Title: Pest and disease hazards and sustainability in African agriculture
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Title: Pest and disease hazards and sustainability in African agriculture
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Expl Agric. (1996), volume 32, pp. 199-211
Printed in Great Britain
Copyright ( 1996 Cambridge University Press


PEST AND DISEASE HAZARDS AND SUSTAINABILITY IN
AFRICAN AGRICULTURE

By ABE GOLDMAN

Department of Geography, University of Florida, Gainesville, FL 32611-7315,
USA

(Accepted 4 September 1995)

SUMMARY
Surveys of the relationship between pests and diseases and crop sustainability in several areas of
Kenya, Nigeria, and other regions o'fAfrica indicate that the production of numerous crops has
declined sharply as a result ofmajor pest and disease outbreaks, and others are threatened with
major decline because ol a surge in virulence of an endemic pest or disease, the introduction of a
virulent exotic pest or pathogen, or because a system of control used previously has collapsed.
Many of the crops tlha have declined were already experiencing reduced economic demand. In
other cases, crop sustainability has been preserved by vigorous farmer responses or by the
intervention of national and international research institutions. Most pests and pathogens,
however, remain within tolerable bounds most of the time, though this often requires the use of
chemical or cultural controls, or the availability of adequate land to compensate for losses. As
land availability declines, more strenuous management efforts may be needed to sustain
productivity.
INTRODUCTION

Discussions of agricultural sustainability in tropical Africa and elsewhere have
focused mainly on issues of resource degradation, particularly in terms of soils
(Lal, 1988; National Research Council, 1991). There has been relatively little
attention given to pests and diseases as threats to the sustainability of agricultural
systems, although much historic evidence indicates that pests and diseases have
had major impacts on the sustainability of crops, domesticated animals, human
populations, and numerous land use practices. Examples in sub-Saharan Africa
include the catastrophic outbreaks of rinderpest in the 1880s and 1890s (Pank-
hurst and Johnson, 1988; Waller, 1988) and the long-term impacts of trypano-
somiasis, onchocerciasis, and other vector-borne diseases on many aspects of land
use (Ford, 1971; Giblin, 1990; McMillan, 1995). In more mundane terms,
farmers in some surveys have ranked vertebrate and other pests as their single
most important agricultural problem (Porter, 1979).
Literature on sustainability often touches on pest and disease issues, but
concern has been limited to a few themes. A common assertion is that mono-
cropping increases the likelihood of pest and disease infestations and losses, while
mixed crop stands and crop rotations help to reduce them; as a result, it is argued
that monocropping should be avoided in favour of polyculture and/or rotation
(Altieri, 1987; Hussey, 1990; Page and Bridge, 1993). It is also common to view
pesticide use as a threat to sustainability because of its many negative




- .ABE GOLDMAN


consequences, including potential pest resurgence and secondary pest outbreaks
as well as human and animal poisonings and environmental pollution (Kenmore
et al., 1987; Luna and House, 1990; Reganold et al., 1990).
The actual impact of pests and diseases on the sustainability ofcrops and larger
agricultural systems, especially in developing regions, has been little considered
in the literature. Pest and disease losses clearly reduce potential output, but most
agricultural systems and most crops persist despite the presence of numerous
biotic threats, suggesting that only certain pests or pathogens, or certain con-
ditions, pose serious threats to crop sustainability. The ultimate impact of pests
and diseases depends both on their potential damage and on the responses of
farmers and others. In some cases, a major decline in crop production has
occurred as a result of pest or disease pressures, but in others, vigorous responses
by farmers or others have enabled a crop to persist despite serious threats. In yet
other cases, chronic losses to pests or diseases have been tolerated and there has
been little farmer intervention.
This paper examines the relationship between pest and disease hazards and
crop sustainability in several areas of Kenya, Nigeria, and other regions ofAfrica,
considering cases of both crop decline and crop persistence.


