Emerging infectious diseases of crop plants in developing countries: Impact on agriculture and socio-economic consequences
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REVIEW Emerging infectious diseases of crop plants in developing countries: impact on agriculture and socio-economic consequences
Maurizio Vurro & Barbara Bonciani & Giovanni Vannacci
Received: 10 February 2010 / Accepted: 29 March 2010 / Published online: 16 April 2010 # Springer Science+Business Media B.V. & International Society for Plant Pathology 2010
Abstract Emerging infectious diseases (EIDs) caused by plant pathogens can develop into unexpected and very serious epidemics, owing to the influence of various characteristics of the pathogen, host and environment. Devastating epidemics, having social implications by increasing the rate of urbani- zation, occurred in the past in Europe, and many other EIDs still occur with high frequency in developing countries. Although the ability to diagnose diseases and the technolo- gies available for their control are far greater than in the past, EIDs are still able to cause tremendous crop losses, the economic and social impact of which, in developing countries, is often underestimated. In the present article, four of the most important EIDs in developing countries are considered from the standpoint of their origin, characteristics, symptoms, mode of spread, possible control strategies, economic impact and the socio-economic consequences of their dissemination. They are Cassava Mosaic Virus Disease, capable of reducing yields by 80–90% and causing the suspension of cassava cultivation in many areas of East Africa; Striga hermonthica, a parasitic weed affecting cereals in an area of at least 5 million hectares in Sub- Saharan Africa; Xanthomonas Wilt of Banana, a bacterial disease that caused around 50% yield losses at the beginning of 21st century in Uganda and is threatening the food security of about 70 million people owing to its impact on an important staple crop; and race Ug99 of the rust fungus Puccinia graminis f. sp. tritici, which is having a tremen- dous impact on wheat in Uganda, and is also threatening most of the wheat-growing countries of the world. Keywords Emerging infectious diseases . Cassava Mosaic Virus Disease . Striga hermonthica . Banana Xanthomonas Wilt . Wheat Rust Ug99 Introduction “Emerging infectious diseases” (EIDs) are those caused by pathogens which, for a number of different reasons, develop into epidemics that may be both unexpected and devastating. Some of the best known epidemics appeared during the 19th century and coincided with the advent of more intensive agriculture and reduction in the duration of sea voyages. The latter allowed increased international trading and, as a corollary, the introduction into new areas of foreign species of plants and parasites, resulting in more frequent upsetting of agro-environmental balances.
- ) Istituto di Scienze delle Produzioni Alimentari, Consiglio Nazionale delle Ricerche, Via G. Amendola 122/O, 70126 Bari, Italy e-mail: maurizio.vurro@ispa.cnr.it
Sometimes EIDs developed into pandemics over whole nations and even continents, causing famine and favouring human diseases, socio-economic disasters and technical crises for the management of whole agricultural communi-
Authors have contributed equally to the preparation of the manuscript. Dr. Bonciani prepared the paragraphs relating to the economic and social impact of the case-studies considered.
M. Vurro (
B. Bonciani Dipartimento di Scienze Politiche e Sociali, Università di Pisa, Via Santa Maria 46, 56100 Pisa, Italy G. Vannacci Dipartimento di Coltivazione e Difesa delle Specie Legnose “G. Scaramuzzi”, Università di Pisa, Via del Borghetto 80, Food Sec. (2010) 2:113–132 DOI 10.1007/s12571-010-0062-7 agent of potato late blight, was the primary cause of the great Irish famine of the nineteenth century. The pathogen originated in the Andes and was observed in North America in 1843 (Gomez-Alpizar et al. Due to intense trade it reached Europe two years later (Fry et al. ). The disease caused significant yield losses, which were partic- ularly catastrophic in Ireland owing to the wetness of the climate and the almost total dependence of a large proportion of the population on potato (Large Out of a population of 8 million approximately one million died of starvation and 1.5 million emigrated, of which about a quarter died in transit (Klinkovski ). Thus, the first massive migration of modern history was caused by a plant disease.
The most recent of the great famines occurred in East
Bengal in 1943, where the failure of the rice crop caused the starvation of an estimated 2–3 million people. The
aetiology of the disease is disputed but many attribute it to the fungal pathogen, Cochliobolus miyabeanus, the dis- semination of which was favoured by the environmental conditions pertaining at the time (Padmanaban ). Others suggest that high levels of iron or aluminium or an outbreak of the brown planthopper were the cause of the problem (Strange
The great Southern Corn Leaf Blight epidemic was caused by a variant strain of the fungus, Cochliobolus heterostrophus, named race T, which was specifically virulent for maize containing a cytoplasmically inherited gene for male sterility (Tcms). Because of the advantage conferred by the gene in breeding this self-fertile crop, it had been incorporated into about 85% of the American crop by 1970. As a result and aided by favourable climatic conditions a pandemic developed with the epicenter in the “corn belt” causing enormous damage in 1970–71. The pandemic was halted by the withdrawal of susceptible varieties and the establishment of new hybrids. Southern Corn Leaf Blight is infamous for having shocked the world feed market and for having set a record in terms of economic losses produced on a single agricultural crop in a single season (Scheffer Subsequent to the disaster, the reason for the specificity and high virulence of C. heterostrophus race T for Tcms maize was determined to be the production of a so-called host-selective toxin by the fungus.
