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6.2 Diseases of Tree Legumes

J.M. Lenné and E.R. Boa


Introduction
Acacia
Albizia
Calliandra
Erythrina
Faidherbia albida
Gliricidia sepium
Leucaena
Prosopis
Sesbania
Potential Control Strategies
Future Research Needs
References


Introduction

Tree legumes include an extensive range of multipurpose, widely adapted species (Nair et al. 1985) which are useful components of agricultural and silvicultural systems (NFT 1989). They are important throughout the tropics as sources of forage, firewood, charcoal, green manure and timber (NFT 1989, Hughes and Styles 1989). Over the last 15 years there has been a rapid explosion of reports of diseases affecting tree legumes. Over 80% of references cited in this section date from 1979 or later. However, we know very little about many important aspects of these diseases including control strategies.

This section consolidates information on diseases of the most important tree legumes used for forage. Emphasis is given to foliage diseases which may directly reduce forage production and quality. We report some of the preliminary findings from recent surveys of diseases of Calliandra, Gliricidia and Leucaena in Central America. Potential control strategies of common diseases and future research needs specifically for forage trees in animal production systems are briefly discussed. A recent review (Lenné 1992) of diseases of multi-purpose woody legumes in the tropics complements this section.

Acacia

Seed and seedling diseases

Seedling blight, defoliation and dieback due to Glomerella cingulata (anamorph Colletotrichum gloeosporioides) caused serious losses to A. mangium in nurseries in Papua New Guinea (FAO 1981) and Indonesia (Lee and Goh 1989) and to Acacia spp. in India (Mohanan and Sharma 1988, Sharma and Bhardwaj 1988) especially under humid conditions (Mohanan and Sharma 1988). Leaf spot caused by Cylindrocladium quinqueseptatum, a common foliar pathogen of trees, caused defoliation of seedlings and young trees of A. auriculiformis and A. mangium in India (Mohanan and Sharma 1988). The rust Uromyces digitatus reduced growth of A. auriculiformis in nurseries and young plantations in Indonesia (Turnbull 1986). Powdery mildew, Oidium sp., severely affected A. mangium in Hawaii (NAS 1983a) and caused up to 75% mortality of A. mangium seedlings in nurseries in Thailand (Chalermpongse 1990).

Fusarium solani, Phytophthora spp., Pythium spp. and Rhizoctonia solani may cause serious damping-off of tree legume seedlings (Lee 1985, Sharma and Bhardwaj 1988, Zakaria 1990). In Malaysia, damping-off damages A. mangium where seedlings are crowded or soils too damp (Lee 1985) with losses as high as 30% (Liang 1987, Zakaria 1990). In Malaysia, seedlings of A. mangium are also affected by brown root disease (Phellinus noxius) and charcoal rot (Macrophomina sp.) resulting in stunting, chlorosis and death (Lee 1985). Root-knot nematodes, Meloidogyne spp., caused high mortality of seedlings of A. mangium in Sarawak, Malaysia (Chin 1986).

Foliage diseases

Limited information is available on the importance of the many foliage (leaf twig and branch) diseases caused by at least 10 fungal genera which have been widely recorded on Acacia spp. (Browne 1968, Gibson 1975, Mohanan and Sharma 1988, Lenné 1992).

Cercosporella theae caused dark sunken lesions on leaves and twigs and may cause defoliation of A. mangium in Malaysia (Browne 1968, Gibson 1975, FAO 1981). Exserohilum rostratum caused dark irregular lesions with pale centres on foliage of young A. auriculiformis in India (Mohanan and Sharma 1988). Powdery mildew, including Oidium sp., was moderately severe on A. auriculiformis and A. mangium in Australia (Ryan and Bell 1989).