SURVEY AREAS
Surveys were conducted in several areas of Kenya, southeastern Nigeria, and
northern Nigeria. The Kenyan surveys were carried out in three contrasting
regions of the country (Goldman, 1986, 1991, 1993a). Kigumo Division is a high
altitude coffee growing region in Murang'a District in central Kenya with a high
population density, 308 km-2 for the division as a whole, but ranging up to 600
km-2 in some areas (Kenyan Ministry of Economic Planning and Development,
1981). The mean size of household land holdings in the area is 1.6 ha. The region
has a high yield potential with favourable rainfall and soil characteristics and a
relatively well-developed infrastructure, due largely to the prosperity that derives
from the production of coffee and other market crops. Land use pressure and the
resulting stress on soil fertility, as well as market conditions, are among the main
agricultural concerns.
Makueni Location is a semi-arid cotton and food producing region in Macha-
kos District in eastern Kenya. The population density (52 km- ) is not a
significant constraint. Most households have sufficient land, with a mean house-
hold land holding of 13.4 ha (Kenyan Ministry of Economic Planning and
Development, 1981). The infrastructure, which has benefited from international
and national assistance, was moderately good at the time of the surveys.
Mbita Division in South Nyanza District in western Kenya is the most remote
of the three areas with the least well developed infrastructure for transport,
marketing, or input supply. Some areas are highly drought prone. Much of the
land neighbours Lake Victoria. The population density is 81 km-2 for the area as
a whole (with a mean household land holding of 8.2 ha), though there is




Pests, diseases and sustainability in Africa


considerable variation and some regions have a considerably higher density (229
km-2 on Rusinga Island).
Two Nigerian surveys were conducted, one in Imo State in south-eastern
Nigeria and the other across the northern Guinea savanna belt of northern
Nigeria (Goldman, 1993b; Goldman and Smith, 1995). Both were village level
rather than household surveys, with groups of farmers asked to characterize
conditions and practices in their village.
The Imo State surveys included a sample of 58 villages. Imo State is an area
with an extremely high population density, despite the acidic sandy ultisols that
cover most of the region, ranging from about 200 to over 1000 km-2 in the regions
surveyed. Root and tree crops dominate the agricultural systems of this humid
tropical region, and numerous pest and disease problems affect the major root
crop staples, cassava, yam and cocoyam (both Colocasia esculenta and Xanthosoma
sagittifolia). Although the length of fallow periods is declining and soil fertility is
stressed, there is minimal or no use of fertilizer in most areas (Goldman, 1993b).
The northern Guinea savanna survey included a sample of 27 villages in four
northern states: Bauchi, Kaduna, Kano and Sokoto. The main crops are
sorghum, millet, maize and cowpeas, and formerly cotton and groundnuts, but
major changes have been occurring in the area, including the adoption of
fertilizers, ox ploughing and new varieties of maize, as well as rice in some regions.
Maize in particular has become a major food and cash crop over the last decade
(Goldman and Smith, 1995). The population density ranges from moderately low
to moderately high (approximately 50 to 200 km-2).


IMPACTS OF PESTS AND DISEASES ON SUSTAINABILITY
The crops in these areas whose sustainability has been threatened by pest and
disease hazards were identified by farmers and can be divided into three
categories: crops that have already declined significantly mainly because of pest
and disease losses; crops that are or have been threatened with similar collapse,
though in some instances the threat has been averted; and crops whose sustaina-
bility is dependent on pesticide use.

Crops whose production has already declined
The first category of crops, those that have already declined significantly
(Table 1), involves examples of three main causal dynamics: a major surge in
population or virulence of an endemic pest or disease, the collapse of prior modes
of control for an endemic pest or disease, or the erosion of tolerance of losses
caused by an endemic pest or disease on a crop in limited demand. In most cases,
socio-economic trends contributed to the crops' decline, but severe losses to pests
or diseases acted as a major precipitating factor.
Groundnuts in northern Nigeria, which until the early 1970s were the main
export crop of the savanna region, are a good example of this category. Through-
out the 1960s, Nigeria was the largest producer and exporter of groundnuts in





