Citrus greening bacterium and its vectors/et ses vecteurs Diaphorina citri & Trioza erytreae

Bulletin OEPP/EPPO Bulletin 18, 497-507 (1988)

FICHES INFORMATIVES O E P P SUR LES ORGANISMES D E QUARANTAINE E P P O DATA SHEETS O N QUARANTINE ORGANISMS

List A2/Liste A2 No. 151

and its vectors/et ses vecteurs Diaphorina citri & Trioza erytreae

(incorporating/incorporant nos 37 and 46) Citrus greening bacterium

Homoptera: Psyllidac

Note

Citrus greening bacterium is transmitted by the psyllids Diaphorina citri (Kuway.), and Trioza erytreue (Del G.). These insects have already been placed on the EPPO A1 list of quarantine organisms (as nos 37 and 46 respectively). However, since they probably present at least as large a risk as vectors of greening as they do as quarantine pests in their own right, it was decided to cover the vectors in the same data sheet as the pathogen.

Diseases

Greening has received different names according to the countries affected but in each case the same type of Gram-negative bacterium is involved, even though strain differences may exist (cf. section on Biology for heat-tolerant (Asian) and heat-sensitive (African) forms of the disease and serological relationships). The disease is known under the following names: greening (Africa), huang long bin, i.e. yellow shoot disease (China), likubin, i.e. leaf mottle disease (Taiwan), leaf mottling (Philip- pines), citrus decline (India), vein phloem degeneration (Indonesia).

Principal hosts

Rutuceous hosts: citrus generally. The bacterium may persist and multiply in most Citrus spp. but most severe symptoms are found on sweet orange (C. sinensis), mandarin (C. reticulata) and tangelo (C. reticulata x C. paradisi), somewhat less severe symptoms on lemon ( C . limon), grapefruit (C. paradisi), Rangpur lime (C. limoniu), Palestine sweet lime ( C . limettioides), rough lemon (C. jambhiri), kumquat (Fortunella spp.) and citron (C. medica) (McClean & Schwarz, 1970). Symptoms

Note

La bacttrie responsable du greening des agrumes est transmise par les psylles Diaphorina citri (Kuway.) et Trioza erytrene (Del G.), qui figurent deja dans la liste A1 d’organismes de quarantaine de l’OEPP (no 37 et 46, respectivement). Leur importance en tant que vecteurs potentiels du greening est certainement au moins egale ii leur potentiel d’occasionner des dkgdts directs et il a ete decide en consequence de les presenter a nouveau dans la m&me fiche informative que le pathogene.

Maladies

Selon des pays affectes, le greening a r e p des noms differents mais dans tous les cas la m&me bacttrie de type Gram-negatif est impliquee. Toutefois des differences de souches peuvent exister (voir la section sur la Biologie pour les formes de la maladie toltrantes (asiatiques) et sensibles (africaines) a la chaleur, ainsi que pour leurs relations serologi- ques). La maladie est connue sous les noms sui- vants: greening (Afrique), huang long bin ou maladie des pousses jaunes (Chine), likubin ou maladie des feuilles marbrees (Taiwan), leaf mottling (Philippines), citrus decline (Inde), vein phloem degeneration (Indonesie).

Plantes- h6tes principales

Rutaceae: agrumes en general. Le bacterie peut survivre et se multiplier dans la majorit6 des Citrus spp. Si les symptbmes les plus stveres sont connus sur oranger (C. sinensis), mandarinier (C. reticu- lala), tangelo (C. reticulatu x C. paradisi), ils le sont moins sur pamplemousse (C. paradisi), citron- nier (C. limon), lime Rangpur (C. limona), lime douce de Palestine (C. limettioide..r), rough lemon (C. jambhiri), kumquat (FortuneNa spp.) et cedrat (C. medica) (McClean & Schwarz, 1970), et faibles

497

498 EPPO Data Sheets

are even weaker on lime (C. aurantiifolia) and pomelo (C. grandis).

The vectors are confined to Rutaceae, occurring on wild hosts (Clausenu anisata, Vepris undulata) as well as on citrus (especially lemon and lime). Murraya paniculata, a rutaceous plant often used for hedges, is a preferred host for D. citri. Non-rutaceous hosts: experimental transmission of citrus greening bacterium from citrus to periwinkle (Catharanthus roseus) by dodder (Cuscuta campestris) has been achieved (Garnier & Bovt, 1983).

Geographical distribution

The African (heat-sensitive) form of the disease has been reported from Comoro Islands, Ethiopia, Kenya, Madagascar, South Africa, Swaziland, Zimbabwe and recently Yemen Arab Republic (Bovt & Gamier, 1984). The Asian (heat-tolerant) form is present in China, India, Indonesia, Malay- sia, Nepal, Pakistan, Philippines, Taiwan, Thai- land, as well as Saudi Arabia (Bove & Gamier, 1984). Both forms of the disease exist in Reunion and Mauritius and probably south-west Saudi Arabia (Bovt & Gamier, 1984). The Mediterra- nean area and most of the Middle East (e.g. Iran) are still free from the disease. The infested area which is closest to the Mediterranean zone extends south of Mecca, along the Red Sea.