Land use by English settlers and population growth had led, in Ceylon (now Sri Lanka), to an enormous expansion of coffee cultivation during the first 50 years of the nineteenth century. In 1868, prosperity from the crop had reached a maximum but then Hemileia vastatrix, a rust fungus, was found which was likely to have spread from Ethiopia, the centre of origin of both the plant and its rust. Initially damage was thought to be light but may have been sate for lower production with increased prices. But the disease spread to all the plantations and production losses quickly became economically unsustainable. By 1905 the area planted to coffee in Ceylon had shrunk from 275,000 acres in 1878 to around 3,500 in 1905 (Mills Because of the pandemic, coffee had to be replaced, luckily with success, by tea.
Thanks to technological advances in, for example, diagnostics, agronomic practices and the use of specific disease management strategies, the risk of epidemics occurring with catastrophic consequences has been sharply reduced in developed countries compared to developing countries (Waage et al. Unluckily, the stability of the agricultural systems reached with great difficulty is very often upset by the sudden appearance of novel parasites and pathogens, as well as by environmental and technological alterations in management practices. If we consider the many factors relating to the pathogen, the host or the environment that can affect a disease, it is easy to understand that the “emergence” of a disease is the coincidence of a number of unfortunate events. Further, if we also include consideration of socio-economic conse- quences in the evaluation of the seriousness of a plant disease, we will find that some devastating epidemics, reported only in history books for Europe, are still occurring very frequently in many developing countries.
As with pathogens of humans and domestic or wild animals, the emergence or re-emergence of phytopathogen- ic agents is very often due to man’s activities, such as their introduction into novel areas as a consequence of mass tourism, global trade, farming changes and environmental changes. Although only a fraction of a pathogen commu- nity is introduced together with a newly-introduced plant species, this seems to be the most important cause of expansion of an emerging disease to a new area. Moreover, if the pathogen responsible has a wider host range than the plant species introduced it may infect indigenous plant species, which may be particularly vulnerable as they will not have co-evolved with the pathogen.
Although introductions of alien pathogens may occur owing to the trading of vegetables, germplasm, grafts or whole living plants, introductions via international seed trading is a particularly important vehicle for pathogen introduction and dissemination. For example, it has been estimated that at least 2,400 different plant pathogens were contained in the seeds of 380 plant genera (McGee and that up to one third of the plant pathogenic viruses are transmissible through seeds to at least one of their hosts (Stace-Smith and Hamilton ).
The simple introduction of a pathogen into a new area is a necessary but not sufficient requirement to produce an epidemic. Many factors influence the success or failure of environmental conditions or the genotypes of the potential host plants. In particular, in the case of pathogens which are transmitted by vectors, it is the introduction of the vector into a new area that may be the “real” cause of the occurrence.
Lacking the elements favouring their further dissemina- tion, some pathogens may remain restricted to their area of introduction, making very limited impact. For example, Citrus Tristeza Virus (CTV) was probably introduced into South America in the 1920s but only became economically devastating in the 1950s owing to the introduction from Asia of a very efficient vector, the aphid Toxoptera citricidus. Since then, in Brazil, more than 6 million citrus trees have been destroyed (Bar-Joseph et al.
Pierce’s Disease (PD) of grapevine, caused by the bacterium Xylella fastidiosa, was reported in California as being not serious for more than a century but in 1997 a new vector, Graphocephala atropunctata, was introduced into Califor- nia. This allowed the rapid development of the disease in the vineyards, with damage estimated in 1999 at 6 million dollars (Anderson et al. In some cases the reciprocal situation can occur, i.e. plants species introduced in novel areas are affected by endemic pathogens. For instance, cassava mosaic viruses are not known in South America, the centre of origin of cassava, but as will be described below, they have caused havoc in the crop in Africa to which the plant was exported in the 16th century (Strange
Climatic changes have very often been connected with the appearance of epidemics in humans and animals, but very little is known about their effects on plant EIDs (Garrett et al. Changes in the incidence and severity of plant diseases are likely to occur and these will vary according to the particular pathosystem. In addition, incidence and severity will also be influenced by other factors, such as the use of transgenic plants, the availability of new chemicals and changes in land management.