ABE GOLDMAN


Table 1. Crops whose production has already declined significantly, and the pests and diseases responsible

Region Pest/disease

Groundnut Northern Nigeria Rosette disease
Cocoyam (Xanthosoma Southeast Nigeria Root rot blight complex
sagittifolia)
Sorghum and millet Eastern Kenya (Makueni) Birds
Sunflower Eastern Kenya (Makueni) Birds
Cassava and sweet potatoes Eastern and central Kenya Wild pigs, squirrels, and other
(Makueni and Kigumo) vertebrate pests; termites
Green and black gram Eastern and western Kenya Aphids and bollworms
(Makueni and Mbita)


Africa. Price reductions caused some decline in 1970 and 1971, and a larger
decline occurred with the droughts of 1972-73 (FAO, Production Yearbook, various
years). A far greater catastrophe occurred in 1975, however, when 0.7 x 106 ha of
groundnuts were decimated by an epidemic of viral rosette disease carried by
cowpea aphids (Aphis craccivora) and production fell far below even that of the
drought years (Yayock et al., 1976). Rosette and various fungal diseases had been
endemic problems of groundnut, but this outbreak was on a far greater scale than
any previously experienced. A second epidemic occurred in 1985, again causing
almost total loss in many areas (Misari et al., 1988). According to farmers' survey
responses in 1989, disease pressure remains high even in non-epidemic years.
Groundnut production in Nigeria in the 1970s and 1980s was about one-third to
one-fourth of that in the 1960s and is now mostly for local use and processing. The
decline in exports has been even more dramatic-from about 170 000 t in the late
1960s to a few hundred in the 1980s -and the country has imported groundnuts in
many recent years (FAO, Trade Yearbook, various years). Demand for groundnuts
for local processing remains strong, but many farmers plant only limited amounts
because disease losses can still be high and new maize varieties and other food
crops have now become the major cash crops (Goldman and Smith, 1995).
A similar collapse in crop production has occurred in south-eastern Nigeria
with the spread of the soil-borne cocoyam root rot blight complex (caused by
several pathogens, with Pythium myriostylum as the main agent), which has
particularly affected the Xanthosoma sagittifolia species of cocoyam (Th6berge,
1985; Caveness et al., 1987). Almost all the surveyed villages in Imo State reported
very high losses and sharp reductions in planting of Xanthosoma. The other main
cocoyam species, Colocasia esculenta, is less affected, and some resistant varieties
have been identified by farmers (Goldman, 1993b).
A major crop decline can also occur if the system to cope with an existing pest
begins to break down, as in the case of the radical decline of sorghum (Sorghum
bicolor) and bulrush millet (Pennisetum typhoides) in much of eastern Kenya where
sorghum and millet were formerly major crops in the semi-arid regions. Although
they continue to be grown in other dry areas of eastern Africa (including the
survey region of Mbita), they have been almost entirely replaced by maize in





Pests, diseases and sustainability in Africa 203
Table 2. Crops whose production is currently threatened, and the pests and diseases responsible

Region Pest/disease

Cassava West and central Africa Cassava mealy bug (Phenacoccus
manihoti) and green spider mite
(Mononychellus tanajoa)
Plantain West and central Africa Black sigatoka disease (Mychosphaerella
fjiensis)
Cowpeas Eastern Kenya (Makueni) Cowpea aphids (Aphis craccivora)