Many rusts occur on leaves, twigs and pods of Acacia spp. Information on host range, symptoms, distribution, biology and ecology has been reported (Bakshi and Singh 1967, Browne 1968, Gibson 1975, Dick 1985, Morris 1987). Many have restricted host ranges (Gibson 1975). An exception is Uromycladium tepperianum which is hosted by 118 Acacia and Albizia spp. (Browne 1968). More than 60 species of rusts are confined to Acacia spp. In Australia, New Zealand and southeast Asia, most rusts belong to the genus Uromycladium while in tropical America, India, Myanmar and Africa, Ravenelia is most common. Uromyces sp. is widespread on A. auriculiformis in Indonesia (Santosa et al. 1984). Although rusts have been described as an 'indeterminate threat' to Acacia spp. (FAO 1981), few rusts are presently considered important.

In Malaysia, the importance of pink disease, caused by Phanerochaeta salmonicolor, varies among sites on A. mangium. In peninsular Malaysia, occasional crown and tree death has been observed (NAS 1983a, Lee 1985) but in Sarawak and Sabah, severe pink disease has been reported (Chin 1982, Khamis 1982) where the fungus attacks the bark, girdling branches and causing dieback (Zakaria 1990). Rhizoctonia solani causes web blight of A. auriculiformis and A. nilotica in India (Mehrotra 1990) resulting in premature defoliation as severe as 30-70%. Three biotypes of the fungus have been identified (Mehrotra 1990).

Sandal spike, caused by a mycoplasma-like organism, affects Acacia spp. in India only (Browne 1968, Ghosh 1981, Sen Sarma 1984, Nayar 1988). Symptoms include foliage dwarfing, shortened internodes, excessive branching, pale green or reddish leaves standing out stiffly from twigs, suppressed flowering and small fruit. Spiked trees usually die within 3-5 years of attack (Browne 1968, Nayar 1988). An extensive review of the disease is provided by Nayar (1988). Virus-like disorders have been observed on Acacia spp. (Seliskar 1964); however, none have been well characterised.

Root diseases

Root diseases caused by Macrophomina phaseolina, Armillaria mellea and Ganoderma spp. may seriously affect Acacia spp. (FAO 1981). These pathogens have been recorded on A. auriculiformis and A. nilotica in India and Pakistan (Bagchee 1945). Red rot disease, caused by Ganoderma sp., affects A. mangium in Malaysia (Lee 1985). In Papua New Guinea, A. auriculiformis is affected by root rot caused by Ganoderma and Phellinus spp.; in one study area, approximately 10% annual mortality has been recorded (Skelton and Howcroft 1987).

Brown root disease caused by Phellinus noxius affects A. mangium in Malaysia (Khamis 1982) and the Solomon Islands (Ivory 1990). The fungus rapidly extends along the roots to the collar then upwards to the stem base. Trees yellow, wilt and die. In older trees, often only part of the crown is affected. Wilt and root rot due to Fusarium solani has been reported on A. nilotica and A. auriculiformis in India (Bagchee 1945, 1958) while Botryodiplodia theobromae is associated with root disease of A. auriculiformis in India, A. mangium in Malaysia and A. nilotica in India and Kenya (Gibson 1975). Six Acacia spp. including A. tortilis and A. nilotica were susceptible to Meloidogyne javanica and M. incognita in Senegal (Pros 1986). Acacia auriculiformis is attacked by nematodes in Zanzibar (NAS 1979).

Albizia

Seed and seedling diseases

Rhizoctonia solani caused destructive damping-off of seedlings in nurseries of A. lebbeck in India (Mehrotra 1989) and Sri Lanka (Bandara 1990). Leaf rusts caused by Ravenelia spp. are important seedling diseases of Albizia spp. in India (Sharma and Bhardwaj 1988).

Foliage diseases

Endothella albiziae caused defoliation of Albizia spp. including A. lebbeck in Africa, the Philippines and Pakistan (Gibson 1975). Camptomeris albiziae caused leaf blotch and foliar necrosis of A. lebbeck in tropical Africa, India, Sri Lanka, Pakistan, the Dominican Republic (Browne 1968, Gibson 1975) and Bangladesh. A similar leaf blotch, Camptomeris albiziicola, has been recorded on A. lebbeck in India (International Mycological Institute, unpublished data).

