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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
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° 00’N and 07° 50’E) 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 (Coc’hran 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.

REFERENCES

  1. Adipala, E., Byabakama, B.A., Ogenga-Latigo, M,W. and Otim-Nape, G.W. 1998. Effect of planting date and varietal resistance on the development of cassava mosaic virus disease in Uganda. African Plant Protection 4:71-92.
  2. Bock, K. R. and Curthrie, E. J. 1978. African mosaic disease in Kenya. In: Proceedings of the Cassava Protection Workshop held at CIAT Agricultural Tropical (CIAT). Brekelbaum, T., Bellotti, A. and Lazano, J. C. (Eds.). CIAT. Series CE - 14:41 - 44.
  3. Byabakama, B.A., Adipala, E., Ogenga-Latigo, M.W. and Otim-Nape, G.W. 1997. The resistance of improved cassava varieties to African cassava mosaic disease in Uganda. African Journal of Plant Protection 7:45-57.
  4. Byabakama, B.A., Adipala, E., Ogenga-Latigo, M.W. and Otim-Nape, G.W. 1999. The effect of amount and disposition of inoculum on cassava mosaic virus disease development and tuberous root yield of cassava. African Plant Protection 5:21-29.
  5. Coc’hran, W.G. and Cox, G.M. 1957. Experi-mental Designs. 2nd Edition. John Wiley and Sons, New York. 611 pp.
  6. Hahn, S.K. 1978. Breeding of cassava for resistance to bacterial blight. PANS 24:480 - 485.
  7. Hahn, S. K., Terry, E. R., Leuschner, K. and Singh, T.P. 1981. Cassava improvement strategies for resistance to major economic disease and pests in Africa. In: Tropical Root Crops Research Strategies for the 1980s. Proceedings of the First Triennial Symposium of the International Society for Tropical Root Crops - Africa Branch. Terry, E.R., Oduro, K.O. and Caveness, F. (Eds.), pp. 25 - 28. International Development Research Center, (IDRC), Ottawa, Canada.
  8. Osiru, D.S.O., Sserubombwe, W.S., Sseruwagi, P., Thresh, M. and Otim-Nape, G.W. 1999. Effects of cassava mosaic virus disease on the growth and yield of cassava - some highlights from Makerere experiments. African Crop Science Journal 7:511-522.
  9. Otim-Nape, G. W. 1993. Epidemiology of the African cassava mosaic geminivirus diseases in Uganda. Ph. D. Thesis, University of Reading, UK. 256 pp.
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