W.M. Ciesla
William M. Ciesla is Forest Protection Officer, Forest Resources Division, FAO, Rome.
Neem (Azadirachta indica) is a favourite tree throughout the Sahel, and with good reason. It grows vigorously under semi-arid as well as humid tropical conditions and responds to many needs. Its spreading crown provides welcome shade from the tropical sun; it is a source of fuelwood; in parts of the Sahel, neem wind-breaks protect millet, sorghum and other crops from the desert winds; and its leaves have insecticidal and medicinal properties that are widely appreciated.
Approximately two years ago, neems in portions of the Sahel were observed to have a sickly appearance. They began to lose their leaves, the foliage that remained in the once thick crowns turned a yellow colour and new leaves were stunted. Some trees have died. Foresters and research scientists in the Niger and Nigeria are working with colleagues from France, the United Kingdom' the United States and FAO to determine the cause of the damage and what might be done to protect one of the few trees available for multipurpose plantings in the Sahel.
Native to India and Myanmar, the neem is a member of the family Meliaceae, the mahoganies. It has compound leaves of nine to 15 leaflets which are dark green in colour. The fruits are yellow-green to green, smooth, olive-shaped and about 2 cm in length, with a sweet pulp enclosing a seed.
Neems can grow up to 30 m in height and 70 cm in diameter, with broad, spreading crowns that retain their foliage the year round. This characteristic is one of the main reasons that neem is so highly valued in its native India.
Neems are characterized by compound leaves and olive-like fruits
Shade is not the only valuable characteristic of neem, however. The tree has so many current uses and potential for future uses that, in some circles, it is referred to as a "miracle tree". The wood is used for fuel, in construction and for furniture. The flower dusters attract bees and honey produced from neem blossoms has a good flavour. Neem has been called "the village pharmacy". In India, people use a neem twig to dean their teeth and prevent gum disease. Other medicinal uses include the treatment of skin disorders and, as a tonic, the treatment pains, fevers and infections.
Neems are popular shade trees in villages throughout the Sahel
Extracts of neem have insecticidal properties. By simply placing fresh neem leaves in water overnight, a natural insecticide can be produced which can be sprayed on foliage to prevent feeding by insects. Neem is generally the only plant that still has green foliage after a locust plague. Neem leaves are placed in beds, books, grain bins and cupboards to keep away insects. At a more sophisticated level, Azadirachtin and several other compounds extracted from the leaves and seeds of neem act as feeding repellents and disrupt insect growth and reproduction.
Neem oil, which is produced from the seed, is used in the manufacture of soaps, lubricants and fuels for lighting and heating while the residue that remains after the oil is removed can be used as a fertilizer (National Research Council, 1992).
Neem was introduced into Ghana between 1919 and 1927. It quickly gained popularity and, from there, was spread throughout West Africa. Today, towns and villages throughout the Sahel, even in the most remote areas, contain neem. In many cases, they comprise 100 percent of the shade trees in a village.
One of the most notable neem planting programmes is that in the Majjia Valley of the south-central Niger. At the beginning of the century, the Majjia was heavily wooded but growing populations, and the consequent increase in demand for fuelwood, fodder and construction material, stripped the valley bare. Strong winds removed topsoil which, when deposited on newly planted crops, covered and destroyed them. In 1974, CARE International began a programme to help the people of the Majjia Valley plant wind-breaks. Neem was the principal species used and local residents were encouraged to establish neem nurseries for the programme. Today, there are more than 500 km of double-row windbreaks in the Majjia Valley which protect the millet and sorghum crop. Crop yields in protected areas have increased by 13 to 23 percent since the planting programme began (Ibraham and Yara, personal communication). In addition to protecting crops, the wind-breaks provide fuelwood and shady lanes connecting villages to farmers' fields.