Cercospora glauca and C. albiziae caused leaf spots of A. lebbeck in USA, China and Nepal and in India, Sudan and Tanzania (Gibson 1975, Bakshi 1976), respectively. Colletotrichum lebbeck has been widely recorded on pods and leaves of A. lebbeck in Pakistan, the Philippines and Jamaica (Gibson 1975) causing grey, circular shot-hole lesions. Phyllosticta albizinae may seriously defoliate young plantations of A. lebbeck in India (Bakshi 1976). Rhizoctonia solani causes leaf web blight of A. lebbeck in India (Mehrotra 1990).

Many rusts belonging to the genera Ravenelia and Uredo are recorded only on Albizia spp. (Gibson 1975). Uromycladium tepperianum and Sphaerophragmium acaciae, have wider host ranges including Acacia spp. (Gibson 1975). Information about these rusts is well documented (Browne 1968, Gibson 1975, Bakshi 1976); however, in most cases, their importance has not been determined.

Dieback and canker of Albizia spp. are caused by Nectria ditissima on A. lebbeck in Madagascar (Gibson 1975). In Mauritius, leaf mosaic of A. lebbeck is believed to be caused by a virus (Seliskar 1964, Gibson 1975).

Stem diseases

Fusarium solani caused grayish-black cankers on 15-20 year old trees of A. lebbeck in India (Bakshi 1976). The pathogen invades through wounds and later develops a stratified canker through repeated killing of the cambium (Bakshi 1976). In severe infections, the canker may extend 2-4 m almost girdling the stem and causing drying of the crown. Trees may snap at the region of the canker.

Root diseases

The most important disease of A. lebbeck is vascular wilt caused by Fusarium oxysporum f sp. perniciosum which is widespread in the USA and has also been reported from Argentina and Puerto Rico (Gibson 1975). The fungus invades fine roots causing gummosis of the vessels. Two races of the pathogen exist (Gibson 1975).

Calliandra

Of 26 disease records on at least nine Calliandra spp., 18 were rusts, mostly Ravenelia spp. from Mexico, Central America, the Caribbean and Brazil (Lenné 1990). Dieback caused by Nectria, Thyronectria and Phomopsis spp. was common in the Caribbean and West Africa; leaf blotch caused by Camptomeris calliandrae occurred in Costa Rica; and pink disease (Phanerochaete salmonicolor) has been noted in Papua New Guinea (Lenné 1990). Rough coppicing may facilitate infection by fungi such as Xylaria spp. and pink disease which may infect and kill weakened stumps (NAS 1983b). In recent surveys, blossom blight (unknown cause) and associated reduced pod formation was observed in Honduras and Guatemala (E.R. Boa and J.M. Lenné, unpublished data). Yet, in both its native range and Indonesia, C. calothyrsus has not suffered any serious diseases to date.

Erythrina

Fungal diseases have been recorded on at least 15 Erythrina spp. throughout the tropics including leaf spots, mildews, moulds, scorches and blights (Lenné 1990), yet no information is available on their importance and no serious diseases have been documented on E. indica or E. poeppigiana. Scab caused by Elsinoe erythrinae causes defoliation in Brazil and rusts caused by Dicheirinia binata, Phakopsora pachyrhizi (soybean rust) and Uredo erythrinae are widely reported on Erythrina spp. in Mexico, Central and South America, and the Caribbean (Lenné 1990, Figueiredo et al. 1983).

Erythrina witches' broom is widespread on E. micropteryx and E. corallodendron in Venezuela (Seliskar 1964) and may affect other species. Virus-like symptoms have been recorded on E. senegalensis in Sierra Leone and referred to as Erythrina mosaic (Seliskar 1964). Vein clearing and banding/bulging between the veins has been observed on E. lithosperma in Sri Lanka associated with cocoa trees affected by cocoa vein banding virus (Seliskar 1964).

Root rot pathogens including Armillaria mellea on E. crista-galli in the USA and E. subumbrans in Tanzania, A. tabescens on Erythrina sp. in the USA, Botryodiplodia theobromae on E. orientalis in India, Fusarium spp. on E. indica in India and E. subumbrans in Malaysia, and Rhizoctonia ramicola on Erythrina sp. in the USA have been noted yet no information exists on their importance (Lenné 1992). Wilt caused by Fusarium solani has been recorded on E. stricta in India and E. subumbrans in Malaysia (Lenné 1992).

