2. FACTORS THAT HAVE INFLUENCED DEVELOPMENT OF CASSAVA IN UGANDA

2.1. Biophysical constraints.

As for many, other African crops. the productivity of cassava in Uganda (7-8 tonnes ha' in 1987-89) is much lower than the world average (9-10 tonnes ha-' in 1987-89). These low yields are due to constraints that challenge the production and utilisation of the crop. Such constraints are (a) the use of inferior and low yielding varieties; (b) lack of good quality planting materials; (c) pests and diseases; (d) deteriorating land availability and soil conditions; (e.) lack of credit facilities and farm inputs; (f) poor price incentives; (g) labour bottlenecks and poor cultural practices (h) bitterness and cyanogenic glucosides hinder the utilisation of the crop, (i) bulkiness and perishability hinder commercialization of the crop. G) poor methods of processing and utilisation (Ocitti p'Obwoya and Otim-Nape, 1986. Otim-Nape and Zziwa, 1990):

Farmers in over 54% of the villages interviewed in many parts of Uganda identified pests and diseases as the main hazards in cassava production (Otim-Nape and Zziwa, 1990; Ocitti p'Obwoya and OtimNape. 1986). The major pests are briefly outlined below.

2.1.1. Pests

Cassava mealybug (Phenacoccus manihoti (Matile-Ferrero)) The cassava mealybug which was accidentally introduced into Africa in the early 1970s in the present Zaire has spread all over Africa (Hahn and Williams, 1973). By early 1992 it was identified in eastern and western Uganda (Tororo and Masindi districts) probably from the neighbouring Kenya and Zaire respectively. This pest causes severe damage to cassava leading to considerable yield losses; it is still a serious dry season pest in some parts of the country particularly in Kumi, Masindi (Buliisa), Pallisa and Soroti districts, most probably because of the harsh, intensive and prolonged dry seasons in these areas. Early planting in areas with heavy and long first rains sustain minimal damage because the number of the mealybug is markedly reduced below economic in ury level during the rainy seasons (Fabres, 1980; Herren 1981). This enables the plants to establish and withstand the attack of the mealybug in the succeeding months of the dry season. In countries with clearly defined yearly dry and wet seasons, several workers have recommended early planting for the control of cassava mealybug (Leuschner, 1980; Akinlosotu and Leuschner. 1981). However. this recommendation may not be widely adopted in Uganda because of interfering with the different cropping patterns.

There are now some high yielding varieties such as Migyera, Nase I and TMS 4 (2) 1425 which are tolerant to P. manihoti and recovers quickly with the first rains after attack by the pest during the dry season. Chemical treatments of late planted cassava with systemic insecticides (Furadan 5G) or foliar insecticides (Ultracide 40 EC, Rogor) did not lead to significantly higher yields than those from the control plots. The subsequent discovery of Epidinocarsis lopezi in south America and its introduction and release in Afrcia, signaled an integrated approach to the control of this pest throughout the region. Life-table analysis confirmed that E. lopezi is the key mortality factor in reducing mealybug population (Neuenschwander, 1990). A combination of crop resistance, optimal insecticide use mostly to disinfest planting material, early planting, weed control and biological control by use of E. lopezi can sustain an effective control of the mealybug infestation at the farm level and raise yields and production.

The green spider mite (Mononychellus tanajoa Bonder, Tetranychideae) is currently one of the most important arthropod pest of cassava in Uganda (Otim-Nape and Odongo, 1984). This pest was inadvertently introduced in Uganda, where it was first reported in 1971 (Nyiira, 1975). The amount of crop damage by cassava green spider mite depends on the fertility of the soil, cultivars used in particular localities and more so on the rainfall pattern. Heavy infestation of susceptible cultivars especially during the dry season in poor soils can cause total leaf defoliation resulting in yield reduction of up to 46% (Nyiira. 1975). An annual yield loss of 17-33% (Nyiira, 1975) amounting to US $ 7.0 million in 1984 has been estimated (Otim-Nape and Odongo, 1984). The use of acaricides to control green mites is economically inviable considering the period of the growth cycle of cassava (8-16 months), toxicity to users, cost of the acaricide, its deleterious environmental side effects and the possibility of selecting for acaricide resistance.

