UC Riverside International Organization of Virologists Conference Proceedings (1957-2010)

Title Survey in East and West African Countries South of Sahara

Permalink https://escholarship.org/uc/item/71757972

Journal International Organization of Citrus Virologists Conference Proceedings (1957-2010), 10(10)

ISSN 2313-5123

Authors Aubert, B. Garnier, M. Cassin, J. C. et al.

Publication Date 1988

DOI 10.5070/C571757972

Peer reviewed

eScholarship.org Powered by the California Digital Library University of California Citrus Greening Disease Survey in East and West African Countries South of Sahara

B. Aubert, M. Garnier, J. C. Cassin and Y. Bertin

ABSTRACT. A survey covering eight African countries south of the Sahara, was undertaken for Citrus Greening Disease (CGD) and its psylla vector erytreae (Del Guercio) on citrus orchards and indigenous belonging to the . Samples of columellas and leaf rnidveins were collected on presumably contaminated material for electron microscopic examination. Both the extent of the vector and the spread of the disease were then correlated with ambient conditions of temper- ature and relative humidity for assessing the outlines of the greening belt in these African countries. Index words. African citrus psylla, greening organism, psylla .

Citrus Greening Disease (CGD) is by placing psylla nymphs in hatch- known to exist in subsaharan Africa, ing boxes (50 to 70 nymphs per where it is disseminated naturally by box). Winged individuals chal- the African psylla vector Trioza eryt- cidoid obtained from the reae (Del Guercio). In Africa, both the hatching boxes, were then dis- vector and the greening or- patched dry for taxonomic identifi- ganism (G.O.) do not tolerate hot and cation. The exoskeleton of the dry climates (1, 2). The extent of the nymph psylla mummies was also vector and the spread of the disease examined to check the characteris- are therefore dependent on ambient tics of the exit hole as well as the conditions of temperature and rela- meconium left by the attendant tive humidity. CGD is in fact re- chalcidoid insects. The mummies stricted to fresh and moist highlying of parasitized nymphs exhibit a areas, with different ranges of eleva- conspicuous hole in the middle of tion according to local microclimates. the thorax when ectoparasites, The present study is an attempt to e.g. Tetrastichus dryi Waterston, give the first description of the green- are dealt with. The emergence of ing belt in different African regions the endoparasite, e.g. Psyl- south of the Sahara. laephagus pulvinatus Waterston, MATERIALS AND METHODS usually takes place through a hole chewed in the abdomen, whereas The survey was conducted in lateral position of the ekit hole is three West African countries, and generally associated with hyper- five East African countries and in the parasitism (fig. 2). Comores Islands (fig. 1). 4) visual greening symptom charac- Citrus orchards and local rutace- terized by sectorial dieback, zinc ous flora were checked following al- pattern deficiences, uneven colora- titudinal transects, and examined for tion of the fruits, lobsided fruits the presence of: with aborted seeds. 1) living psylla nymphs andlor adults Samples of columellas and leaf on new flush (search of 10 minutes midveins were collected on presuma- per tree) bly contaminated material, then fixed 2) traces of leaf galls and nymph immediately in a 4% buffered solution mummies left by T. erytreae of glutaraldehyde, pH 7.4, and dis- (search of 10 minutes per tree) patched to the laboratory. The Sam- 3) attendant primary and secondary ples were subsequently postfixed in a parasites (hyperparasites), caught 1% osmium tetroxide solution, dehyd- 232 Tenth ZOCV Conjkrence

millibars and can be obtained by combining midday maximum tem- perature and lowest relative humidity. SD values above 30 Prospected mmb are considered as lethal for areas the African psylla vector, owing to the high rate of mortality of T. erytreae eggs and first instar nymphs (2). RESULTS

Fig. 1. Greening and sur- Results of the survey are sum- vey in Africa south of the Sahara. marized in table 1 for East and West Africa. rated up to absolute alcohol and then EAST AFRICA embedded in Epon for ultrathin sec- Ethiopia. In Ethiopia, greening tions. Observations of phloem sieve symptoms and T. erytreae are usually tubes were made by transmission noticed above 1,500 m of elevation (5, electron microscopy for the presence 8). Most of the recent citrus - of endocellular bacteria asociated ations have been established in the with CGD. middle Awash River valley between Meteorological records were col- 1,100 and 1,200 m (especially in Nura lected from local national networks, Era and Merti Jeju). But citrus or- when available at a given representa- chards were also traditionally planted tive site. Attention was focused on in the lower Awash River (e.g. Melka maximum temperature and maximum Werer 730 m) or in the upper Awash vapor pressure saturation deficit. River (e.g. Awasa-Koka 1,600 m). Furthermore, backyard citrus trees 1) Temperature: besides average are commonly found at higher eleva- monthly maximum temperature, tion, for instance at Debre Zeit (1,850 the number of days per annum m) and near Addis Ababa (2,400 m). having a maximum temperature The present survey was carried out less than 25 C (TDl25) was com- on orchards located along the altitud- puted. The latter index had been inal gradient which prevails between used previously for predicting Melka Werer and Addis Ababa. symptom expression of CGD (6): it Table 1 shows that the TI25 index was found that severe symptoms increases from zero to 100 days be- occurred when TDl25 exceeds 60 tween 1,200 and 1,600 m. There is a days but usually mild symptoms or concomittant appearance of severe no symptoms appeared below a CGD symptoms on the trees at the TDl25 of 30 days. latter altitude. This field diagnosis 2) Mean maximum air saturation was subsequently confirmed by the deficit (SD): SD is expressed in examination of the phloem ultrastruc-