D. citri occurs throughout the Asian zone of greening (and also in Brazil and Japan), while T. erytreae occurs throughout the African zone, in Yemen Arab Republic (and also in Cameroon, St Helena). Both occur together in Mauritius, Reunion and Saudi Arabia.

Biology

Citrus greening bacterium

Transmission of the greening agent by graft inoculation was first reported in China (Lin, 1956). Lafleche & Bovk (1970) found that the disease was characterized by the presence of an apparently mycoplasma-like microorganism, not a virus, in the sieve tubes of affected plants. However, when soon thereafter the agent of citrus stubborn disease was discovered (Igwegbe & Calavan, 1970) and found to be a mycoplasma (Fudl-Allah & Calavan, 1972; Saglio et al., 1971a) and more precisely a spiroplasma (Saglio et al., 1973), comparison between

sur lime antillaise (mexicaine) (C. aurantiifolia) et pomelo (C. grandis).

Les insectes vecteurs sont infeodts aux Rutaceae et s’attaquent des especes spontanees (Clausenu unisata, Vepris undulata) aussi bien qu’aux agrumes (citron et lime surtout). Murraya paniculata, une rutacee souvent utiliste pour faire des haies, est un hbte de predilection pour D. citri. Autres h6tes: la transmission expCrimentale de la bacterie du greening de citrus a pervenche (Catha- ranthus roseus) a ete realisee au moyen de la cuscute (Cuscuta campestris) (Gamier & Bove, 1983).

Distribution geographique

La forme africaine (thermosensible) de la maladie a ete signalee en Afrique du Sud, Ethiopie, Iles Comores, Kenya, Madagascar, Swaziland, Zim- babwe et rtcemment en Republique arabe du Yemen (Bovt & Gamier, 1984). La forme asiatique (thermotolerante) est prksente en Chine, Inde, Indonksie, Malaysie, Nepal, Pakistan, Philippines, Taiwan, Thailande, ainsi qu’en Arabie Saoudite (Bove & Gamier, 1984). Les deux formes de la maladie existent a 1’Ile de la Reunion et a 1’Ile Maurice ainsi que probablement au sud-ouest de 1’Arabie Saoudite (Bovt & Garnier, 1984). A l’heure actuelle, la maladie n’est pas presente dans la region mtditerraneene, ni dans la plupart du Moyen-Orient, par ex. Iran. La region contaminee la plus proche de la zone mediterraneenne est la cbte de la mer Rouge au sud de la Mecque. D. citri est largement repandu en Asie partout ou

le greening est present (ainsi qu’au Japon et au Bred). T. erytreae est, de m&me, largement disst- mine dans la zone du greening en Afrique, et est present aussi en Republique arabe du Yemen, au Cameroun et a Ste Helene. Les deux especes se retrouvent ensemble a l’Ile Maurice, a la Reunion et en Arabie Saoudite.

Biologie

Bacterie du greening des agrumes

La transmission de l’agent du greening par greffage d’inoculation a Btt dkrite pour la premiere fois par Lin (1956) en Chine. En 1970, Lafleche & Bove (1970) trouvent que la maladie est caracthisee par la presence, non pas d’un virus, mais d’un micro- organisme apparemment de type mycoplasme dans les tubes cribles des plantes malades. Cependant, avec la dtcouverte, peu apres, de l’agent de la maladie du stubborn des agrumes (Igwegbe & Calavan, 1970) et du fait qu’il s’agissait d’une veritable mycoplasme (Fudl-Allah & Calavan,

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the greening agent and Spiroplasma citri showed the greening agent to be different from a mycoplasma (Saglio et al., 1971b). Since then, considerable work has confirmed this view and shown the greening agent to be a bacterium with a peptidogly- can-containing membranous cell wall of the Gram- negative type (Moll & Martin, 1974; Garnier et al., 1976; Gamier & Bove, 1977; Garnier et al., 1984a, b). The bacterial nature of the greening organism explains why penicillin treatment of infected plants results in symptom remission (Bove et al., 1980; Aubert & Bove, 1980). A report claiming culture of citrus greening bacterium has appeared (Garnett, 1985). There is however at this time no experimental evidence to show that the cultured organism is really the greening bacterium.

