fertiliser was applied to the maize
and cassava at 3 and 8 weeks after planting (WAP).
Two main data sets were taken and analysed. CMD severity scores
were taken 3, 6, 9 and 12 months after planting using a 1 - 5 scale where:1
= No symptoms, 2 = a mild chlorotic pattern on entire leaflets, amild distortion
only at the base of leaflets with the remaining leaflets appearing green and
healthy, 3 = conspicuous mosaic pattern throughout leaf, narrowing and distortion
of the lower half of leaflets, 4 = Severe mosaic distortion of 2/3 or more leaflets
and a reduction of leaf size, and 5 = severe mosaic distortion of 4/5 or more
of leaflets, twisted and mis-sharpened leaves. Fresh tuberous roots were harvested
6, 9 and 12 months after planting (MAP). The disease and yield data were analysed
by season (year) and stage of cassava growth using analysis of variance (ANOVA)
procedures (Cochran and Cox, 1957).
RESULTS
CMD severity scores at 3, 6, 9 and 12 MAP in sole and intercropped
cassava are shown in Tables 1, 2, 3 and 4, respectively. There were no significant
effects of fertiliser on any of the genotypes at 3 MAP. However, genotypes effects
were significant in pure stands with TMS 89/00077 expressing the mildest symptoms
score (1.19 - 1.38).
In the cassava grown as an intercrop, increased rates of fertiliser
resulted in a non-significant increase in CMD severity. Also, CMD severity was
lower in the local cassava grown as an intercrop as opposed to in the sole crop,
but the difference was not significant. In general, CMD severity at 3 MAP (Table
1) was comparatively higher in NR 7721 and the local genotype than in TMS 98/00077
and TMS 30572 in both cropping systems.
TABLE 1. Cassava mosaic disease severity on
cassava morphotypes in sole and cassava-maize mixtures at different NPK
15:15:15 rates at 3 months after planting |
Genotypes
|
1995/96 (sole cassava)
|
1996/97 (sole cassava)
|
1996/97 (cassava-maize mixture)
|
Fertiliser levels (kg ha-1)
|
0
|
200
|
400
|
600
|
0
|
200
|
400
|
600
|
0
|
200
|
400
|
600
|
Obubit okpo 2.50
|
3.00
|
3.00
|
2.50
|
2.25
|
2.75
|
2.75
|
2.25
|
2.25
|
2.50
|
2.50
|
2.25
|
|
TMS 30572
|
1.50
|
1.75
|
2.75
|
1.75
|
1.75
|
1.75
|
2.00
|
2.00
|
1.50
|
2.25
|
2.25
|
2.50
|
NR 7721
|
4.00
|
2.25
|
2.75
|
2.75
|
3.25
|
2.50
|
2.25
|
2.75
|
2.25
|
2.50
|
2.50
|
2.75
|
TMS 89/00077
|
1.25
|
1.25
|
1.50
|
1.50
|
1.00
|
1.25
|
1.25
|
1.25
|
1.00
|
1.25
|
1.25
|
1.25
|
Overall mean
|
2.31
|
2.06
|
2.50
|
2.25
|
1.88
|
1.94
|
2.06
|
2.13
|
2.75
|
2.13
|
2.25
|
2.3
|
LSD0.05
|
0.68
|
0.97
|
NS
|
0.75
|
0.84
|
NS
|
NS
|
0.64
|
0.43
|
0.71
|
NS
|
0.5
|
SE±
|
0.31
|
0.23
|
0.06
|
0.25
|
0.22
|
0.26
|
0.21
|
0.22
|
0.17
|
0.18
|
0.17
|
0.1
|
CMD severity scale, 1 = no symptom, 5 = severe mosaic |
At 6 MAP, CMD severity in sole cassava in 1995/96 decreased
by 0 - 25% in the local genotype; 0 - 60% in TMS 30572; 44 - 62% in NR 7721;
and 0 - 33% in TMS 89/00077. Though fertiliser levels did not significantly
affect the CMD severity among the genotypes, severity reductions were greater
at 400 kg ha-1 of NPK applied to NR 7721 and TMS 30572. In the case of the local
genotype and TMS 89/00077 the most decrease in CMD severity at 6 MAP occurred,
respectively, where 200 kg ha-1 and 600 kg ha-1 fertilisers were applied.
At 6 MAP in 1996/97, CMD severity was generally low in both
cropping systems (Table 2) although there was a decrease in CMD severity compared
to increase level at 3 MAP. Cassava in the intercrop tended to manifest greater
reductions in CMD severity than the sole crop.