much of Machakos District. This major crop change was mainly due to the
damaging effects of birds on these open-headed crops once the availability of child
labour, traditionally used to keep birds from devouring the ripening seeds,
declined sharply with the expansion of primary education in Kenya after indepen-
dence in 1963. As the proportion of sorghum and millet declined, bird attacks
became concentrated on the remaining stands of the crop, and the extent of loss
became intolerable to farmers in one area after another.
A similar process has been occurring in northern Nigeria, though it is at an
earlier stage. Early-maturing millet varieties were reported to be declining in
about one-quarter of the villages surveyed in 1989 as a result of the expansion of
maize as a new food and cash crop together with increasing losses due to birds.
These disasters have occurred despite the fact that the crops concerned were
grown in polycultural and/or rotational systems, demonstrating that devastating
pest or disease outbreaks can occur even under these traditional systems.
Groundnuts, cocoyam, and sorghum were all important food and income crops,
and it took intense disease or pest pressures to precipitate their decline. Less
severe losses have been sufficient to cause the sharp decline of crops of lower
importance. Examples include sunflower in eastern Kenya (due mainly to birds),
cassava and sweet potatoes in eastern and central Kenya (due mainly to
vertebrate pests and termites), and black and green grams (Phaseolus aureus and P.
mungo) in eastern and western Kenya (due to various insect pests). Apart from
sunflower, these were older food crops being replaced by similar but more
attractive new crops (as potatoes have almost entirely replaced cassava and sweet
potatoes in highland central Kenya). Pest pressure on the older crops contributed
to their demise.

Crops whose production is currently threatened
The second category of crops (Table 2) includes those that are currently
threatened, mainly as a result of newly introduced pests or diseases or a surge in a
previous pest or disease. Some of these threats have generated major international
and national level responses, which seem likely to be successful. Among the most
dramatic have been the cassava mealy bug (Phenacoccus manihoti) and green spider
mite (Mononychellus tanajoa), both exotic pests introduced in the 1970s from South
America. These posed a major threat to cassava virtually throughout Africa, not





ABE GOLDMAN


Table 3. Crops whose continued production is contingent on pesticide use

Region Pest/disease

Cocoa Nigeria and W. Africa Black pod disease (Phytophthora
palmivora)
Coffee Central Kenya Coffee berry disease (Colletotrichum
coffeanum) and leaf rust (Hemileia
vastatrix)
Cotton Eastern and western Kenya American bollworm (Helicoverpa
armigera); stainers (Dysdercus spp); red
spider mites (Tetranychus spp)
Potatoes Central Kenya Late blight (Phytophthera infestans) and
bacterial wilt (Pseudomonas solanacearum)



only in humid forest zones such as southeast Nigeria, but also in parts of southern
and eastern Africa where cassava has become an important staple food (Hansen,
1994). The threat of mealy bugs has, however, been largely controlled by a highly
successful biological control project which resulted in the identification and
import of natural enemies from South America, notably the parasitic wasp,
Epidinocarsis lopezi, in addition to other parasites and predators (Neuenschwander
et al., 1989; IITA, 1992; Norgaard, 1988). Biological control of the widespread but
less destructive spider mites has as yet been less successful (Yaninek et al., 1994).
Another highly destructive introduced disease, caused by black sigatoka
(Mycosphaerellafijiensis), has been affecting plantains and some bananas in West
and Central Africa, and will perhaps eventually affect those in East Africa. This
disease was apparently imported from the Caribbean where it destroyed plantain
production in many places. A plant resistance breeding programme shows some
signs of success in controlling the disease (IITA, 1992; Vuylsteke et al., 1994), but
until resistant planting material is developed and distributed to farmers over a
large area, the disease is likely to spread and severely affect many plantain and
banana-growing areas.
Another threatened decline involves cowpeas in eastern Kenya, where losses
caused by aphids (Aphis craccivora) were so high in 1983 that many farmers
expected to have little or no seed available for the next season. Unless this level of
infestation and loss can be controlled, the crop may experience a long term
decline.