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Declines: complex diseases Experts who have examined diseased neems in the Sahel refer to the condition as a "decline". Decline may be defined as a disease "caused by the interaction of specifically ordered abiotic and biotic factors to produce a gradual general deterioration, often ending in the death of trees" (Marion, 1981). Declines are therefore complex diseases and it is difficult to identify the factors responsible for them from the symptoms. According to Manion (1981), declines involve three sets of factors. Predisposing factors, which are generally static phenomena such as soil type, site or the genetic make-up of a population of trees. Inciting factors, which are short in duration but result in a significant weakening of trees already under stress. Examples include defoliation by insects, drought or exposure to pollutants. Contributing factors, which prevent recovery of affected trees. Examples include secondary insects and fungi, viruses and other biotic factors that produce visible signs and symptoms and prevent the tree from recovering. Contributing factors are often blamed for a condition when, in fact, they are only secondary agents. Symptoms associated with declines may include: reduced growth; degeneration of root systems; reduction in stored food reserves; yellow or abnormally small foliage; loss of foliage; branch dieback; water sprouts from adventitious buds; and eventual death of the tree. To date, declines have been reported primarily in temperate and boreal forests. Noted cases are a decline of birch Betula sp. in eastern Canada during the 1950s, a decline of oaks Quercus sp. in the Appalacian mountains of the United States and declines of several species of hardwoods and conifers in central Europe. There is recent evidence, however, that declines are also a problem in tropical forests. For example, there have been recent reports of extensive branch dieback of Acacia nilotica in the Sudan. |
Although a tree that is the source of a natural insecticide might be expected to be relatively free of insect pests, there are cases where damage has been reported. Several species of scale insects infest neem, the most important being the yellow scale or cochineal, Aonidiella orientalis (Homoptera: Diaspididae), which attacks the leaves and young stems. This insect, believed to have been introduced on citrus trees, was first recorded on neem in Africa in Cameroon in 1972 and later in the Sudan. During the mid 1980s, the insect spread from northern Cameroon to Chad and northeastern Nigeria. Heavy infestations cause a drying of the foliage and give the tree a burnt appearance. A heavy infestation reported in 1987 in the Lake Chad Basin was believed to be related to stress caused by a lowering of the water-table in the Basin during a prolonged drought. Damage by this insect is still of concern around Lake Chad.
In Nigeria, 14 insect species have been reported as pests although none have been widespread. Several species of fungi attack the roots, stems and foliage of mature trees and seedlings but none are considered serious pests (National Research Council, 1992).
In November 1990, during the course of a countrywide inventory of neem, trees displaying disease symptoms were found near Sani-Tanko, a settlement about 80 km northwest of Dakoro in the south-central Niger (USAID/Niger, 1992). The most conspicuous of these symptoms is a loss of older foliage, often preceded by leaf-yellowing. The foliage loss gives trees an open appearance, with clumps of foliage occurring at the branch apices. This condition has been described as "giraffe neck". New leaf growth is abnormally small and often has a yellow cast.
Other symptoms associated with neem decline include shortened internodes near the apex of the branches, exudation of gum from branch tips, branch dieback and tree mortality. Some workers have reported the occurrence of a deep red colour in the cambium layer on branches greater than 1 cm in diameter as being associated with the decline. There is disagreement, however, over whether this is a symptom of decline or a normal characteristic of the tree.
There is also some debate as to whether the observed decline is a sudden phenomenon or whether it had been going on for some time and was only detected recently. There are photos of neem plantations exhibiting thin crowns and abnormally small foliage dating back as far as 1963 (CTFT, 1963; Lauridsen, Kanchanaburgura and Boonsermsuk, 1991).
In June and July 1991, an evaluation of the situation (financed by USAID) found declining neems over a large area of the southern Niger (Batra, 1991). The evaluation eliminated insects as a possible cause of the decline (many people had been confusing the decline of neem with damage caused by the cochineal scale (A. orientalis) and suggested that a virus might be involved. Batra (1991) later recovered a fungus of the genus Verticillium sp. from several trees and concluded that this might be a causal factor. Further investigation, by the Sahelian centre of the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) near Niamey, recovered two species of fungi, Nigrospora and Curvilaria, from diseased neems but has thus far failed to confirm their role in the decline (Waliyar and Hess, personal communication, 1991).
R. Catinot, former director of the Centre Technique Forestier Tropical (CTFT), working with the Government of the Niger in developing a national forestry plan within the framework of the Tropical Forests Action Programme (TFAP), also investigated the decline (Catinot, personal communication, 1991). His observations, based on many years of experience in West Africa, led him to conclude that the decline is not caused by a biotic agent such as a fungus, virus or bacterium, but is the result of site-specific problems such as moisture stress or poor nursery and plantation management practices.