TABLE 2. Cassava mosaic disease severity on cassava morphotypes
in sole and intercropped cassava at different NPK 15:15:15 rates at 6 months
after planting |
Genotypes |
1995/96 (sole cassava) |
1996/97 (sole cassava) |
1996/97 (cassava-maize mixture) |
Fertiliser levels (kg ha-1) |
0 |
200 |
400 |
600 |
0 |
200 |
400 |
600 |
0 |
200 |
400 |
600 |
Obubit okpo |
2.00 |
2.25 |
2.50 |
2.50 |
1.50 |
1.75 |
2.00 |
1.50 |
1.25 |
2.25 |
1.75 |
2.00 |
TMS 30572 |
1.25 |
1.75 |
1.25 |
1.50 |
1.25 |
1.00 |
1.25 |
1.50 |
1.00 |
1.50 |
1.00 |
1.25 |
NR 7721 |
1.50 |
1.25 |
1.00 |
1.50 |
1.50 |
1.50 |
1.00 |
1.75 |
1.75 |
1.25 |
4.00 |
1.75 |
TMS 89/00077 |
1.25 |
1.25 |
1.50 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
Overall mean |
1.50 |
1.63 |
1.56 |
1.63 |
1.31 |
1.31 |
1.31 |
1.44 |
1.25 |
1.50 |
1.19 |
1.50 |
LSD0.05 |
NS |
NS |
0.75 |
0.56 |
NS |
NS |
0.62 |
NS |
0.43 |
NS |
0.62 |
0.43 |
SE± |
0.13 |
0.15 |
0.20 |
0.17 |
0.12 |
0.12 |
0.15 |
0.16 |
0.11 |
0.18 |
0.4 |
0.13 |
CMD severity scale, 1 = no symptom, 5 = severe mosaic |
At 9 months after planting (Table 3) CMD severities were lower
in all the genotypes than the earlier months in both cropping systems. At 12
MAP (Table 4) slight increases were observed in CMD severity on all the genotypes
except TMS 89/00077. However, enhanced soil fertility did not significantly
affect CMD severity in either cropping system.
TABLE 3. Cassava mosaic disease severity on cassava morphotypes
in sole and intercropped cassava at different NPK 15:15:15 rates at 9 months
after planting |
Genotypes |
1995/96 (sole cassava) |
1996/97 (sole cassava) |
1996/97 (cassava-maize mixture) |
Fertiliser levels (kg ha-1) |
0 |
200 |
400 |
600 |
0 |
200 |
400 |
600 |
0 |
200 |
400 |
600 |
Obubit okpo |
1.00 |
1.00 |
1.25 |
1.00 |
1.50 |
1.25 |
1.00 |
1.00 |
1.50 |
1.25 |
1.25 |
1.00 |
TMS 30572 |
1.00 |
1.00 |
1.00 |
1.00 |
1.50 |
1.00 |
1.00 |
1.25 |
1.25 |
1.25 |
1.00 |
1.00 |
NR 7721 |
1.00 |
1.25 |
1.25 |
1.00 |
1.00 |
1.25 |
1.00 |
1.00 |
1.50 |
1.25 |
1.00 |
1.00 |
TMS 89/00077 |
1.00 |
1.00 |
1.00 |
1.00 |
1.50 |
1.00 |
1.00 |
1.25 |
1.50 |
1.25 |
1.25 |
1.00 |
Overall mean |
1.00 |
1.06 |
1.13 |
1.00 |
1.50 |
1.13 |
1.00 |
1.13 |
1.38 |
1.25 |
1.13 |
1.00 |
LSD0.05 |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
SE ± |
- |
0.06 |
0.09 |
- |
0.13 |
0.09 |
- |
0.09 |
0.13 |
0.11 |
0.09 |
- |
CMD severity scale, 1 = no symptom, 5 = severe mosaic |
TABLE 4. Cassava mosaic disease severity on cassava genotypes
in sole and intercropped cassava in 1995/96 and 1996/97 at 12 months after
planting |
Genotypes |
1995/96 (sole cassava) |
1996/97 (sole cassava) |
1996/97 (cassava-maize mixture) |
Fertiliser levels (kg ha-1) |
0 |
200 |
400 |
600 |
0 |
200 |
400 |
600 |
0 |
200 |
400 |
600 |
Obubit okpo |
1.75 |
1.50 |
1.50 |
1.00 |
2.25 |
2.00 |
1.75 |
2.50 |
2.25 |
1.75 |
1.75 |
2.00 |
TMS 30572 |
1.00 |
1.00 |
1.00 |
1.00 |
2.00 |
1.50 |
1.75 |
1.50 |
1.75 |
1.50 |
1.50 |
1.50 |
NR 7721 |
1.00 |
1.50 |
1.75 |
1.00 |
1.50 |
1.75 |
1.00 |
2.00 |
1.75 |
1.50 |
1.25 |
1.50 |
TMS 89/00077 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
1.50 |
1.25 |
1.50 |
1.50 |
1.25 |
1.25 |
1.00 |
Overall mean |
1.19 |
1.25 |
1.31 |
1.00 |
1.69 |
1.69 |
1.44 |
1.88 |
1.81 |
1.50 |
1.44 |
1.50 |
LSD0.05 |
0.39 |
NS |
0.66 |
- |
0.68 |
NS |
NS |
0.70 |
NS |
NS |
NS |
NS |
SE± |
0.10 |
0.11 |
0.12 |
0 |
0.15 |
0.12 |
0.16 |
0.15 |
0.14 |
0.13 |
0.16 |
0.10 |
CMD severity scale, 1 = no symptom, 5 = severe mosaic |
In general, severity of CMD varied significantly with stage
of cassava growth in all the genotypes except TMS 89/00077 which had consistently
low scores. This trend was similar in both sole and intercropped cassava. There
was a significant genotype x fertiliser rate x growth rate interaction on fresh
tuberous root yields as shown in Table 5. Yield increases were observed from
6 to 12 MAP as well as with fertiliser levels and cropping system in all the
varieties. TMS 89/000/77 consistently outyielded the other genotypes.