Crops whose sustainability is contingent on pesticide use
The third category of crops involves those whose sustainability is contingent on
intensive pest or disease management, including the use of pesticides; their
continued production also depends on the infrastructure supporting research, the
provision of pesticides and associated credit and marketing services. These
include many crops grown mostly for export, notably coffee, cocoa, and cotton,
which are major sources of foreign exchange in many African countries (Table 3).
Many of these crops are planted by small-scale farmers as well as in large-scale




Pests, diseases and sustainability in Africa


settings and are important income sources for farm households. The category also
includes various vegetable, fruit and flower crops that are increasingly being
grown for export.
Coffee in highland East Africa is highly dependent on fungicide use to control
coffee berry disease (Colletotrichum coffeanum) and other diseases, which would
otherwise devastate the crop in most areas. It is somewhat less dependent on
insecticides for the control of leaf miners (Leucoptera spp) and other pests. Cocoa in
West Africa faces severe threats from black pod disease (Phytophthora palmivora),
for which fungicide sprays are necessary, and from the viral swollen shoot disease,
which requires uprooting and has killed millions of trees.
Cotton is almost everywhere threatened by a wide range of insect and other
pests. In eastern Kenya, American bollworm (Helicoverpa armigera) and stainers
(Dysdercus spp) are the main cotton pests, although an outbreak of red spider mites
(Tetranychus spp) also became an important problem in the early 1980s. The red
spider mite may be a secondary pest, becoming a problem only when the use of
broad spectrum insecticides pyrethroidss and DDT) eliminated its natural
enemies (Murega and Khaemba, 1985). But many pest problems in this region
pre-date the use of pesticides, as shown by colonial agricultural reports which cite
the damage caused by stainers and other pests as the reason for the failure of
attempts to introduce cotton to Machakos District in the 1930s (Kenya Colony,
1958).
Cotton stainers affect output quality rather than yield, but can drastically
reduce farmers' profits and render the crop uneconomic. Similar concerns affect
fruit, vegetable and flower production for export. Management of these and other
cash crops is highly sensitive to economic returns, and pesticide spraying is one of
the first expenses that farmers reduce when financial pressures appear. As disease
and pest losses mount, farmers may stop tending or planting the crop altogether.
There are many indications of dependence on pesticides and of the economic
attraction of their use. Coffee farmers in Kenya say that during periods when they
felt that coffee production was not sufficiently profitable, chemical use was
reduced, and output declined substantially (Goldman, 1986). In Nigeria during
most of the 1970s and 1980s, government economic policies, including inflated
exchange rates, reduced the profitability of cocoa production, and chemicals were
either unprofitable or not available. Cocoa production declined precipitously
(FAO, Production Yearbook, various years) and much of what Nigerian farmers did
produce was smuggled to neighboring countries where higher prices were
available.
Food crop production may also be dependent on pesticide use. In Kigumo,
potatoes have become a major crop for both home consumption and sale. In this
highland region, potatoes are severely affected by late blight (Phytophthora
infestans), and nearly all farmers growing potatoes spray them with copper
fungicides to control the disease, usually using chemicals obtained from the coffee
cooperative societies. In the absence of chemicals and spraying equipment, potato
cultivation in this area would probably be sharply reduced. Some areas are also





ABE GOLDMAN


affected by soil-borne bacterial wilt (Pseudomonas solanacearum), and many farmers
try to obtain more resistant varieties, mainly from other potato-growing regions
such as Meru District.
The development of resistant cultivars by researchers, and/or the search for
them by farmers, has played an important role in maintaining the sustainability of
threatened crops. One example of successful resistance breeding was the develop-
ment in Tanzania of hairy-leaved cotton varieties that are resistant to jassids
(Empoasca spp). These varieties became widely distributed in East Africa,
althoughjassids were generally not severe enough pests to threaten crop sustaina-
bility (de Pury, 1968). Some of the sorghum cultivars grown in Mbita in western
Kenya are planted because of their high tannin content, which makes them
unpalatable to birds. Research institutes have produced goosenecked varieties
whose seeds are less accessible to perching birds (Doggett, 1982). Some of the
sorghum cultivars in this area are selected by farmers because of their greater
resistance to storage pests. A somewhat less successful example of resistance
breeding is the 'Blue Mountain' variety of Arabica coffee, which incorporates
some resistance to coffee berry disease. This had not been widely adopted in
central Kenya at the time of our survey because it would have required the
uprooting of existing trees and considerable loss of current production.