In 1994, one species of exotic phytoseiid mites Typhlodromalus aripo was introduced and released at Namulonge. The functional and numeric responses of T aripo on green spider mites population effectively controlled green mite population development and crop damage on release fields. It is likely that this could form basis of biological control of the pest. Integrated control measures involving the use of resistant varieties. cultural practices, and natural enemies have been recommended.

Whiteflies (Bemisia tabaci (Genn)). In Uganda B. labaci is important as a vector of African cassava mosaic disease (ACMD). The possibility of developing cassava varieties that have some relative resistance to B. tabaci could form the basis of an integrated approach to its control and to the control of ACMD. The use of cultural practices such as barrier crops is still being investigated. Early planted cassava was reported to suffer higher virus contamination than late plantings. An integrated approach to control is being developed.

Elegant grasshoppers (Zonocerus variegatus (Thunb.)

In Uganda grasshoppers are reported on cassava in the drier areas of West Nile, Soroti and Kumi districts. Their attack is mostly during dry season and at the onset of first rains. Their feeding damage can cause total defoliation of the crop and with heavy infestation, green stems are consumed, leaving only the white wood. Chemical control using Dieldrin, fenitrothion etc. have been effective to control this pest in Uganda. A possibility of biological control is being investigated.

Other pests include the cassava scales (Aonidomytilus albus (Cockerell), Root knot nematodes (Meloidogyne incognita), termites and vertebrate pests such as wild pigs, monkeys, mole rats etc.

2.1.2. Diseases

Contrary to earlier views (Jameson, 1970) diseases are now by far the biggest constraints to cassava production in Uganda. Many diseases which infect cassava are summarised in Table 4 and are briefly outlined below.
Cassava brown streak disease caused by a whitefly-transmitted virus was first reported in Uganda in 1945 (Nichols 1950) at Bukalasa experimental station. central Uganda. It was assumed to have been introduced in'1934 in cassava stems from Amani Tanzania. An eradication campaign was carried on between 1945 - 1950 and since then there has been no report of this disease and the campaign seems to have been successful (Emechebe, 1976).

The African cassava mosaic disease (ACMVD) caused by a whitefly transmitted geminivirus (Bock and Wood. 1983( was first reported in Uganda in 1928 (Hall, 1928; Martin, 1928). It is considered the most important and ser, as disease of cassava in the country (Otim-Nape 1990). A severe epidemic devastated crops in eastern region from 1933-1944 (Jameson, 1964). Vigorous breeding and selection for mosaic-resistant varieties carried out at Amani. Tanzania. resulted in genotypes that were widely tested and released, in Uganda. as varieties Bukalasa 8, Bukalasa 11, etc. They were multiplied and distributed to farmers (Jameson, 1964). A bye-law instituted in the 1950s made it mandatory for farmers to uproot all infected and susceptible local varieties and replace them with the new ones (Jameson, 1964).

2.1.3. The current epidemics qf severe cassava mosaic disease in Uganda

Since 1988, severe epidemics have traversed the country from north to south and caused devastating losses and food shortages. Comprehensive surveys carried out in 1990-1992 (OtimNape. 1993) and again in 1994 in all cassava-growing districts revealed that ACMD occurred throughout the country. There was almost total infection in most parts of the country where symptoms were very severe. Healthy planting material of local Ugandan varieties introduced to the high incidence areas encountered high inoculurn pressure and became heavily infected within a few months of planting.

Movement of the epidemic. Observations on the progress of the epidemic across Uganda revealed that since 1988, it moved c. 140 km southwards towards Kampala. By May 1997, the epidemic reached Kampala and continued to spread southwards along a broad front at a rate of c. i 5-20 km per annum. The front is characterized by large population of whiteflies and by a high incidence of ACMD mainly due to recent infection by the whitefly vector. The lower leaves of plants infected in this way seem healthy while the youngest leaves show severe symptoms. They are reduced in size and show marked distortions and malformation which give infected plants a paint-brush- like appearance. The plants harbour numerous adult -,vhiteflies on the young shoots and large nymphal populations on the undersides of the lower leaves (G. W. Otim-Nape, unpublished).