Changes in farming systems have determined the appearance of a number of EIDs both of crop and wild species. In some developing countries, the lowering of the value of traditional crops and the higher demand for non- traditional crops has caused an increased cultivation of the latter. The introduction of highly productive selected monocultures has reduced genetic variability, increasing the risk of exposure to pathogens. For example, in 1970, outbreaks of Southern corn leaf blight and yellow corn leaf blight destroyed 17% of all US maize crops, 85% of which were of the same variety, susceptible to these diseases (Pring and Lonsdale ).
In the next sections, four EIDs of great importance to developing countries, in particular to Sub-Saharan Africa (SSA) will be considered as models, chosen for their
Species of the genus Striga, although often referred to as parasitic weeds, are actually pathogens and are therefore considered. Although there is an extensive bibliography available regarding the biology, symptoms, distribution and crop losses of some pathogens, data on their economic and social impact are scarce. As a result, several of the estimates of impact on crop production losses due to disease cited in this paper are at best based on assumptions rather than certifiable data. This is much more evident for pathogens affecting crops which are neither widely grown nor exported and are therefore of interest only to local populations. Here the lack of a network of technical assistance for monitoring, surveying and controlling disease means that such information is far from complete and this therefore represents a considerable handicap for crop protection. These problems are particularly severe in developing countries where when a disease is reported as “new”, it is often already widely distributed in the environment without any control, increasing the risk of pandemics.
Cassava Mosaic Virus Disease The plant host Cassava (Manihot esculenta Crantz) (Fig.
) is a shrubby
perennial plant belonging to the Euphorbiaceae family. It has been cultivated in South America and particularly in the Amazon basin for millennia for its starchy roots, but it was only introduced into Africa by the Portuguese in the sixteenth century. Thereafter, its drought tolerance and ability to produce yields on even marginal lands was appreciated. As a result, its cultivation spread slowly across Africa, largely by way of the river trade in Central and Western Africa (Legg and Thresh ). There was a considerable expansion of the area devoted to the crop during the colonial period, reaching its present distribution during 1920–1930 as a consequence of the authorities encouraging its cultivation as a food reserve for periods of famine and drought. Moreover, cassava (as well as banana, considered in another section of the article) is ideally suited to SSA, because it requires minimal field management. Cassava now constitutes a major source of food in SSA, and a significant source of revenue from the sale of the fresh or processed crop. The status of cassava cultivation today is changing from subsistence farming to an industrialized system designed to process cassava into a diverse spectrum of products, including starch, sago grains, flour, chips, animal feed and, potentially, biofuel, all derived from a crop that has the ability to grow in poor soils (Thresh ).
Origin, distribution and impact Cassava Mosaic Virus causes the most important disease of cassava in Africa. Symptoms consist of mosaic yellow or yellow-green chlorosis, leaf deformation and stunting (Fig.
). The disease was first reported in Tanzania
(Warburg ) and it was assumed to be caused by a virus, as the pathogen could be transmitted mechanically and was not visible. Only in more recent times has the exact aetiology been determined, with the agent of the disease being identified as a geminivirus (Bock and Woods ). There are some reports of its spread in the early decades of the last century, but none that were “alarming” as regards either disease severity or rate of spread. However, there were some sporadic outbreaks during the period 1920–1940 that led to the initiation of some crop protection pro- grammes, especially the introduction of resistant varieties.
Nevertheless, until the mid-1980s, Cassava Mosaic Disease (CMD) was considered to be just one of several diseases affecting cassava (Otim-Nape ). The situation changed suddenly in 1988 when a serious outbreak was reported in northern Uganda. Owing to social and political insecurity and instability at that time, following the flight of the dictator, Idi Amin, 10 years previously, it was not possible to obtain accurate analysis of the situation. Based on a series of observations, it was hypothesized that higher temperatures and lower humidity in that area of the country had favoured the spread of Bemisia tabaci, a polyphagous white fly insect and vector of the virus, thus indirectly favouring the spread of the virus itself (Otim-Nape
However, this hypothesis soon became untenable because, in subsequent years, the disease spread southward to areas that were more humid and colder at the rate of 20–30 km per year (Otim-Nape et al. Also, symptoms were more severe at the disease front, whereas vector populations were more numerous where the disease was already present.
The effects of virus disease on the farming communities in Uganda became evident in the early 1990s. The initial impact was greatest in the north-eastern areas of the country, particularly because of the cultivar grown, Ebwa- nateraka, which later proved to be the most susceptible to the virus. Here, cassava production between 1990 and 1993 was reduced by 80 to 90% and many farmers suspended its cultivation (Thresh and Otim-Nape In 1993, the failure of the crops of maize, beans and other food crops, owing to drought, compounded the lack of cassava as a food reserve, leading to widespread food shortages and famine-related deaths (Thresh and Otim-Nape A common reaction to this situation was the cultivation of other crops, mainly sweet potatoes. The impact of the epidemic in central and western regions of Uganda was less acute, mainly due to the use of a greater range of varieties, and therefore to the presence of certain varieties more tolerant to the disease. But the effects were still extremely serious. Several attempts have been made to quantify the losses due to the virus, the most reliable estimate being around 600 thousand tonnes per year valued at 60 million dollars (Otim-Nape et al. ).