Two forms of greening disease are known (BovC et al., 1974). One, in the southern part of Africa, is heat-sensitive, as symptoms do not develop in hot climates where temperatures above 30 “C are reached several hours a day. The other form is heat- tolerant and withstands high temperatures. It is present in China, India, Nepal and south-east Asia. The Asian heat-tolerant form of the disease has been discovered in Saudi Arabia, and the African heat-sensitive form in Yemen Arab Republic (Bove &Gamier, 1984). When the African and the Asian forms of the greening bacterium were transmitted from citrus to periwinkle by dodder (Gamier & Bove, 1983), the African form remained heat- sensitive and the Asian form heat-tolerant in periwinkle as in citrus (Gamier, pers. comm.).

Monoclonal antibodies (MA) to citrus greening bacterium and more specifically to the Indian form, have recently been obtained for the first time (Gamier et a/., 1987). By immunofluorescence on sections, the MA reacted not only with the homolo- gous (Indian) form of citrus greening bacterium, but also with the organisms associated with Afri- can, Philippines or Reunion greening. There are thus serological relationships between African and Asian forms of citrus greening bacterium, but it is possible that further work will discover strain differences.

Under natural conditions greening is transmitted in Africa and Yemen Arab Republic by T. erytreae (McClean & Oberholzer, 1965) and in Asia (includ- ing Saudi Arabia) by D. citri (Capoor et al., 1967). However, it was shown experimentally that T. erytreae can transmit the Indian form of greening (Massonie et al., 1976) and that inversely, D. citri is able to transmit the African form of greening (Lallemand el al., 1986).

1972; Saglio et al., 1971a) et plus precistment d’un spiroplasme (Saglio et al., 1973), il devenait possible de comparer I’agent du greening avec Spiro- plasma citri et de constater que l’agent du greening ne pouvait pas etre un mycoplasme (Saglio et al., 1971b). Depuis, tout un ensemble de travaux a confirm6 cette conclusion et a montre que l’agent du greening ttait une bacttrie i paroi membranaire du type Gram ntgatif et possedant un peptidogly- cane (Moll & Martin, 1974; Garnier et al., 1976; Garnier & Bovt, 1977; Garnier et al., 1984a, b). La nature bacttrienne de l’organisme du greening explique pourquoi le traitement des plantes infecttes par la ptnicilline se traduit par une rtmission des sympthmes (Bovt et al., 1980; Aubert & Bove, 1980). Une publication concernant la culture de l’organisme du greening est apparue recemment (Garnett, 1985). Cependant a l’heure actuelle il n’a pas ete dtmontre que I’organisme cultive etait reellement la bacttrie du greening.

Le greening est connu sous deux formes (Bove et al., 1974). L’une, en Afrique australe, est sensible a la chaleur, les symptbmes ne se developpant pas lorsque la temperature depasse 30 “C pendant plusieurs heures de la journte. L‘autre forme est tolerante a la chaleur et rtsiste aux temperatures &levees; elle est presente en Inde, au Nepal, en Chine et dans 1’Asie du Sud-Est. La forme asiatique, toltrante a la chaleur, a ete decouverte en Arabie Saoudite et la forme africaine, sensible a la chaleur, en Republique arabe du Yemen (Bove & Gamier, 1984). Les formes africaines et asiatiques de la bacterie du greening ont etk transmises par la cuscute de citrus a pervenche (Gamier & Bod, 1983). Dans les pervenches comme dans les citrus, la forme africaine est sensible a la chaleur et la forme asiatique est resistante a la chaleur (Gamier, comm. pers.).

Des anticorps monoclonaux (AM) specifiques du greening et plus prtcisement du greening de I’lnde ont t t t obtenus rtcemment pour la premiere fois (Garnier et al., 1987). En immunofluorescence sur coupe, ces AM reagissent non seulement avec la forme homologue (Inde) de la bacttrie du greening, mais aussi avec les organismes associts avec le greening d’Afrique, des Philippines et de la Reu- nion. Bien que ces experiences montrent qu’il existe des relations strologiques entre les formes africaines et asiatiques de la bacttrie du greening, il est vraisemblable que les travaux ulterieurs mettront en tvidence des differences entre diverses souches.

Dans les conditions naturelles le greening est transmis en Afrique et au Yemen par T. erytreae (McClean & Oberholzer, 1965), et en Asie (y compris 1’Arabie Saoudite) par D. citri (Capoor et

500 EFFO Data Sheets

Trioza erytreae

The vector of the African form, T. erytreue, behaves like it with respect to temperature sensiti- vity (Catling, 1973a; Schwarz & Green, 1970). It is very sensitive to extremes of hot, dry weather (the eggs and first iiistar nymphs being particularly vulnerable). It is favoured in cool, moist areas over 500-600 m, where citrus growth flushes tend to be prolonged. Green & Catling (1971) have used maximum saturation deficit as an accurate predic- tor of the geographical distribution of T. erytreae. Sex ratios fluctuate in the field, but females always predominate. There is a pre-oviposition period o f 3-7 days, but this is considerably extended in the absence o f young foliage; longevity is also prolonged under such conditions. Mating occurs 2-4 times a day and eggs may be laid immediately. Eggs are supplied with a sharp point which is driven through the leaf epidermis and is thought to be responsible for maintaining a favourable internal water relationship. Females remain fertile for 11- 16 days in the absence of males, and maximum egg production occurs towards the middle of their life span, which normally lasts 17-50 days; up to 2000 eggs may be laid per female. There is an incubation period of 6-15 days and nymphal development ( 5 instars) takes 17-43 days, both periods being inversely related to mean temperature and directly related to nutritional value of the leaves. The temperature threshold for nymphal development is, apparently, around 10-12 ”C. There is no diapause.