TOLERATED PESTS AND DISEASES

In contrast to the cases discussed above, most pests and diseases do not represent
major threats to crop sustainability most of the time. While crop yields and output
are affected, losses usually remain within limits that farmers can tolerate. Some
intervention using various cultural methods or pesticides may be needed, how-
ever, to keep losses within these limits.
Among the pests that farmers can usually tolerate are maize stem borers in
Kenya (mainly Busseolafusca and Chilo partellus); farmers cite these as the main
pest of maize, which is the most important food crop in all three areas. Walker and
Hodson (1976) suggested that stem borers may be responsible for losses of 15% or
more of potential yield. While stem borers cause a chronic level ofloss, they do not
usually destroy a high percentage of the crop or threaten total crop loss and most
farmers surveyed in Makueni and Mbita accept the losses and take no active
measures against these pests. Although inexpensive stem borer chemicals
(usually trichlorphon ['Dipterex'] granules but previously, and still in some
cases, DDT dusts) were available in both these areas, only 25% of surveyed
farmers in Makueni and 14% of those in Mbita used stem borer dusts. These low
frequencies of use are not due to a lack of financial stake in maize production, as
maize sales comprise one of the main sources of agricultural income. The relative
abundance of land in these areas seems to be a major factor permitting tolerance
of these losses. Instead of using pesticides or other active measures, farmers
simply increase the area planted to these crops to compensate for the expected
losses.




Pests, diseases and sustainability in Africa


In contrast, farmers in densely populated Kigumo Division in central Kenya do
not have the option of increasing the area planted. Intensive pest control is
common, and stem borer chemicals are used by 76% of surveyed farmers in this
region. Use of pesticides is also encouraged by the region's infrastructural
development. The smallholder coffee economy has helped to support the con-
struction of paved roads and an infrastructure for marketing and input supply,
and farmers can obtain pesticides from the coffee cooperative societies and local
shops (Goldman, 1993a).
There is an instructive contrast between the use of stem borer chemicals and the
use of storage pesticides for maize and other foods. Storage dusts, in most cases
2% malathion, but also 4% actellic and 2% DDT dusts, are the most commonly
used of all pesticides in the survey areas. This is because while the damage caused
by stem borers can be compensated for by increasing the area planted, that caused
by maize weevils (Sitophilus zeamais) and other storage pests, which may cause the
complete loss of stored grains, occurs when few compensatory options are
available.
Storage pests do not usually threaten the sustainability of crops, and post-
harvest losses may sometimes be reduced by the use of resistant or tolerant
cultivars. There is also a wide range of traditional techniques for dealing with
storage pests, many of which are still in use (Goldman, 1991). These include the
use of ashes in storage, one of the most widespread traditional methods. This is
still fairly common in Makueni and Mbita though not in Kigumo where it has
been almost entirely replaced by pesticide use.
When available, pesticides may be used on food crops as well as non-food cash
crops. An important example is the widespread use of fungicide sprays on
potatoes in central Kenya already mentioned. In Makueni, many farmers have
used cotton insecticides against pod borers (Maruca testulalis) and other pests on
their pigeonpeas (Cajanus cajan). The use of these chemicals on food crops is linked
to farmers' perceptions of their safety. When synthetic pyrethroids (cypermethrin
and permethrin) were the main cotton chemicals available and considered safe by
farmers, almost half of those interviewed sprayed their pigeonpeas. But in 1983, a
new pesticide was supplied that combined a pyrethroid and an organophosphate
(profenophos) for the control of red spider mites (Tetranychus spp). This chemical
had a powerful odour, made some farmers dizzy, and there were tales that it
caused poisoning of domestic animals. All of the survey farmers avoided its use on
their pigeonpeas.
A final example suggests some problems of regulation of hazardous chemicals.
In southeast Nigeria, farmers' yam fields are heavily attacked by yam beetles
(Heteroligus spp), and many have been using aldrin as a soil insecticide. Recently,
the use of aldrin was restricted in Nigeria because it is now considered hazardous.
Farmers received no explanation for its restriction, and many began using acid
from old dry cell batteries as a substitute for the control of yam beetles. The
widespread use of this locally developed practice may be as hazardous as the
practice it replaced. This experience indicates the importance of informing





ABE GOLDMAN


farmers about the reasons for pesticide restrictions and the potential dangers of
such substitutes, particularly when the hazards are long term and not easily
observable by farmers.