A) Primary Ectoparasite B) Primary Endoparasi te

n C) Hyperparasite

Fig. 2. Examples of leaf psylla mummies exhibiting different types of chalcidoid exit holes. TABLE 1 CITRUS GREENING DISEASE SURVEY IN EAST AND WEST AFRICA m8 Climatic conditions g Greening Primary V) SD (mmb)" parasite k, Altitude T" Visual Trioza Secondary h 'aE Country Location (a) maxZ TD/25Y Mean Max Min Symptom GOV erytreaeW Endow Ectow parasitew R Ethiopia Melka Werer 730 34.3 0 32 - - S Lat. 4"-18"N Awash 916 33.4 0 30 39 24 - - 'u Metahara 950 32.3 0 25 31 19 - - a Nura Era 1,200 30.8 0 25 30 12 - - c5 Tibila 1,400 - - - - - + + s AwasaIKoka 1,600 24.3 100 16 24 8 + + + + - + S' Debre Zeit 1,850 26.1 120 13 16 6 + + + + - + 2 Addis Abeba 2,400 22.8 337 14 13 7 + + s Rwanda- Muronge + + + B Burundi Karama 1,403 27.9 25 + + + + + t Lat. 4"-7"s Butare 2,312 20.1 360 + + + + + + Malawi Chileka 767 27.5 70 16 26 10 + + + Lat. 15"s Chiteze 1,149 26.7 120 19 29 11 + + + + + Bvumbe 1,400 23.8 190 11 18 8 + + + + + Cameroon Garoua 183 34.4 0 37 61 18 ------Lat. 2-9"N Sangmelima 730 28.8 30 17 28 10 + + + - - Nkoundja 1,220 28.0 59 21 39 9 + + + + + Gabon Okoloville 380 28.0 30 22 12 40 - - + - - - Lat. 1"s

Namibia Swakopmund 5 29.3 32 29 40 17 - - - - - Lat. 19-22"s Grootfontein 1,400 27.2 45 28 41 17 ------Comores Grande Comore 50 29.9 - - Diapho- + - - Lat. 12"s Anjouan 350 25.7 - - rinasp. + - - -- "Mean annual maximum temperature. YTotal number of day per annuum having a maximum temperature less than 25 C. "Saturation deficit (SD) in millibars (mmb) at mid day; mean = annual average; Max = highest monthly saturation deficit; Min = lowest monthly saturation deficit at midday. w- = not seen; + = considered organism or symptom present. '- = apparently healthy phloem + = presence of endocellular bacteria in the phloem tissue. Tenth IOCV Conference ture. attesting to the presence of en- group known to develop on the docellular GOs in the sieve tubes (fig. Rutaceae flora (3). 3). Natural parasites obtained from Mean SD maximum is only 16.7 T. erytreae include Psyllaephagus mmb at 1,600 m with highest peaks of pulvinatus Waterston, Psyl- only 24 mmb, this means a low rate laephagus sp. and a secondary para- of air desiccation strongly favourable site: Cheiloneurus cyanonotus to psylla build-up. Between 1,000 and Waterston. To date, no Tetrastichus 1,200 m, SD values of 30 mmb bring sp. has been obtained from T. eryt- about high mortality rate with low reae in Ethiopia. chance of psylla proliferation. Ethiopian endemic Rutaceae flora Rwanda, Burundi. Trioxa eryt- include the following species reae, as well as symptoms of CGD, anisata (Wild), Vepris and endocellular phloem restricted glomerata (EH. Hoffrn.) Fagaropsis GOs, were found in Rwanda and chalybea (Engl) Verdoon, Toddalia Burundi above 1,400 m of elevation. asiatica (L.) Harm and Zantoxylum An example of GOs observed in the chalyteum Engl.. But so far only C. columella of a contaminated anisata was found harbouring T. eryt- tree at Butare is shown on fig. 4. Wild reae in Ethiopia and a more complete rutaceous plants of these countries in- survey is needed for the other clude , Citropsis species, mainly the Fagariops. Trioxa schweinfurtxi, and Balsamocitrus sp. species described recently on Ficus but more inspections are needed to sycamorus, Pygium africanum and check for the presence of T. erytreae Stephania abyssinica (7) belong to reservoirs on this indigenous flora. different psyllid species, since Trioxa The examination of 100 T. erytreae erytreae is the only member of this nymphs in Gisovu, Karama and Bu-

Fig. 3. Greening organism in the sieve tube of an orange tree, Koka, Ethiopa, 1,600 m. (X 25,000). Diseases Induced by Proearyotie Pathogens