Diaphorina citri

D. citri (Catling, 1970; Schwarz & Knorr, 1973) has a short life cycle and high fecundity. It is more prevalent in hot coastal areas. Pairing starts soon after emergence, the insects being most active during March to April in India (Pande, 1971) and May to June in the Philippines (Catling, 1970). Eggs are laid singly from March to May inside half- folded leaves of the buds, in leaf axils or other suitable places on the young tender parts of the tree. Females have a pre-oviposition period of about 12 days, and are capable of laying up to 800 eggs within 2 months. Eggs hatch within 3-23 days

al., 1967). Cependant il a 6tt montre experimentale- ment que le vecteur africain pouvait transmettre la forme asiatique (Inde) du greening (Massonie et al., 1976) et que, inversement, le vecteur asiatique etait capable de transmettre la forme africaine du greening (Lallemand et al., 1986).

Trioza erytreae

Le vecteur de la forme africaine, T. erytreae, manifeste une thermosensibilite semblable ;i celle de la bacterie (Catling, 1973a; Schwarz & Green, 1970). II est trts sensibile aux conditions chaudes et seches (en particulier, les oeufs et les nymphes du ler stade) mais, par contre, est favorist dans les zones 1 altitude superieure B 500-600 m, oh les conditions sont plus fraiches et humides, et les periodes de pousse des agrumes sont plus longues. Green & Catling (1971) se sont servis du deficit maximum de saturation comme indice de prevision de la distribution geographique de T . erytreae. La proportion des sexes en verger est variable, mais les femelles sont toujours preponderantes. La periode de prtponte dure 3-7 j , mais peut Etre beaucoup plus longue en l’absence de jeune feuillage. Dans ce cds, la longevite est augmentee d’autant. Les insectes s’accouplent 2-4 fois par jour et les oeufs sont pondus immidiatement. Les oeufs potent une pointe aigue que ptnetre l’epiderme de la feuille et maintiendrdit un iquilibre hydrique favorable. Les femelles restent fertiles 11-16 j en I’absence de males. La ponte maxiinale se situe vers le milieu de leur vie, qui dure normalement de 17 d 50 j; chaque femelle peut pondre jusqu’a 2000 oeufs. Les periodes d’incubation, de 6 a 15 j, et de developpement larvaire ( 5 stades), de 17 a 43 j, varient en fonction inverse de la temperature moyenne, et en fonction directe de la valeur nutritive du feuillage. Le seuil de temperature pour le developpement des larves est de 10-12 ’C. 11 n’y a pas de diapause.

Diaphorina c h i

D. citri (Catling, 1970; Schwarz & Knorr, 1973) a un cycle biologique court et une feconditt elevee. L‘espece est surtout presente dans les zones c8tieres chaudes. L’accouplement commence peu apres l’emergence et les insectes sont plus actifs en mars- avril en Inde (Pande, 1971) et en mai-juin aux Philippines (Catling, 1970). Les oeufs sont pondus individuellement de mars ri mai a l’interieur des jeunes feuilles encore enroultes des bourgeons, dans les aisselles des feuilles ou a d’autres sites sur les parties jeunes et tendres de l’arbre. La phiode de preponte est proche de 12 j, et les femelles pondent

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(in summer or winter respectively) and nymphs pass through 5 instars in 11-30 days. In an insectary, at 25-26”C, females laid 8 eggs per day, and nymphal development took 11-1 5 days. In dry periods, adults are numerous, but nymphs usually absent. The complete life cycle thus takes 14-48 days, there being up to 10 overlapping generations per year.

The adults overwinter and can live for up to 6 months. They are very active and jump on the slightest disturbance. Nymphs will move away when disturbed but normally lead a sedentary existence clustered in groups. Population fluctuations are closely correlated with flushing rhythm of citrus trees, since eggs are laid exclusively on young flush points.

Economic importance

Citrus greening is an extremely severe disease. In South Africa, in 1965, fruit losses from the disease were 30-100% in individual orchards; many of these had subsequently to be abandoned or removed. Earlier outbreaks occurred in 1932- 1936 and 1939-1946. Current annual losses are esti- mated at ZAR 35 million (Van den Berg et ul., 1987). In Reunion, large areas of citrus cultivation had to be abandoned (Catling, 1973b) and also in Thailand (Schwarz & Knorr, 1973). In the Philip- pines, mandarin production fell from 11,700 t in 1960 to 100 t i n 1968 (Commonwealth Department of Health, 1982). In south-western Saudi Arabia, sweet orange and mandarin practically disap- peared over the 1970s (Bove, 1986). In Asia, an FAO-UNDP programme has recently been estab- lished to try to control the disease.