When the impact of the epidemic became very clear, the real causes that had led to the outbreak of a disease that had hitherto been relatively innocuous were investigated. Serological and molecular techniques demonstrated the existence of virus variants which differed in virulence. Two are prevalent in Africa, African cassava mosaic virus (ACMV), and East African cassava mosaic virus (EACMV) (Swanson and Harrison ). However, in Uganda, a variant, which appeared to be a recombinant hybrid of EACMV and ACMV has been detected and is referred to
Fig. 2 Cassava plant with the typical symptoms of cassava mosaic virus on the right and a healthy plant on the left. Courtesy of Dr. a distinctive strain of EACMV (EACMV-Ug) (Deng et al.
The enhanced severity of the disease was related
both to higher virus titres in cassava plants infected with UgV and the widespread occurrence of plants with mixed ACMV/UgV infections in which symptoms were most severe (Harrison et al. ). The whitefly vector, although polyphagous, was much more prolific on infected plants. Also, in seeking less crowded areas, whiteflies enhanced the rate of dissemination of the virus. Moreover, the decline in cassava cultivation increased the capability of whiteflies to spread the disease to non-infected areas (Legg and Thresh
The severe CMD epidemic subsequently expanded rapidly to Kenya, affecting in a few years (from 1995 to 1998) virtually all the areas of cassava cultivation (Legg et al. ). Field observations estimated yield reductions of approximately 140 thousand tons per annum in those areas. The disease subsequently spread to the Sudan and Congo but, probably because of political instability in these regions, an accurate assessment of the magnitude of the pandemic was not possible, although it seems very serious in these areas (Legg Social and economic aspects Cassava is a main staple food in tropical areas and its production is extremely important in the poor central, eastern and southern African region for the role that it plays with other food crops in the local diet. Its adaptability to different environments as well as its tolerance to long periods of drought make it one of the most important staple foods in many parts of the world where soil stresses and human conflicts constrain production.
There are several reasons for cassava’s importance for food security in African countries. In order to understand
CMD is one of the most serious and widespread diseases throughout cassava growing areas in those African regions. It has threatened food security of millions of people through its impact on cassava production. According to FAO, in Africa cassava is the primary source of food for an estimated 70 million people, contributing over 500 kcal per day per person (FAO Cassava has been defined as ‘the crop of the poor’. Its contribution to rural development and poverty alleviation of marginal popula- tions is an important reality (Howeler et al.
In many Asian and African countries, cassava is the real catalyst for development of rural areas, its production representing the main source of income for the poorest rural households. In those areas in which food security con- ditions remain alarming, further development of the cassava production sector and of disease control and prevention could be very important contributions to the achievement of poverty reduction (FAO ).
CMD is the most important disease of cassava in Africa, Currently the presence of the disease has been registered in many countries of SSA, such as Angola, Burundi, Central African Republic, Democratic Republic of the Congo, Gabon, Kenya, Malawi, Mozambique, Rwanda, Southern Sudan, Tanzania, Uganda, Zambia and Zimbabwe. Accord- ing to FAO ), CDM threatens the whole cassava production system in the Great Lakes regions. In those areas the disease has reduced cassava yields of affected farms by up to 80 percent. The highest levels of the disease have been reported in Burundi, Malawi, North and Central Uganda, North Zambia, Tanzania and the central areas of Kenya. In the Democratic Republic of the Congo, it is estimated that the disease can cause losses as high as 90% (FAO ).
the socio-economic impact of cassava mosaic disease on the continent we have to consider that 50 percent of the current world production of cassava takes place there. Cassava is extremely important for the poorest smallholder farmers cropping on marginal and sub-marginal lands. First of all, it provides them with their main source of income. Secondly, it contributes to their living standard as cassava is the source of simple food products which are cheaper and more accessible than those of rice, wheat and maize (Nweke and Ezumah ). Cassava plays a major role in the alleviation of famine because of its efficient production of food energy and year-round availability. The crop is tolerant to drought and does not require the use of fertilizer or purchased seeds that are very expensive and not accessible to poor farmers (Nweke Another reason for cassava’s importance is that it can remain in the soil untended for up to 2 years without losing its nutritional properties, an important property in the event of temporary civil strife (Nweke et al. ).