Faidherbia albida

Rhizoctonia solani causes leaf web blight of F. albida in India (Mehrotra 1990). Faidherbia albida is a good host of root-knot nematodes, Meloidogyne javanica and M. incognita (Pros 1986).

Gliricidia sepium

Cercosporidium gliricidiasis, chocolate or brown leaf spot, is widely recorded on G. sepium throughout Central and South America, the Caribbean (Lenné 1990), Africa (Lenné and Sumberg 1986), southeast Asia and the Pacific. Recent surveys confirmed its common occurrence in Honduras and Guatemala (E.R. Boa and J.M. Lenné, unpublished data). Under humid conditions, it causes defoliation. Colletotrichum gloeosporioides, expressed as small, dark, rounded leaf spots, is more common than C. gliricidiasis in Nigeria (Lenné and Sumberg 1986). Gliricidia sepium was defoliated by Cladosporium sp. in Costa Rica (Glover and Heuveldop 1985) and the pathogen has also been recorded in Jamaica and Venezuela (Lenné 1990). Scab (Sphaceloma sp.), manifested as brown scab-like lesions on petioles and stems, was found for the first time on G. sepium in Honduras during recent surveys. Its relation to other legume scabs is being determined. Leaf spots caused by other fungi are listed in (Lenné (1990). Surveys have also found leaf scorch/scauld and powdery black leaf spot at several sites. Investigations are in progress to determine the causal agents.

Although widely grown throughout the tropics, G. sepium has apparently remained free of serious diseases. Recent surveys in Central America, however, noted the common occurrence of serious 'little leaf disease' (thought to be caused by a mycoplasma-like organism) in fenceline and natural populations of G. sepium (E.R. Boa and J.M. (Lenné unpublished data). This was associated with extensive dieback and tree death, especially in Honduras. Investigations are in progress to verify the causal agent(s). Twig, stem and branch dieback of G. sepium in Central America, Asia and Africa have been associated with various fungi in the past (Lenné 1990). The same fungi may also be involved in Honduras and Guatemala. Recent surveys showed that pink disease occurs in Papua New Guinea (Lenné 1990).

Virus-like symptoms, including leaf curl, shoe-string leaves, foliar distortion, mosaic and mottle have recently been noted in many natural stands of G. sepium in Central America. Further investigation of these populations is planned. Viruses are potentially very serious to future development of G. sepium especially if they are seed-borne.

Leucaena

Leucaena leucocephala is the most intensively researched, widely used and best known forage tree. Many diseases caused by fungi, bacteria, a possible virus and nematodes have been recorded on Leucaena species throughout the tropics and detailed information is available in a recent review (Lenné 1991). The most important diseases of Leucaena spp. and new information from recent surveys in Central America are summarised in this section.

Foliage diseases

Camptomeris leaf spot (CLS), caused by Camptomeris leucaenae, reduces forage production and quality of L. leucocephala throughout Central and South America, the Caribbean, India, Taiwan and Philippines (Lenné 1991). The pathogen is specific to Leucaena and has been recorded on seven species. CLS is characterised by chlorotic patches on upper surfaces of leaflets and blotches of profuse sporulation in crowded black pustules on lower surfaces. Coalescence of lesions results in chlorosis and defoliation (Lenné 1980a). The most susceptible species are L. leucocephala and L. collinsii (Moreno et al. 1987). The protein content of severely affected leaves was reduced by 18%. Resistance has been identified in accessions of L. lanceolata, L. trichodes, L. diversifolia, L. macrophylla, L. pulverulenta and L. shannoni and may be widely available in these and other species (Lenné 1991).

Blight canker, caused by the fungal complex Calonectria rigidiuscula (anamorph Fusarium decemcellulare) and F. roseum, has been recorded in Taiwan (Chang and Chen 1984, Chang and Tao 1984). Symptoms include wilt, blight, branch death, canker and tree decline. It is sporadically distributed in Taiwan with higher incidence in plantations on sandy soils (Chang and Chen 1984).