Heavy infestations with T. erytreue cause severe leaf distortion, but not the defoliation or dieback typical of D . citri, which can also cause serious damage to growing points, leading to dwarfing as well as lack ofjuice and taste in fruit. Heavy D. citri populations can cause blossom and fruitlet drop.

In the Transvaal (ZA), citrus is treated against greening by injection of tetracycline (up to 20 g per adult tree) with high-capacity compressors work- ing at 10 kg cm-2. Such methods have been tried out, but not widely used, in Asia. Dimethoate can be used against the vectors on orchards with low infection rates, while the highly toxic aldicarb is used in South Africa as a soil-applied biocide (Bove, 1986). In Reunion, the vectors have been successfully controlled by introduction of parasites (Tetrastichus dryi Waterston, from South Africa, for T. erytreae; T. rudiutus Waterston, from India, for D. citri) (Aubert et al., 1980). In Saudi Arabia

jusqu’i 800 oeufs en 2 mois. Les oeufs eclosent en 3-23 j (en ete ou en hiver, respectivement) et les larves passent par 5 stades en 11-30 j. En insecta- rium, a 25-26 “C, des femelles ont pondu 8 oeufs par jour et le dtveloppement larvaire s’est ttendu sur 11-15 j. Dans des periodes seches, les adultes sont abondants, mais les larves manquent. Le cycle complet dure de 14 a 48 j et il peut y avoir jusqu’d 10 generations se chevauchant par an.

Les adultes hivernent et peuvent survivre pendant 6 mois. 11s sont tres actifs et sautent a la moindre provocation. Les larves s’eloignent si on les perturbe, mais restent generalement immobiles en petits groupes. Les fluctuations de population sont ttroitement likes au rythme de pousse des agrumes, puisque la ponte est limitee aux jeunes organes rtcemment formes.

Importance economique

Le greening des agrumes est une maladie extrime- ment severe. En Afrique du Sud, en 1965, les pertes de fruits occasionnees par la maladie etaient de 30 a 100% selon les vergers. De nombreux vergers ont dij Etre abandonnes ou arraches. Des attaques equivalentes ont i t& observkes en 1932-36 et 1939- 46. Les pertes annuelles actuelles sont estimees a ZAR 35 millions (Van den Berg et al., 1987). A la Reunion, de grandes surfaces agrumicoles ont dij Ctre abandonntes (Catling, 1973b) ainsi qu’en Thallande (Schwarz & Knorr, 1973). Aux Philip- pines, la production de mandarines a chute de 11,700 t en 1960 a 100 t en 1968 (Commonwealth Department of Health, 1982). Au sud-ouest de 1’Arabie Saoudite, la culture de l’oranger et du mandarinier a pratiquement dispdru au cours des annees 1970 (Bovt, 1986). En Asie, un programme FAO-PNUD a ett: recemment mis en place pour essayer d’enrayer la maladie.

Les feuilles fortement attaqutes par T. erytreae sont deformees, mais ne subissent pas la chute et le deperissement caracteristique des attaques de D. citri, qui endommage aussi les pointes des pousses et provoque un rabougrissement accompagnk d’al- teration du gotit des fruits. Une forte infestation de D. critri provoque la chute des fleurs et des jeunes fruits.

Au Transvaal (ZA), on traite les agrumes contre le greening par injection de tetracycline (jusqu’ii 20 g par arbre adulte) a l’aide de compresseurs a haute capacite, fonctionnant a 10 kg cm-*. Ces traitements ont tte experimentes en Asie, mais n’y ont pas encore ete largement utilists. I1 est possible de traiter au dimtthoate contre les vecteurs dans des vergers ayant un faible taux d’infection. En Afrique du Sud, I’aldicarbe, produit a haute toxicite, est


(Bovt, 1986), however, T. radiatus is present but does not keep D . citri populations down to a low level.

In the citrus-growing areas of the EPPO region, citrus greening bacterium could only present a problem if introduced with one or other of its vectors. There is no suggestion that native Mediter- ranean vector could exist. T. erytreae and especially D. citri could probably establish and spread in Mediterranean countries without difficulty. Besides, both these psyllids have significant damage potential in themselves. Though biological control may be possible, there is no guarantee that it could keep populations to a sufficiently low level to prevent transmission of greening. In view of its severity it is essential to keep the disease (and its vectors) out of the Mediterranean zone and to prevent their spread in the Middle East.