The current widespread existence of CMD on the African continent is alarming the whole international community (developed countries, research and humanitar- ian organizations) because of the precariousness of live- lihoods there owing to the high incidence of civil unrest and the reduced capability to face natural crises. These have resulted in millions of refugees crossing borders, putting a strain on the recipients’ food supply and also contributing to the spread of the disease by the transport of infected vegetative material.
Today, food security and livelihoods of millions of people are under severe threat because of CMD. For a long time, the cultivation of cassava as an economic activity in those regions was neglected with little research being carried out to improve the crop and control disease. Today, greater attention from the international community is needed in order to increase cassava production and control considerable contribution to the achievement of sustainable food security and poverty alleviation.
There have been interesting political interventions to buffer the impact of the disease indirectly. In Nigeria great strides have been made, as a result of a Government initiative, through collaboration with the International Institute of Tropical Agriculture (IITA), to penalize impor- tations of wheat flour and promote the of use of cassava flour. All products using wheat flour now contain a defined percentage of cassava flour, hence providing a market for local producers (personal communication). This is impor- tant considering that the population of Nigeria amounts to 130 million (20% of all Africans). This has buffered
Nigeria’s food security from fluctuations in global wheat prices. Further, huge strides have been taken to develop
processing technologies to turn cassava into chips for human and animal feed and, perhaps more novel, into industrial starch for the textile and food industries (it has even been used in oil drilling activities). This example provides a positive indication of what is possible if the political will is present to create policies and an associated enabling environment for the exploitation of cassava varieties selected for market orientations accompanied by appropriate processing technologies. This approach also reduces the current constraint of massive post harvest losses due to the sale of perishable goods and transport on appalling roads by poorly maintained vehicles.
Striga hermonthica Origin, distribution and impact Striga hermonthica (Family Scrophulariaceae) is a hemi- parasitic weed, currently the main biotic “problem” for cereal crops in the SSA region (Fig.
. It is very
widespread, infesting land in western, central and eastern SSA, from Gambia in the west, to Ethiopia, Kenya and Tanzania in the east (Parker and Sahel in West Africa, including northern areas of Cameroon; besides S. hermonthica, economically the most important, there are three other main species of Striga in SSA which are agriculturally important: S. asiatica is economically signif- icant in eastern and southern regions; S. forbesii is limited to certain areas of Zimbabwe; S. gesnerioides, is found in areas of Nigeria and Tanzania. Sorghum, millet and maize are particularly susceptible to S. hermonthica, whereas all grasses are attacked by S. asiatica and S. forbesii. Legumes are attacked by S. gesnerioides, with cowpea (Vicia unguiculata) being very susceptible.
S. hermonthica is native of the tropical grasslands of the “old world” and reached its highest biodiversity in regions and rice. It then spread widely and became a scourge for the production of cereals (including maize) in areas where fertility is low and water availability is limited or erratic.
Maize was introduced into Africa many years ago, replac- ing sorghum and millet species more tolerant and better adapted to the scarce water resources available. The reasons for this introduction are many, such as increased produc- tivity compared to sorghum, at least in favourable years, consumer preference due to greater palatability, and a covered panicle reducing the risk of predation by the weaver bird, whose flocks of millions can destroy entire plantations of sorghum (Doggett
S. hermonthica seeds germinate only in the presence of the host plant, owing to the release of germination stimulants by the roots of hosts. The germ tube grows towards the root, attaches to it by a haustorium, and begins to steal nutrients and water from the host. The small seeds survive for many years in the soil, and therefore crop rotations have little effect on control once a certain threshold of the seed bank has been reached. When infestation is heavy, the plant seems also to be “poisoned” during the underground phase, making the injury more serious than the “mere” withdrawal of nutrients. It is not clear whether the compounds responsible are directly produced by S. hermonthica, or are the result of metabolism by the crop. Plants that initially appear healthy suddenly turn yellow and wither, as if they had been bewitched— giving the parasite its common name “witchweed”. After a phase of underground growth, when the parasite accumu- lates nutrients, it emerges from the soil, where its own
Fig. 3 Maize field destroyed by infestation of Striga hermonthica in Benin. Courtesy of Dr. Fen Beed, IITA, Kampala, Uganda An estimate in 1991 showed that there were at least 5 million hectares infested in six countries of Central-West Africa with an average loss in production of 12% (Sauerborn ). In northern Ghana, the estimated losses of the sorghum and millet crops reached an average of 20%. Overall losses in economic terms were estimated at more than $300 million although, considering the incompleteness of information, this may be an underestimate by a factor as high as ten. Recent estimates report a rapidly deteriorating situation with an increase in the total area infested to almost 50 million hectares. Nigeria is the country worst affected, with over 8 million hectares infested. According to these estimates, the infested area in Ghana would be between 12 and 27%, while in Central and Eastern Africa more than 6 million hectares of maize are infested by Striga. A report on the distribution of S. hermonthica in 25 African countries estimated that infestation of maize fields varied from 20 to 30% of the total in Togo, Mali and Nigeria and up to 65% in Benin (De Groote et al. In the province of Nyanza, in Kenya, clean fields planted with maize yielded about 1.5 t/ha, whereas the yield was about 750 kg/ha in moderately infested fields and only about 300 kg/ha in severely infested fields (Manyong et al.