Stem diseases

Gummosis, the exudation of gum from the main trunk, stems and branches of Leucaena spp., with associated leaf senescence, dieback and stunted growth, is described as the most serious disease of L. leucocephala in India and Sri Lanka (NAS 1984). Yet, little is known of its pathology and economic importance (Lenné 1991). Putative causal organisms vary among countries and regions within countries and include Phytophthora drechsleri (Van Den Beldt and Hodges 1980) and at least four species of Fusarium (Lenné 1991). Some researchers consider gummosis to be a physiological phenomenon (Anderson 1984, Hegde 1984). To date, gummosis has been recorded on L. leucocephala and L. diversifolia only with Peruvian, Hawaiian and giant types of L. leucocephala being seriously affected in India.

Serious stem canker caused by Pirex subvinosus has resulted in wilting, necrosis and death of L. leucocephala at Kununarra, Western Australia since 1980 (Shivas and Brown 1989). It was first reported as a pathogen of L. leucocephala in India (Sankaran and Sharma 1986). From 1984 onwards, up to 10% of trees have been killed by the fungus in Kununarra (Shivas and Brown 1989). The pathogen is considered a serious threat to cultivation of L. leucocephala in Australia.

Pod diseases

Pod diseases reduce seed production and infect seed (Lenné 1991). Pod rot caused by Fusarium sp. has been recorded in Colombia, Brazil and India on L. collinsii, L. diversifolia, L. macrophylla, L. pulverulenta and L. shannoni with high incidence (Lenné 1991). Pod rot, caused by Pseudomonas fluorescens biotype II, has been recorded on L. leucocephala in Belize, Brazil, Colombia, Mexico and Panama (Lenné 1980b, (Lenné et al. 1981) and recently in Guatemala. Under humid conditions, pods and seed rot rapidly reduce seed production (Lenné 1980b, (Lenné et al. 1981). Leucaena esculenta, L. leucocephala and L. pulverulenta were more susceptible than L. diversifolia and L. shannoni in inoculation studies (Lenné et al. 1981). Seed infection with P. fluorescens can be as high as 95% (Moreno et al. 1987).

A probable Ravenelia sp. rust occurs on pods of L. salvadorensis in Honduras causing blister-like pustules filled with brown spore masses. It has not, however, been observed on other Leucaena species growing at the same site. The rust has also been found associated with overgrowth of a Fusarium sp. which could be involved in a pod rotting complex (C.E. Hughes, personal communication). Although the rust does not appear to affect growth or forage production, it may affect seed production of L. salvadorensis.

Root rots

Root rots caused by Ganoderma and Fusarium spp., Pseudolagarobasidium leguminicola and Pirex subvinosus kill L. leucocephala throughout Asia and Australia (Lenné 1991). Root rot, caused by Ganoderma lucidum, has been reported on L. leucocephala in India (Raina 1980, 1983, Pathak 1986) while G. applanatum and G. tornatum affect L. leucocephala in Papua New Guinea (Straw 1984). Ganoderma lucidum causes wilting, drying of apical meristems, stem blackening, defoliation, root rot and tree death. Seven year-old trees had 10-15% mortality at moist sites (Pathak 1986). In Taiwan, a root and stem rot caused by Pseudolagarobasidium leguminicola has been recorded sporadically in L. leucocephala plantations especially on clay loam soils in the mountains (Chang and Chen 1984, 1985). Symptoms include decreased crown growth, root rot, dieback, girdling of the lower stem and death. Root rot caused by Fusarium oxysporum and F. moniliforme var. subglutinans is an increasing problem in forage and shade plantings in Sri Lanka (Bandara 1987). Root and collar rot, caused by F. solani, has been recorded in India and Mauritius (IMI, unpublished data). In the southwest Pacific, L. leucocephala was severely affected by brown root rot caused by Phellinus noxius (Ivory 1990).

Prosopis

A large number of fungi have been recorded on P. juliflora but no information is available on their importance. These include leaf spots caused by Pleospora scirrhioides widely reported from India, Pleospora sp. from Pakistan and Septoria prosopodis from India, rusts caused by Ravenelia spp. from Mexico and USA, and dieback caused by Diplodia dalbergiae, D. prosopidina, Nectria flavistroma and N. purtonii from Pakistan and India (Browne 1968, IMI, unpublished data).