Means of entry

This can be on citrus planting material. However, since the two known psyllid vectors of the disease are not present in the Mediterranean area, parts of the Middle East, and America, introduction of infected plant material into the above areas should not in theory result in spread of the disease, by means other than propagation of the infected plant material. This is precisely the situation encountered in the Hergeissa area of Somalia (Bove & Garnier, pers. comm.). However, hitherto unrecognized vectors of greening could reveal themselves. Also, citrus material (budwood, grafted trees, rootstock seedlings) from infected areas can carry eggs and/or nymphs of the psyllid vectors. Such 5th or 6th instar nymphs, as well as the adults born from these nymphs, are capable of transmitting the greening agent to citrus. This is probably the way by which the Asian form of citrus greening bacterium was introduced into Saudi Arabia.

The situation in Brazil and possibly other South American countries, is different. D. citri is present in Brazil, but greening itself has never been reported from North, Central or South America. Hence introduction of infected plant material, even without the vectors, would be most dangerous.

The rutaceous plant Murraya paniculata, fre- quently used as an ornamental bush or hedge, is one of the best hosts of D . citri. This plant can carry eggs or nymphs of the vector and therefore its

applique au sol (Bove, 1986). A la Reunion, il a it6 possible de limiter des populations des vecteurs par l’introduction de parasites (Tetrastichus dryi Waterston, d’Afrique du Sud, contre D. erytreae; T . radiatus Waterston, d’lnde, contre D . citri) (Aubert et ul., 1980). En Arabie Saoudite (Bove, 1986), T. radiatus est present mais ne limite pas les populations de D . cilri.

Dans les zones agrumicoles de I’OEPP, la bacterie du greening des agrumes ne presenterait un probltme que si elk etait introduite avec l’un de ses vecteurs. I1 n’y a aucune indication qu’une vecteur indigene miditerraneen puisse exister. T. erytreae et specialement D. citri devraient pouvoir s’etablir et se disstminer dans les pays mtditerraneens sans difficulte. En outre, les deux psylles sont eux-memes de dangereux ravageurs. M&me si la lutte biologique se rkvtle possible, il n’est pas certain qu’elle puisse reduire les populations au niveau ou la transmission du greening deviendrait impossible. En vue de sa severit&, il est essentiel d’eviter la penetration de la maladie et de ses insectes vecteurs dans le bassin Mediterranken et leur extension au Moyen-Orient.

Voies d‘introduction

L‘introduction se ferait par le materiel vegetal destine a la plantation. Puisque les deux psylles vecteurs de la maladie ne sont pas presents dans 1es regions de la Mtditerranee, de certaines parties du Moyen-Orient et de l’Amirique, l’introduction de materiel vegetal infecti: ne devrait pas en theorie se traduire par une dissemination de la maladie autre que par la multiplication du materiel vegetal infect&. C’est preciskment la situation rencontree dans la rtgion de Hergeissa en Somalie (Bovk & Garnier, comm. pers.). Cependant, des vecteurs nouveaux, non encore soupqonnes, pourraient se reveler. En outre, le materiel vegetal fgreffons, arbres greffes, porte-greffe) en provenance de regions infecttes peuvent vthiculer des oeufs et/ou des nymphes des psylles vecteurs, Les nymphes des stades larvaires 5 et 6 ainsi que les adultes nes de ces nymphes sont capables de transmettre l’agent du greening aux agrumes. C‘est vraisemblablement ainsi que la forme asiatique de la bactirie du greening des agrumes a ete introduite en Arabie Saoudite.

La situation au Bresil, et sans doute dans d’autres pays d’Amtrique du Sud, est differente. D. citri est present au Bresil mais la maladie du grecning elk-mtme n’a jamais t t t signal&, ni au Bred, ni ailleurs en Amtrique. Dts lors l’introduction de materiel vegetal infecti, mCme dipourvu de toutes formes de vecteur, serait trts dangereuse.

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introduction into disease and vector-free regions could be dangerous.

Entry on fruit is extremely unlikely. Seed transmission does not occur.

Identification

Symptoms

General aspect of affected trees. Open growth, stunting, twig dieback, sparse yellow foliage; severe fruit drop; on certain trees and certain countries (China), symptoms are seen initially on one limb of an affected tree (yellow branch aspect); severe decline mainly with Asian greening.

Symptoms on fruits. Some fruits are under- developed, lopsided, and poorly coloured. When pressure is exerted with a finger, a greyish white waxy mark appears sometimes on the rind surface. Seeds are often aborted. The greening symptom, mainly seen in Africa, is seen on fruit which mature only on the side exposed to the sun, the unexposed side remaining dull olive green (Commonwealth Department of Health, 1982).

Symptoms on leaves. Mottling and zinc defi- ciency-like symptoms are the most common and characteristic. Mature leaves often show irregular patches between the main veins. The veins are often prominent and yellow.

Symptoms on trunk, limbs and shoots. No symptoms apparent.