DISCUSSION
These studies indicate that losses caused by pests and diseases have been
responsible for sharp declines in the production of both major and minor food and
cash crops in all of the surveyed regions. Much literature on agricultural
sustainability, which has been strongly influenced by the experience and priori-
ties of industrialized countries, has not given adequate attention to such direct
impacts of pest and disease hazards. Because these are the outcome of the
interaction of the biotic hazards and farmers' responses, assessment of the need
for outside assistance should include evaluation of farmers' existing responses to
the pest or disease hazard.
The cases of severe crop decline identified in this paper are characterized by
three main features: the potential of the pest or disease to cause a high proportion
of crop loss over a relatively short time; the absence of an effective or economically
feasible chemical response; and, in many cases, a low or declining level of demand
for the crop, either for consumption or market. Often a biological or other
perturbation is involved-including an unexpected surge in the level or intensity
of a pest or disease, or the introduction of a highly destructive exotic pest or
pathogen. In some instances, existing modes of coping with an endemic hazard
have been undermined by significant socio-economic changes.
Polyculture and crop rotation practices have not necessarily ensured the
sustainability of crops that are faced by severe threats, but maintaining a diversity
of crops and other economic activities has helped to limit the impacts of individual
crop losses on farm households. None of the cases of crop decline has led to major
social or economic collapse. In response to an experienced or threatened crop
decline, farmers have often sought resistant cultivars or used other crops as
substitutes to provide comparable food or income benefits. Assistance from
national and international agriculture research institutions has often been essen-
tial for the development and dissemination of resistant cultivars and new crops.
Pesticides have been important to the sustainability of a number of crops,
particularly export crops that face virulent and extensive pest or disease hazards
as well as market-imposed quality criteria. Production of some domestic food
crops has also been dependent on pesticide use. The sustainability of these crops is
contingent on the institutions that supply chemicals, credit, and other inputs to
farmers, and on economic conditions that favour input use.
The losses caused by most pests and diseases remain within tolerable bounds
most of the time and do not represent major threats to the sustainability of crops
or systems. Considerable efforts, however, may need to be expended to keep some
hazards within tolerable bounds. These may include various traditional and





Pests, diseases and sustainability in Africa 209

cultural responses as well as pesticide use. The degree and types of effort farmers
devote to the management of pest and disease hazards depends on the significance
of the crop and of the hazard, farmers' ability to tolerate the losses involved, and
the range of feasible control options available. In regions with abundant land,
chronic and fairly predictable losses are often dealt with by increasing the planted
area. High population density can reduce farmers' tolerance of routine losses and
increase their willingness to use intensive pest control measures. Losses that are
not easily compensated for, such as those caused by storage pests, have elicited
vigorous responses in almost all areas, including both traditional and modern
modes of response.
Pest and disease hazards that have the potential to cause large scale losses on
crops of economic or dietary importance, and for which existing types of farmer
response will probably not be sufficient, are those most likely to threaten crop
sustainability, particularly when virulent exotic pests and diseases are involved.
These should generally be given the highest priority for international and national
assistance. Crop declines caused by the coincidence of socio-economic changes
and pest or disease losses are very difficult to reverse and should generally not be
given priority for assistance.
Finally, despite the negative effects that pesticides can cause, their use should
not be excluded from consideration, either on the basis that African smallholders
will not be able to afford them, an argument which is contradicted by many
examples, or because of instances of overuse in some parts of the world.
Particularly where the level of current use is very low, the benefits of a moderate
increase in pesticide use to enhance the sustainability of certain crops are likely to
outweigh the negative impacts.



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