Management Generally speaking, one of the most widely used conven- tional solutions for the management of weeds is the use of herbicides. There are systemic herbicides that, when sprayed on leaves, are absorbed and move through the vascular system until they reach the roots where they may come into contact with parasitic plants, controlling them. Initially herbicides were applied at low doses, so as not to affect the crops, but without positive results. Then the use of hybrids of crops resistant to herbicides was attempted (Joel et al. ). This strategy, although extremely valuable, would still require supply and adoption of commercial seeds and machines for treatments, which would be too expensive for most African farmers. Recently, the use of seeds pre-treated with herbicides has been proposed. The herbicide spreads systemically in the plant after germination, protecting it from attack by Striga. The advantages of this approach are that it does not require machinery or technical knowledge and the consumption of herbicide is much lower, and therefore more environmen- tally compatible (Gressel ). Herbicide seed coating could also be combined with biocontrol agent coating, to improve the efficacy and reduce the risks of resistance occurring due to selection pressure. A study on the acceptability of this technology in which treated seed was distributed has recently been done with the help of nongovernmental organizations (De Groote et al. ). providing the treated seed were not able to meet the demand in the following year. Even greater benefits were reaped by supplying farmers with bags containing fertilizer along with the treated seed. Another advantage of this technology, in addition to its affordability, is that the cultivation of legume intercropping is not precluded, as would be the case with traditional herbicide treatments (Kanampiu et al. ).
Researchers have tried for decades to find resistance genes in maize effective against S. hermonthica but as the centres of origins of the two species are on different continents, the Americas and Africa, respectively, they have not co-evolved and it is therefore probable that maize has no intrinsic resistance (Gressel and Valverde ). Only recently, an indigenous species Zea diploperennis, similar to maize, has been found to have a modest level of resistance (Amusan et al. ). In the case of sorghum the situation is different as its centre of origin and diversity, like that of Striga, is in Africa (Ethiopia) and consequently there are likely to be resistance genes in wild populations. Recently, significant progress was made as plants were found and characterized that produce small quantities of stimulants that hamper the development of the haustorium or block the penetration of the germ tube. The combination of a few of these factors allowed sorghum to have normal yields, even if some Striga plants grew and set seed. Breeding these traits into local cultivars has been facilitated by the use of genetic markers, resulting in sorghum lines with high resistance (Ejeta et al.
).
Often African farmers have intercropped legumes with maize, both as a nitrogen source for the crop and to ensure a food supply in the event of total loss of the maize crop due to drought or Striga. Legumes are generally not antagonistic to S. hermonthica but recent work has shown that the legume shrub, Desmodium uncinatum, originally investigated as a catch crop for the maize borer, Sesamia cretica, had an excellent effect on the control of Striga. Unfortunately, owing to its lack of adaptability it has the disadvantage of only being able to grow in restricted areas and can only be used, if freshly harvested, as animal feed (Khan et al. ). Studies are underway to identify the factors responsible for control- ling Striga in order to identify other crops with similar properties which are better suited to the different cultural needs and environmental characteristics of African regions (Hooper et al. ).
Interesting results were achieved with the use of isolates of Fusarium as mycoherbicides specific to Striga, particu- larly strains of F. oxysporum (Ciotola et al. ). The conidia of Fusarium can be applied in the form of a powder to the soil, or directly mixed with the seeds of the crop. Initially it was thought industries and programmes were initiated in this direction. However, as the culture of the microorganism is not easily accomplished, recent trends favour the organization of a centralized unit for the production and distribution of the material, which may provide higher quality and reliability of the microbial product (Venne et al.
Social and economic aspects In the colonial period, the spread of Striga (and of S. hermonthica in particular) had been limited for several reasons: in the fertile soil symptoms of the host were not serious, local labour was used to remove Striga plants and prevent the production of seeds, and crop rotations helped to reduce its impact and spread. After the colonial period, the situation progressively worsened. National governments wanted cheap grain for the inhabitants of cities, and therefore prices were kept low by decree or by importing grain donated or discarded by Western countries. Fertilizers have never been used traditionally, and Striga plants compete better in low-fertility soil (Ransom et al. ). Once the seed bank of the parasite reaches a critical level the situation becomes hopeless: restoring soil fertility is ineffective because both the parasite and the crop take advantage; weeding by hand-pulling becomes impracticable (Ransom et al. ).