Pod spot and seed blight of P. juliflora caused by Macrophomina phaseolina is described as a new disease of mesquite from northeast Brazil (Muchovej et al. 1989). Affected pods showed brown discoloration while infected seeds did not germinate. Prosopis juliflora is an excellent host of root-knot nematodes Meloidogyne javanica and M. incognita in inoculation studies (Pros 1986). Weak reproduction of Scutellonema cavenessi also occurred on P. juliflora (Pros 1986).

Sesbania

Pathogens affecting common Sesbania spp. worldwide have been reviewed (Evans and Rotar 1987, Murphy 1990). Severe disease problems have been reported occasionally (Evans 1986).

Seedling and foliage diseases

Colletotrichum capsici collar and seedling blight has been recorded on S. grandiflora in India (Srinivasan 1952). Severe leaf spot and defoliation caused by Pseudocercospora sesbaniae occurs widely on S. grandiflora in India (Kumar and Joshi 1983), Myanmar and the Philippines (IMI, unpublished data) and on S. sesban in India (Josh) and Kumar 1986). Disease incidence was 47.2% on one cultivar. The rusts Uromyces poonensis on S. grandiflora in India (Josh) and Kumar 1986) and Uredo sesbaniae on S. sesban in India have been reported (Evans and Rotar 1987). Protomycopsis thirumalacharii causes angular black leaf spot of S. grandiflora in India (Pavgi 1965, Haware and Pavgi 1976) while Erysiphe polygon) (powdery mildew) has been recorded on S. sesban in India (Evans and Rotar 1987).

Xanthomonas campestris pv. sesbaniae causes leaf and stem spots and defoliation of S. sesban in India (Bradbury 1986). Sesbania mosaic virus caused decreased nitrate reductase activity (Srivastava 1986) and interfered with nodulation (Rao and Shukla 1988) of Sesbania spp. in India (Sreenivasulu and Nayudu 1982). An unnamed, sap-transmitted virus caused mild mosaic and mottling on leaves and defoliation of Sesbania spp. (Singh and Srivastava 1985).

Root diseases

Sclerotium rolfsii caused wilt of S. sesban in a forage experiment under dry conditions in Hawaii (Evans 1986, Evans and Rotar 1987). Wilt occurred after cutting with cumulative incidence up to 50%. It has also been recorded on S. sesban in Malaysia (Turner 1971). Fusarium oxysporum f. sp. sesbaniae causes root rot and wilt of S. sesban in India (IMI, unpublished data).

Sesbania grandiflora is very susceptible to nematodes (NAS 1980). Root-knot nematode Meloidogyne incognita is potentially destructive to Sesbania spp. in India (Trivedi et al. 1986) and M. javanica has also been recorded on Sesbania spp. in Pakistan (Munir et al. 1986). The cyst nematode Heterodera trifolii occurs on S. grandiflora in Hawaii (Holtzmann and Aragaki 1963).

Potential Control Strategies

Because almost no information exists on actual losses caused by diseases of forage tree legumes, the economic importance of most diseases is unknown and potential control strategies only can be discussed.

Seed and seedling diseases affect many tree legumes and have been well documented for Acacia. Pathogens are capable of destroying seedlings before and after emergence (Gibson 1975). Damping-off is the most important seedling disease. It is caused by a wide range of fungi and can rapidly cause appreciable losses (Gibson 1975). Chemicals are commonly, successfully and economically used to control seed and seedling diseases of tree legumes in nurseries (Ibnu and Supriana 1987, Mohanan and Sharma 1988, Chalermpongse 1990, Pongpanich 1990, Zakaria 1990, Lenné 1991). If the pathogen is seed-borne, control by seed dressing can be effective. Chemical treatment reduced fungal and bacterial infection of seed from infected pods of Leucaena (tonne 1991). Removal of seed debris (Yuan et al. 1990), soil drenches and heat or steam sterilisation of the seed bed prior to sowing are also useful measures (Gibson 1975). Cultural control by reducing the density of seedlings, restricting watering and modifying the seedling environment so that it is less suitable for disease development are valuable control practices (Gibson 1975, Sharma and Sankaran 1987). Yet, diseases of seedlings and young trees sown in extensive plantings for forage production will be difficult and costly to control by normal nursery practices. Seed treatments, especially with systemic fungicides which may be incorporated in pellets with rhizobium, may be the major viable option in the field.