Loss of older foliage, giving trees a more open crown, is one of the early symptoms of neem decline
In April 1992, two teams of international experts, joined by national counterparts from the Niger and Nigeria, made a series of investigations into the causes of the neem decline. Dr Eric Boa, a forest pathologist with the United Kingdom's Natural Resources Institute, visited a large portion of the damaged area under the sponsorship of the United Kingdom Overseas Development Administration. Under the sponsorship of USAID, Drs Charles Hodges and Jerome Beatty, two forest pathologists from the United States, visited the Niger as a follow-up to Dr Batra's earlier mission. Neither group was able to confirm earlier hypotheses that a virus might be involved in the decline, nor were they able to isolate any other biotic agents that they could link to the problem. Root systems of a number of trees were carefully excavated and examined but failed to reveal evidence of a root disease that might explain the symptoms.
Hodges and Beatty came to essentially the same conclusion reached earlier by Catinot; that the decline of neem is due to site-related stress such as low soil moisture, competition, intercropping and soil compaction. Dr Boa also tended to agree with the stress hypothesis but did not totally discount the involvement of a biotic factor such as a plant virus or a mycoplasma-like organism. Both teams of scientists recommended the establishment of a programme to monitor the progress of symptoms over time and provide reliable data on the effects of this condition on tree growth and mortality (Boa, 1992; Hodges and Beatty, 1992). The Ministry of Agriculture and Livestock Rearing of the Niger has begun to develop a plan for such a programme.
In June 1992, the author had the opportunity to make a series of observations on neem decline in the Niger, from which a number of points can be made regarding the occurrence and severity of the decline:
· trees of all ages are affected;
· single trees tend to have fewer symptoms than trees growing in groups;
· the most severe symptoms appear in forest plantings, followed by windbreaks;
· trees growing in villages tend to be less severely affected, although decline symptoms do appear, especially in areas which have heavy use, such as outdoor market places;
· trees that have been pollarded tend to have healthy foliage and dense crowns;
· at the present time, tree mortality attributable to the decline is minimal.
The pattern of the occurrence of decline, coupled with symptoms such as foliage loss, smaller than normal foliage and yellow foliage, suggest moisture stress as a key factor in the decline. Using the concept that three sets of factors - predisposing, inciting and contributing- are involved in declines (see Box), the following sequence of events may be hypothesized as having brought about the symptoms now seen in the Niger and other countries of the Sahel.
Predisposing factors. The Sahel characteristically has long dry periods, with a July-September rainy season. While neem is reportedly well adapted to the arid and semi-arid tropics, long-term stress could result from the growing of neem in plantations where trees compete for available moisture, a narrow genetic base with little variability in susceptibility to stress, poor planting techniques and localized soil nutrient deficiencies in combination with long dry periods.
Inciting factors. A subtle, yet significant, change in temperature and/or precipitation could be an inciting factor in the decline of neem. A simplistic analysis of rainfall patterns at selected stations across the southern Niger during the period 1940-1991 (Hodges and Beatty, 1992) showed a decrease in rainfall in each decade, beginning with the 1950s. Rainfall from 1982 to 1985 and in 1987 was especially low; at most stations it was below the level commonly believed to be necessary for the growth of neem (approximately 450 mm per year).
As trees increase in size, so do moisture requirements. Consequently, as plantings become older and larger, there is more competition for available moisture, inciting a further stress on the trees.
An exceptionally heavy crop of neem flowers occurred throughout the Niger in April 1992. Heavy flowering could place a short-term stress on trees, especially in a moisture limiting environment, and result in foliage loss. Heavy flower and seed crops are also known to be a response to stress; however, stress crops tend to have a low seed viability and fruit size is usually smaller than normal.
Contributing factors. Factors that could contribute to further weakening of neem include soil compaction, especially in areas of heavy human and livestock activity, invasion by secondary fungi, such as Nigrospora and Curvilaria, and browsing by livestock.
Declining neem continue to appear over an increasingly large area of the Niger. In addition, the presence of the decline has been confirmed in several neighbouring countries, including northern Nigeria, Cameroon, Chad and Mali. Concern about neem decline is increasing. CARE International has suspended its neem planting programme. Three Niger Government agencies responsible for resource management and plant protection the Environment Directorate in the Ministry of Water Resources and the Environment, the Directorate of Plant Protection in the Ministry of Agriculture and Livestock Rearing and the Institut National de Recherches Agronomiques du Niger together with international technical assistance and donor agencies active in the Niger, have requested that FAO coordinate activities related to the decline of neem.