In the developing countries in general, and especially those in Africa, control of weeds is delegated to women, who are subjugated to a life in the fields, spending as much as 80% of their available time there performing this manual practice (Akobundu ). This is why women often prefer to accept other jobs, where these are available, even if they are poorly paid, rather than working in the fields. Weeds are a major reason why land becomes unproductive and when this occurs, men leave manage- ment of the land in the hands of women, children or the elderly. The abandonment of land and the proliferation of weeds lead to a further worsening of the situation. Men move to cities in search of work, with the consequence of the spread of sexually transmitted diseases. Men and women living with HIV are debilitated, and thus have even less ability to manage the lands, and the situation becomes worse still when their children are orphaned. There is thus a vicious circle: no fertilizer to limit the initial spread of Striga, fewer manual workers to remove the parasitic plants because the men leave the farms for the cities, spreading diseases such as HIV-AIDS and malaria, less farm work owing to disease and consequently increasingly larger areas becoming more severely infested with Striga (Ejeta ). This situation is further complicated by the fact that the level of damage is unpredictable, some years being worse than others. Thus, caloric needs of families, well below the level of survival
(Gressel and Valverde ). Banana Xanthomonas Wilt (BXW) The plant host Bananas and plantains (Musa spp.) are the fourth most important staple food in the world, after rice, wheat and maize. The annual world production is estimated at 100 million tonnes, of which only 10% enter the commercial circuit, demonstrating how this culture is more important locally than for export (FAOSTAT ). Approximately one third of world production is concentrated in the SSA regions, where it provides about 25% of the food to over 70 million people. Banana is a common feature of the agricultural and cultural landscape in Africa, despite being introduced from Asia only several hundred years ago. Moreover banana is a perennial, providing continuous ground cover and preventing soil erosion that would otherwise occur if annuals were cultivated, especially those that are uprooted for their roots and tubers.
Fruits are used at different stages of ripening and for different purposes. For this reason they are named: dessert, plantain, cooking and juicing bananas. Dessert is a snack and in some countries exported (Ghana, Ivory Coast, Cameroon, Kenya, Malawi, Zambia and under irrigation in Somalia). Plantain (West and central Africa) is a staple and commands a higher premium as it takes longer to produce than cooking bananas. Cooking tends to be grown in the Great Lakes region and is a key staple. Juicing is also grown in the Great Lakes region and is processed into alcohol and used for income generation.
The eastern regions (Burundi, Kenya, Rwanda, Tanzania and Uganda) are the major producers and consumers of bananas in Africa. Uganda is the second largest producer in the world after India (FAOSTAT Bananas have huge economic and social importance in the Great Lakes area as they represent both a source of food security and of profit (Edmeades et al. In countries such as Uganda and Burundi, they provide more than 30% of daily caloric needs, reaching even 60% in some areas. Also, for some agricultural areas they are the main export crop and therefore constitute a very important source of income (Abele et al. Annual banana consumption is about 190 Kg per person in Uganda, 140 Kg in Rwanda, 90 Kg in Kenya and 20 Kg in Tanzania (FAOSTAT ). Bananas constitute an important source of income for approximately 30% of the farmers who generally sell from 25% to 50% of their yield, especially in the west of Uganda (Okech et al. More than seven not surprising that they use the term “matooke” to indicate both “food” in general as well as “boiling banana” in particular. Origin, distribution and impact Among the many threats to banana plantations, such as reduced soil fertility, insects or phytopathogenic agents, the disease caused by the bacterium Xanthomonas campestris pv. musacearum, known as Banana Xanthomonas Wilt (BXW) is one of the most important emerging risks. This disease was initially reported in Ethiopia about 40 years ago on Ensete sp. (Yirgou and Bradbury a genus closely related to Musa. It was reported in Uganda in 2001 on banana and from there it has spread rapidly to all regions of
Fig. 5 Typical symptoms on fruit transects showing brown staining,
Africa where the crop is grown. No varieties have complete
caused by Banana Xanthomonas Wilt. Courtesy of Dr. Fen Beed,
genetic resistance but they differ in degree of susceptibility
IITA, Kampala, Uganda
(Tripathi et al. The cultivar Pisang Awak, originating in Malaysia, is the most susceptible (Tushemereirwe et al. bees, fruit flies) (Tinzaara et al. by mechanical
Symptoms consist of a progressive yellowing and
transmission due to the use of infected tools; in the roots withering of leaves, and rapid and premature ripening of when the soil is contaminated by infected plant debris fruits (Fig.