Tree pathologists have generally minimised the importance of foliar diseases because they rarely affect timber production, usually the most valuable forest product. However, if the tree legume is grown as a source of forage, effects of foliage diseases on forage production and quality become more significant. Limited work has been done on control of foliage diseases of tree legumes. Potential control strategies include cultural control through cutting and grazing, selection of resistant germplasm and manipulation of natural biological controls. Camptomeris leaf spot of Leucaena was reduced by periodic 8-weekly cutting of forage in Colombia (Moreno et al. 1987). Management of the regrowth of tree legumes primarily used for forage is a widely applicable disease control measure. Natural biocontrol fungi have been recorded on leaf blotches (Lenné 1991) and rusts (Khan et al. 1989) and could be manipulated for disease management. Resistance is considered the best long-term strategy for control of foliage diseases of tree legumes and has been achieved for some diseases (Gibson 1975, Lenné 1991).

Avoidance and eradication have been used for common stem and root diseases. Application of fungicides directly to cankers successfully reduced gummosis of leucaena; however, the economics of this practice must be compared with the value of wood and forage. When branches are cut for forage, care should be taken to avoid injuries, which are potential sites for fungal invasion (Sharma and Sankaran 1987, Lee et al. 1989). Clear site selection is recommended to avoid soil-borne root rots (Gibson 1975, Bakshi 1976). Other measures including trenching to isolate infection centres and treatment of roots with fungicides (Bakshi 1976) are expensive, time consuming and often ineffective (Gibson 1975, Ivory, 1990). Soil solarisation was as effective as fungicides in controlling Fusarium root rot of Leucaena in Sri Lanka (Bandara 1987) and may have wider application.

Except for seedling diseases, the state of knowledge of strategies for controlling diseases of tree legumes is inadequate. Yet, a greater range of control strategies is potentially available for tree legumes in many systems. Mixtures of trees and crops in agroforestry systems may promote reduction in diseases by physical barrier effects and genetic diversity for resistance (Beets 1984). Control strategies should always be related to the systems in which the trees are grown and the purpose for which they are to be used. Labour intensive strategies may be feasible at the village level.

Future Research Needs

The extent of knowledge of diseases of tree legumes in the tropics is generally poor. Knowledge of diseases is mainly restricted to records in host lists with limited information on importance and pathogenic variability, and even doubts regarding the true status of the 'pathogen'. Standard texts including Browne (1968) and Gibson (1975) are urgently in need of revision. Conflicting reports of the importance of diseases have been made from the same country. There are many important forage species for which little or no pathological information is presently available. Limited work has been done to assess losses caused by known diseases and the economics of control.

The narrow global genetic base of many tree legumes such as L. leucocephala presents few problems to any rapidly disseminated disease. Widespread damage to L. leucocephala caused by the leucaena psyllid emphasises the importance of diversifying the narrow genetic bases of tree legumes and the need for greater awareness and knowledge of their pests and diseases. In recent years, significant variation has been collected in Acacia, Calliandra, Leucaena and Gliricidia by CSIRO, on, CGIAR centres and the University of Hawaii. This variability is now being evaluated in trial networks. These networks represent an invaluable opportunity to widely survey diseases and pests of tree legumes. Present and future efforts to increase the diversity utilised in key genera should facilitate selection for resistance among superior provenances. Close international and institutional collaboration is needed.

Damaging and potentially damaging diseases have been identified on some important tree legumes. As the use of tree legumes for forage continues to expand in the tropics, disease problems will continue to increase both in occurrence and severity. Attempts should be made now to develop country and region specific information bases through coordinated surveys, efficiently using the existing trial networks, and to develop strategies to control diseases quickly.

References

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