Declining neems in a shelter-belt in the Majjia Valley of the south-central Niger
If neem is to continue to be an important multipurpose tree in the Sahel, there needs to be a thorough understanding of the factors involved in its decline. In addition, measures to improve the overall health of present and future neem plantings are required.
There is agreement among experts who have studied the problem that an inventory to monitor the health of neem is essential. This would provide data on changes in the severity of the decline over time as well as growth and mortality rates. A method of classifying trees according to the severity of symptoms has been developed. The establishment of permanent sample plots, which would be visited at least twice a year to classify trees according to severity of decline, could provide data essential for an understanding of the progress and socioeconomic impacts of neem decline.
Research on possible causes of the decline should be continued, including an in-depth analysis of recent climatic trends in the Sahel. Other research should focus on methods to improve the health of stressed neems, such as irrigation, pollarding and a clearer definition of the conditions under which neem should be planted.
Neem introduced into the Sahel is believed to come from a narrow genetic base. Therefore, there may be little variation in the ability of the present population of neems to resist environmental stress. New neem germplasm should be introduced into the Sahel and evaluated in provenance trials. The generally poor seed viability of neem (usually from two to six months) has limited the introduction of new genetic material in the past. However, recent work done by the CTFT in France and confirmed by investigators in the United Kingdom indicates that special handling of neem seed will extend viability (Roederer and Bellefontaine, 1989). FAO, the CTFT and the Forestry/Fuelwood Research and Development Project in Bangkok, Thailand, have initiated a joint project for international collection and testing of neem germplasm in cooperation with national institutes in Asia and Africa. Provisions should be made for the testing of this material throughout the Sahel.
If a wider number of species were available for tree-planting, the entire population of trees would be at less risk of damage by insects, disease or other destructive agents. The risk of single species dependency can be illustrated by the example of the stately American elm, Ulmus americana, once widely used as a shade tree in eastern and central Canada and the United Sates until Dutch elm disease, caused by the fungus Ophiastoma (= Ceratocystis) ulmi, devastated the species.
Existing and possible new species as well as provenances should be carefully evaluated while work on the identification of possible alternative species should be intensified.
Finally, new knowledge and technology have little value unless they are effectively disseminated and put into practice. A coordinating mechanism is needed to receive and distribute reports by the various specialists studying neem. In addition, periodic workshops should be held to report on new information, exchange knowledge among countries in the region and coordinate action.
FAO is currently working with the Government of the Niger and international technical assistance and donor agencies to develop a coordinated action programme which addresses these needs. Information derived from this programme will be applied throughout the region so that neem will continue to enhance the quality of life of the people in the Niger and other countries of the Sahel.
Batra, G.K. 1991. Vascular decline of neem in Niger. Report submitted to USAID. Washington, D.C., USAID. 26 pp.
Boa, E.R. 1992. Neem disorder and neem scale in Nigeria. Project TO 361. London, Natural Resources Institute, United Kingdom Overseas Development Administration. 41 pp.
CTFT. 1963. Azadirachta indica et Melia azedarach, caractères sylvicoles et méthodes de plantation. Rev. Bois Forêts Trop., 88: 23-29.
Hodges, C.S. & Beatty, J.S. 1992. Evaluation of a disorder of neem in Niger. Report submitted to USAID. Washington, D.C., USAID. 32 pp.
Lauridsen, E.B., Kanchanaburgura, C. & Boonsermsuk, S. 1991. Neem (Azadirachta indica A. Juss) in Thailand. Forest Genet. Resour. Inf, 19: 25-33. (FAO forestry occasional paper)
Manion, P.D. 1981. Tree disease concepts. Engelwood Cliffs, New Jersey, Prentice Hall, Inc. 397 pp.
National Research Council. 1992. Neem: a tree for solving global problems. Washington, D.C., National Academy Press. 141 pp.
Roederer, Y. & Bellefontaine, R. 1989. Can neem seeds be expected to keep their germinative capacity for several years after collection? Forest Genet. Resour. Inf, 17: 30-33. (FAO forestry occasional paper)
USAID/Niger. 1992. The neem disorder in Niger. Washington, D.C., USAID. 12 pp. (updated version, April 1992)