),
(Mwangi and Bandyopadhyay ); leaf necrosis (Fig.
and rotting of male flowers, rachis
and by raindrops containing the bacterium. It can also be and the bunch. Finally, the plants wither and rot. Symptom disseminated by planting infected propagating material. appearance is rapid, becoming evident as early as 3 or
The disease has a devastating impact because it develops 4 weeks after infection. This, however, depends on the type very quickly, giving rise to severe symptoms, leading to the of infection, the environmental conditions, the state of the death of entire plants, including those used for propagation plant and the cultivar. Infection can occur: in inflorescences, (Tripathi et al. ). Moreover, infested fields cannot be when the bacterium is carried by insect vectors (stingless replanted with banana for at least for 6 months, owing to the persistence of the pathogen in the soil. Once the pathogen has initiated infection, damage limitation is extremely difficult and the disease is impossible to cure (Eden-Green ). Since 2001 the disease has spread in some areas in an impressive manner, causing yield losses of
Fig. 4 Early ripening and rotting bunch caused by Banana Xantho- Fig. 6 Foliar symptoms caused by Bacterial Xanthomonas Wilt on up to 60%. This has led the government of Uganda, for example, to set up a task force for the eradication of the disease by cutting down and destroying by fire diseased plantations, removal of male buds to prevent infestation from insects and stopping traders from coming to farms to harvest bunches unless tools are sterilized (Tushemereirwe et al. ). Although these interventions have led to a reduction in the incidence of the disease, there has been little support for them because of the high costs and the difficulty in convincing farmers of their necessity.
It has been estimated that, if not controlled, the pathogen can increase the area infected at a rate of 8% per year (Kayobyo et al. ). The damage caused by the disease each year is estimated at 2 billion dollars, and at least 8 billions if projected over a period of 10 years. A recent study estimated 53% yield losses in banana production in Uganda in 10 years. Production losses caused by the disease threaten the food security of about 100 million people and the income of millions of farmers in the Great Lakes region of Central and Eastern Africa (Tripathi et al.
Management The scenario described in the previous paragraphs has some important consequences for disease management. Usually, disease control measures are based on an economic threshold and are put into practice when the losses outweigh the costs of disease management (Peterson and Hunt ). In this regard, management of bacterial diseases presents several problems, such as mild symptoms in the early stages of an epidemic, and thus reducing the propensity of farmers to take action as the losses resulting from the destruction of plantations would, in the short term, outweigh the benefits. This, allied with the rapid onset of severe symptoms and spread of the pathogen mean that farmers only begin to take action when it is already too late (Biruma et al. ).
The management of tropical diseases in perennial crops such as bananas and plantains is a continual challenge. Management measures include a combination of: preventa- tive interventions to reduce disease establishment; curative control through destruction of infected plants where the disease is already present; and, rehabilitation of areas that were previously infected. Information campaigns, technical assistance and financial input from national governments supported by international and transnational organizations are crucial in these circumstances. From this point of view, the results in different countries have been different. In countries such as Uganda and Tanzania, with active political leadership, disease reduction of more than 90% was achieved. In other countries with different social and
Congo, the spread of the disease has, on the contrary, almost quadrupled (Mwangi et al. ).
Thanks to a suite of international donors, a Task Force was set up over the period from 2005 to 2008, which has helped the poorest countries to mitigate the effects of the disease in terms of both social security and food security, so that the reduction of banana production in countries such as Burundi, Kenya and Tanzania has had a less devastating social impact. This Task Force implemented differing interventions according to severity of the disease in order to: reduce the spread in areas where it was not yet widespread; provide opportunities for cultivating alternative crops in areas where it had had devastating effects; and, prepare for a gradual replacement of bananas with other crops, where the situation was gradually worsening. Properly informed and trained, the majority of small farmers have shown willingness to replace banana cultivation with annual crops resistant to Xantho- monas, such as beans, cassava, maize and potato, which are viable alternatives for cultivation and consumption (Tushemereirwe ).
The use of certain farming practices and cultivation operations can reduce the spread of the disease. For example, timely removal of male buds prevents dissemina- tion of the disease by insect vectors, which in some areas is the most important means of spread. However, this practice has found little application in some areas as farmers, owing to ignorance, refuse to adopt it, considering it to be detrimental to the quality of the crop (Kagezi et al. Once the disease is established, there is no remedy other than to remove and destroy all plants and debris, in which the pathogen can survive for a considerable time. If this is done carefully, the crop can be reintroduced after a fallow period of several months (Turyagyenda et al. The plants are propagated by detaching the many suckers that are formed, and replanting them. Selection of planting material from fields known to be free of BXW can do much to prevent the re-establishment of the disease. Diagnostic methods are also valuable such as the use of semi-selective media or PCR-based diagnostics, preferably controlled through a robust regulatory system (Mwangi et al. Tripathi et al. ). A current constraint to the re- establishment of banana is the lack of systems to supply micro-propagated (tissue culture) or macro-propagated (suckers) planting material, or certification systems to ensure material is disease free.