TABLE 5. Effects of fertiliser x genotype x time of harvest
interaction on mean tuberous root yields (t ha-1) of two cropping
seasons |
Genotypes |
NPK fertiliser levels (kg ha-1) |
6 MAP1 |
9 MAP |
12 MAP |
Obubit okpo |
0 |
3.60 |
8.70 |
10.70 |
TMS 30572 |
6.70 |
13.80 |
18.20 |
|
NR 7721 |
4.30 |
11.90 |
15.50 |
|
TMS 89/00077 |
10.70 |
18.00 |
24.80 |
|
Mean |
6.30 |
13.10 |
17.30 |
|
LSD0.05 |
2.30 |
5.00 |
5.80 |
|
Obubit okpo |
200 |
5.20 |
13.10 |
16.00 |
TMS 30572 |
10.40 |
21.60 |
25.00 |
|
NR 7721 |
5.60 |
14.00 |
22.30 |
|
TMS 89/00077 |
12.10 |
21.00 |
26.60 |
|
Mean |
8.30 |
17.50 |
22.60 |
|
LSD0.05 |
3.50 |
NS |
6.50 |
|
Obubit okpo |
400 |
6.30 |
15.80 |
19.30 |
TMS 30572 |
11.80 |
22.50 |
27.20 |
|
NR 7721 |
6.40 |
15.00 |
21.30 |
|
TMS 89/00077 |
15.2 |
24.20 |
30.70 |
|
Mean |
9.90 |
19.40 |
24.60 |
|
LSD0.05 |
2.70 |
3.40 |
5.90 |
|
Obubit okpo |
600 |
4.80 |
16.60 |
20.00 |
TMS 30572 |
13.10 |
20.20 |
25.10 |
|
NR 7721 |
8.20 |
15.70 |
21.80 |
|
TMS 89/00077 |
17.60 |
28.90 |
36.10 |
|
Mean |
10.90 |
20.40 |
27.80 |
|
LSD0.05 |
4.20 |
7.20 |
4.90 |
|
1MAP = months after planting |
DISCUSSION
The differences in CMD expression between genotypes are attributable
to inherent differences in their response to infection. The decrease in symptom
severity between 3 and 6 MAP could have been due to the drier environmental
conditions which probably did not favour virus multiplication but each genotype
reacted differently. Bock and Guthrie (1978) observed that CMD vectors were
most numerous just before the rainy season and low for the rest of the year.
Field studies in Uganda have also documented decline in MSD
severity after 3 months of growth, and this was attributed to the decline in
young short growth which is not attractive to the whitefly vectors (Byabakama
et al., 1997; Adipala et al., 1998).
Since reduction in severity values alone may not be enough
for susceptibility assessment, initial severity values can be considered. On
the basis of this, TMS 89/00077 with relatively low CMD severity scores in both
sole cassava (1.00 - 1.25) and intercropped cassava (1.00 - 1.75) appeared to
be the most resistant genotype (Tables 1 - 4). The marked reductions in CMD
severities from 6 to 9 MAP (Tables 3 - 4) could be due to a reduction in amount
of susccelent tissue, and hence less whitefly infection (Byamukama et al.,
1997; Adipala et al., 1998), and probably, adult plant resistance. However,
the reasons for increased susceptability at 9 MAP is unclear, but this could
have been related to rejuvenated growth (more succelent tissue) which attracted
the whitefly vectors. However, this must have occurred earlier, since there
is a time lag between infection and symptom expression.
Generally, CMD severity was very low as the scores ranged from
1 to 2 (Tables 1 - 4). Since the CMD scores were low in all the four genotypes,
differences in fresh tuberous yield were probably due to factors other than
CMD.
Hahn et al. (1981) stated that the development of resistant
cultivars provides the most appropriate and realistic approach to the control
of cassava pests and diseases. The absence of significant differences between
genotypes especially at 9 and 12 MAP (Tables 3 and 4) suggests that these materials
could be deployed for managmenet of CMD. However, although CMD levels were comparatively
lower in the intercropped than sole cropped cassava, the difference were statistically
not significant. It would appear therefore that the best control strategy is
to employ host resistance.
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