Impact of the epidemic on cassava production. Fifteen to twenty kilometres behind the front, all plants show severe ACMD symptoms due to the use of cuttings from plants infected by whiteflies the previous year. If this material is used in the absence of adequate stocks of healthy cuttings. the ensuing plants are severely stunted and produce no or very poor yields. Consequently farmers become discouraged and in the absence of adequate amounts of healthy planting material . they abandon growing cassava (Fig. 3) . Annually over 60.000 ha. of cassava, equivalent to over 600,000 mt (U.S. $ 60 million) of fresh cassava roots are being lost in this way (Otim-Nape et al., 1997). The causes of the epidemics are being investigated. More aggressive strain of the virus has been dentified as the most likely cause. The possibility of a new biotype of whiteflies B. tabaci is being investigated (Otim-Nape et al; 1997).

The current epidemic has led to a drastic decrease in cassava production and to the virtual elimination of the Crop in some areas. Moreover, over 500 local cassava genotypes are threatened with extinction and special measures have been required to protect them. The epidemic has had serious consequences on commununities heavily dependent on cassava as a staple food and cash crop. There have been massive food shortages and starvation in some districts, especially in the east and north.

Table 4: Cassava diseases and their pathogens in Uganda.

Disease Pathogen Reference

_A. Viral Diseases

   
1. Cassava brown Streak Cassava Brown Streak I virus Storey, 1936
2. African cassava mosaic African Cassava Mosaic Geminivirus Storey and Nicholas, 1936: Emechebe. 1976
3. Kumi cassava virus Cassava Chlorotic Portex Virus Harrison, 1991 (Pers. comm.)
B. Bacterial Diseases    
1. Cassava bacterial blight Xanthomonas campestris manihotis (Dye) Otim-Nape, 1977
2. Cassava bacterial leafspot Xcampestris p. r cassavae Hansford 1936, Wiebe and Dawson, 1962
C. Fungal Diseases    
1. Cassava anthracnose Colletorrichum gloesporoids manihotis Otim-Nape, 1977
2. Brown leafspot Cercasporidum heningsii Jameson, 1970, Emechebo, 1976
3. Blight leafspot Cercospora Otim-Nape, 1988
4. White leafspot Phaeoramularia Otim-Nape, 1988
5. Botryodiplodia stem rot Botryodiplodia theobromae Otim-Nape, 1984
6. Armilleriella wilt and root rot Armilleriella mellea Emechebe, 1976; Jameson, 1970
7. Verticillium wilt Verticillium Emechebe, 1976
8. Phytopthora root rot Phytopthora Emechebe, 1976
9. Dry root rot Riqidosporus liqnosus Emechebe, 1976
10. White root rot Sclerotium rolfsii Emechebe, 1976
11. Rosellinia root rot Rosellinia necatrix Emechebe. 1976
D. Nematode Diseases    

Bridge et al, 1991

   

The "Kumi" cassava virus disease also proposed to be known as cassava chlorotic virus (CCV) (Harrison, 1991. pers. corn) was first discovered in Kumi district. eastern Uganda in April. 1991 (OtimNape and Thresh 1991. unpublished). The aetiology of the disease. its transmission. distribution in Uganda, economic importance and control of the disease is unknown.

Cassava bacterial blight (CBB) caused by Xanthomonas campestris pv. manihotis was first reported in Uganda in 1976 (Otim-Nape. 1976). It was found widespread in the country and caused severe losses in the savanna areas particularly on susceptible varieties grown on poor soils (OtimNape and Sengooba, 1980). CBB causes up to 70% reduction in yields of cassava tubers and planting materials. The disease is spread through infected planting material, wind driven rainsplash, insects and the movement and use of infected implements (Lozano and Sequeria, 1975, Otim-Nape and Sengooba, 1980). The use of resistant varieties, cultural practices and sanitation are the recommended control methods (Otim-Nape and Sengooba, 1980).