Histological symptoms. Localized zones of nec- rotic phloem are scattered through the vascular system of the leaf. Massive accumulation of starch in the plastids is seen together with aberrations in cambial activity and excessive phloem formation.

Symptoms due to iiectors. T. erytreue severely distorts leaves, which are stunted and galled, and appear dusted with fecal pellets. D. citri stunts and twists young shoots, so that the growing tips present a rosetted appearance, Leaves are badly curled, and may be covered with honeydew and sooty mould, dropping prematurely.

Indexing

Suspect material may be grafted onto sensitive

La rutacee Murraya paniculata, tres utilisee comme arbuste ornemental seul ou sous forme de haies, est l’un des hbtes priviltgits de D. citri. Cette plante peut vehiculer des oeufs et des nymphes et son introduction inconsidCree dans les regions indemnes pourrait ttre dangereuse.

Enfin, il est tres peu problable qu’une introduction se fasse sur fruits. I1 m’y a pas transmission par les semences.

Identification

S ymptbrnes

Aspect giniral des arbres atteints. Feuillage jaune, peu dense; rabougrissement; dessechement des brindilles, chute importante des fruits. Dans certains cas et dans certains pays (Chine), les symp- t8mes apparaissent dabord sur une seule branche (aspect yellow branch). Dtclin severe, principale- ment avec le greening asiatique.

SymptBmes sur fruits. Certain fruits restent petits, ont une columelle incurvte et sont ma1 colores. Quand on exerce une pression sur le fruit avec le doigt, une marque argentee apparait la surface de la peau. Les pepins sont souvent avortes. Le symptbme de ‘greening’ (ou verdissement) est surtout connu en Afrique, sur fruits que mfirissent seulement du c8te expose au soleil, l’autre face restant d’une vert olivitre (Commonwealth Department of Health, 1982).

Symptcimes foliaires. Les symptbmes les plus caracteristiques sont des marbrures et une dtcolo- ration ressemblant a celle due a la deficience en zinc. Sur les feuilles bien developptes, les mdrbrUreS se traduisent par des zones vert clair sur un fond plus fonce. Les nervures sont souvent en relief et de couleur jaune.

SymptBmes sur tronc, branches charpentiires et pousses. Pas de sympt8mes apparents.

SymptBmes histologiques. Presence dans le phloeme de zones necrotiques. Accumulation massive d’amidon dans les plastes. Fonctionnement anormal du cambium. Formation excessive de phloeme.

SymptBmes dus aux vecteurs. T. erytreae d t forme gravement les feuilles, que restent rabougries et en forme de galle. Elks sont saupoudrtes d’excre- ments. D. citri deforme les jeunes pousses, qui se tordent et presentent ainsi un aspect de rosette. Les feuilles sont tordues, parfois couvertes de miellat, et tombent prematurement.

Indexation

Le mattriel soupConnt d’&tre infect6 peut &tre


indicator plants. Preferred indicator plants are Orlando tangelo and sweet orange seedlings. Inoculation should preferably be with pieces of mottled leaves. Because of the variable results in graft transmission, at least 10 seedlings should be used for each tree to be indexed. After inoculation, the indicator seedling should be kept at 24°C (South African form), 32°C (Asian form). The symptoms usually show up after 4-5 months. The presence of a specific fluorescent marker, gentisoyl glucoside (Feldman & Hanks, 1969) in greening- infected tissue has been used for indexing (Schwarz, 1968a, b).

Electron microscopy

As there are no symptoms specific of greening, suspect trees to have to be analysed by electron microscopy to confirm the presence of the characteristic bacteria in the sieve tubes (elongated sinuous rod-like structures 0.15-0.25 pm in dia- meter and several /mi long). Similar structures have been seen in both vectors (Moll & Martin, 1973; Chen et al., 1973).

Serological identification

The monoclonal antibodies against the Indian form of citrus greening bacterium have been successfully used to detect the bacterium in green- house-grown citrus and periwinkles by immunofluorescence and ELISA (Gamier et al., 1987).

Diaphorina citri

Egg: orange and almond shaped; 0.01-0.15 mm. Nymph: light yellow to dark brown and bears well- developed wing pods. Adult: 2.5 mm long, body yellowish brown, legs greyish brown, while wings are transparent with white spots or light brown with a broad, beige, longitudinal band in the centre.

Trioza erytreae

Egg: orange, cylindrical, with a sharp point anter- iorly; laid on leaf margins of young, actively growing foliage. Nymph: dorso-ventrally compressed and varies in colour from yellow, olive green to dark grey; has a marginal fringe of white, waxy filaments (Moran,

index6 par greffage a des plantes indicatrices sensibles. Parmi celles-ci les tangelos Orlando et les orangers issus de semis sont les plus utilists. Le meilleur inoculum est constitue de fragments de feuilles a marbrures. Vu les rtsultats variables de l’inoculation par greffage, au moins 10 plantes indicatrices doivent itre utilisees pour chaque arbre a indexer. Aprts inoculation, les plantes indicatrices doivent itre maintenues a 24 ”C (forme africaine) ou a 32 “C (forme asiatique). Les symptdmes apparaissent aprks 4-5 mois. La presence d’un marqueur specifique fluorescent, le gentisoyl glucoside (Feldman & Hanks, 1969) dans les tissus infectes par le greening a etC utilise pour l’indexation (Schwarz, 1968a, b).

Microscopie dectronique

Comme il n’existe pas de symptdmes sptcifiques du greening, les arbres apparaissant suspects par leurs symptdmes doivent ktre analysis par microscopie electronique pour confirmer la presence, dans les tubes criblks, des bacteries caractkristiques du greening, qui se manifestent sous la forme de structures allongees, sinueuses et bacilliformes, de 0,15-0,25 pm de diamttre et de plusieurs pm de long. Ces mimes structures ont ete detectees chez les deux vecteurs (Moll & Martin, 1973; Chen et al., 1973).

Identification skrologique

Les anticorps monoclonaux contre la forme indienne de la bacterie du greening ont ete utilises avec succts pour la detecter chez les agrumes et les pervenches cultivtes en serre par immunofluorescence et ELISA (Gamier ef al., 1987).

Diaphorina citri

Oeuj orange, en forme d’amande, 0,Ol-0,15 mm. Larue: jaune clair a brun fonce, portant des rudi- ments d’ailes bien dtveloppes. Adulte: 2,5 mm de long, corps jaune-brun, pattes gris-brun, ailes transparentes avec taches blanches ou brun clair et une large bande longitudinale beige.

Trioza erytreae

Oeufi orange, cylindrique, avec une pointe aigue anterieure. Pondu sur les bords de feuilles en pleine croissance. Larve: comprimke dorso-ventralenient; couleur variable de jaune, vert olivitre a gris fond; porte une frange lattrale de filaments blancs de cire

Fiches informotives UEPP 505

1968); largely sedentary; forms distinct colonies and settles on the underside of young leaves, where, after a few days feeding, it produces distinctive cup- shaped, open galls. Adult: winged, pale and delicate initially, later becoming light brown. Males are smaller than females and have a blunt tip to the abdomen, the latter ending in a sharp point in females. When feeding, adults take up a distinctive stance, with the abdomen raised at an angle of about 35” to the feeding surface.

Phytosanitary measures

Import of planting material of citrus from any country in which citrus greening bacterium, or either of its vectors, occur should be prohibited. If any material is imported, this should be done under a strict licence. Healthy budwood can be obtained by shoot-tip grafting, or alternatively by heat treatment (49 “C for SO min) of budwood in tetracycline solution (1000 ppm), which results in a high percentage of greening-free buds. Various other heat-treatment regimes are possible (Common- wealth Department of Health, 1982). Such greening-free material should be kept and propagated under insect-proof screenhouse conditions, and its health status checked by grafting onto sweet orange. It is possible to fumigate citrus budwood material against the vectors (FAO, 1983).

Acknowledgements

M. Garnier and J. Bove have assisted in the preparation of this data sheet.

(Moran, 1968); stdentaire; forme des colonies distinctes sur la face inferieure des jeunes feuilles, sur lesquelles, apres quelques jours dalimentation, apparaissent des galles ouvertes en forme caracteristique de coupe. Adulte: ail&, pBle et delicat d’abord, puis brun clair. Les miles sont plus petits et leur abdomen est obtu, alors que celui des femelles se termine par une pointe aigue. Durant leur alimentation, les adultes se tiennent dans une pose caracttristique, avec l’abdomen sureleve a un angle de 35” par rapport au substrat.

Mesures phytosanitaires

11 est recommande d’interdire toute importation de materiel de Citrus, destine a la plantation, a partir de tout pays contamine par la bacttrie du greening des agrumes ou par l’un ou I’autre de ses vecteurs. Le mattriel import6 par derogation sera assujetti a des conditions sevtres precisees dans un permis d’importation. I1 est possible d’assainir des plants par greffage apical de mtristemes, ou par traitement thermique (49 “C pendant SO min) dans une solution de tetracycline a 1000 ppm (qui permet d’obtenir un pourcentage eleve d’yeux indemnes de greening). Dautres traitements thermiques ont etk dtcrits (Commonwealth Department of Health, 1982). Ce materiel indemne de greening doit itre conservt. et multiplie en cage d’isolement, et son etat phytosanitaire doit &tre vtrifie par indexation sur oranger. I1 est possible de fumiger le materiel vegetal d’agrumes contre les vecteurs (FAO, 1983).

Remerciements

M. Garnier et J. Bove ont participe a la redaction de cette fiche informative.

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Documents

Citrus greening bacterium and its vectors/et ses vecteurs Diaphorina citri & Trioza erytreae