Cassava bacterial leafspot (CBL) (Xanthomonas campestris pv. cassavae) was first reported by Hansford (193 6) as Bacterium cassavae. A type species of B. cassava was later found synonymous to Erwinia larythi (Mann and Taubenhaus), which is ubiquitous saprophyte (Wiehe and Dowson, 1962). However in 1962 Wiehe and Dowson (1962) isolated Xanthomonas cassavae from a similar disease in Malawi. The disease in Uganda was later attributed to this pathogen. Later, Maraite and Weyns (1980) isolated X cassavae from a number of samples from Uganda. Xanthomonas, X campestris pv. cassavae became the new name of the pathogen (Dye et al, 1983). Unlike CBB, CBL causes only angular leafspotting on cassava and is a relatively unimportant disease in Uganda.

Cassava anthracnose (Collectotrichum glosporoides f. manihotis) was first reported in 1983 (Otim-Nape 1983) and is widespread. The disease dissemination is aided by an insect Pselidotheraptus devastans group. Under normal conditions, the disease is unimportant but it can be serious when hailstones predispose the crop to infection. Use of resistant varieties is the recommended control (Otim-Nape, 1983).
Brown, blight and white leafspots and Botryodiplodia stem rot are prevalent in the country but they are unimportant economically. Cases of Armilleriella and Verticillium wilts have been reported. They appear less sporadic and are also unimportant.

The root rot complex (Phytopthora, dry, white and Rosellinia root rots) are quite common but little is known about them. Similarly, cases of root knot nematodes have been reported on the crop, but information and their distribution and economic importance is lacking.

2.1.4 Weeds

Weed can cause significant yield reductions if uncontrolled. Competition exerted by weeds reduce yields and can favour survival of pathogens when the weeds act as alternate hosts. Yield reductions of up to 90% can be achieved when weeding is delayed especially during tuberisation and tuber enlargement. In Uganda, varieties like Migyera which have been developed and released to the farming community has the ability to compete favorably with weeds. Until recently manual digging have been the only control option available to the farming community. However, chemical control using glyphosate (roundup) is becoming common as the cost of labour rises.

2.1.5 Lack of improved varieies. A majority of farmers plant local varieties which are characterized by low yields and susceptibility to diseases. As a result, although farmers may plant large land area to cassava, low yields results into low output. However, several programs to improve varieties resulted in improved cassava varieties like, Bukalasa 8, Bukalasa I I etc, Nase 1, Nase 2, Migyera, etc which have been or are being distributed to farmers.

2.1.6 Soils

Apart from the poor soil types in some parts of the country, another problem is the deteroriating soil fertility as a result of continued use of the same land. Furthermore, poor methods of cultivation has led to increased soil erosion, thereby reducing productivity. There is therefore, need for improved agricultural techniques and increased use of fertilizers whose prices are prohibitive to most farmers.

Figure 5, : Yield of improved and local cassava varieties in six selected districts in 1995.

Figure : Yield of the improved and local varieties, 1995

2.1.7 Support Services

Survey. findings and discussions with farmers suggest that in general, extension services is inadequate. Associated with this is lack of farmer training. In the north-east and northern parts of the country.. draught oxen used to be the most widely used technology.

However, following civil strifes and cattle rustling, farmers' means of opening land has significantly been affected. Simmilarly in most parts of the country. tractor-hire services are minimal. Labour is also generally in short supply as the youth and the males migrate to urban areas.

2.1.8 Weather

Uganda's agriculture is rainfed. Any short fall in the amount of rain expected for agricultural production affects t he output. Over the past years, the Karamoja region, Moyo and parts of northern and central Uganda have had continous rain shortages, resulting into low output levels. On few occassions, however, too much rain has also destroyed crops particularly when the rains come when crops have sprouted or nearing harvest.

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