Fig. 4. Greening organism in the sieve tubes of a lemon tree, Butare, Rwanda, 2,312 m. (X 25,000). tare has shown a high rate of to CGD, and have been found con- parasitism (90 to 92%) in the first two taminated by the GOs. They consti- places induced by what is probably tute an important source of inoculum. Tetrastichus dryi. Endoparasites be- Survey of parasites has revealed longing to the Psyllaephagus group the presence of a new species of Tet- as well as unidentified secondary mstichus obtained from hatching parasites, were obtained from the Bu- boxes and presently under taxonomic tare samples. investigation. This Tetrastichus sp. Malawi. In Malawi, located at a seems less effective than Tetmstichus latitude of 15" south, both CGD dryi for controlling psylla buildup. symptoms and the African psylla vec- The examination of 379 nymphs tor were observed at only 760 m of collected in the citrus collection of altitude. Phloem restricted GOs were Bvumbve has given a percentage of detected from different samples of parasitism of only 27%. Among the hitcolumellas sampled on suspected parasitized nymphs 4% were attacked trees. Since most of the territory of by the ectoparasite belonging to the the Republic of Malawi is located group of Tetrastichus, 7% by an en- above 1,000 m elevation, CGD is doparasite of the group Psyl- widespread all over the country and laephagus, whereas 16% exhibited there is virtually no production of lateral exit holes presumably induced oranges or mandarins. Local rough by hyperparasites (fig. 2 c). lemon seedlings which are quite com- Zambia. In Zambia, the survey mon in rural areas as backyard trees, was carried out only in Lusaka area exhibit several flushes per year which on orange or mandarin orchards. No strongly attract T. erytreae adults. trace of Trioxa erytreae was detected These plants are moderately tolerant and the survey did not show, either Tenth ZOCV Conference

Trioza erytreae or CGD symptoms. Gabon. Typical leaf galls of T. The observation of columella samples erytreae were noticed in several pri- under electron microscope gave nega- vate orchards of Okoloville (380 m), tive results. but no clear-cut symptoms of CGD WEST AFRICA were seen on orange or mandarin Cameroon. In the Cameroon, im- trees, and electron microscopic portant outbreaks of T. erytreae have examinations of phloem columellas been noticed in the volcanic highlands gave negative results. (e.g. Nkoundja area 1,220 m), where Namibia. The survey of several the mean annual saturation deficit is citrus trees in Swakopmund (7 m) and only 21 mmb. Nonetheless extreme Grootfontein (1,400 m) showed value of SD varies from 9 mmb in neither T. erytreae nor CGD symp- rainy season to 39 mmb during the hot toms, and electron microscopic exami- dry season. The latter is too arid for nation of tissue samples failed to de- the survival of eggs and first instar tect the GO. In the areas visited, the nymphs, hence there are strong fluc- climate is apparently too arid for the tuations of psylla colonies throughout psyllid borne CGD, even near 1,500 the year, with only two or three gen- m of elevation. erations concentrated during the wet Comores Islands. A new species flush season. of Diaphorina, close to Diaphorina In the hilly rainforest area of amoena was discovered on the citrus Sangmelima (700 m), permanent trees of Grande Comore, Anjouan and humid and fresh conditions support Moheli. A taxonomic description of psylla outbreaks all year round with this new citrus psyllid was given re- probably six or seven annual genera- cently (4). No typical symptoms of tions. Extremely high populations CGD were noticed in this archipelago (exceeding 200 galls/m2 of canopy and the results of electron micro- area) were observed on citrus trees scopic examination of citrus phloem and Clausena anisata. Apart from C. were negative. Whether the new anisata, the endemic rutaceous flora Comorian Diaphorina is able to trans- has not yet been surveyed. mit CGD remains to be demon- One of the main characteristics of strated. the Cameroon T. erytreae ecosystem, is the absence of effective predators DISCUSSION AND CONCLUSION and a very low percentage of natural parasitism. A new species of Tetras- Although the distribution map of tichus apparently similar to that of Trioza erytreae is presently known to Malawi has been found but its impact cover 20 African countries or ter- on regulation of the psyllid outbreak ritories, less information was avail- is too low. Of 245 psylla nymphs able on the citrus greening disease it- examined, only 25 were found self. During the present survey, the parasitized (i.e. 10%). Greening Organism was detected on Greening symptoms on citrus citrus trees in Malawi, Rwanda, trees were seen in Nkoundja and Burundi, Ethiopia and Cameroon to Sangmelima. However, the Sang- augment electron microscopic evi- melima samples were the only ones to dence of CGD obtained previously reveal phloem restricted GOs. No GO from South Africa, Kenya, Reunion was found in C. anisata mid-veins. and Mauritius. With the exception of In the northern part of the coun- Gabon and certain areas of Came- try near Garoua, the climate is far too roons we found that T. erytreae was arid for allowing psylla buildup or associated with CGD. symptom expression of the disease. In all the inspected areas, the ob- For this reason, all the new disease- served psylla outbreaks were consis- free germplasm recently has been es- tent with the expected populations tablished in this area. deducted from climatic data. But the Diseases Induced by Procaryotic Pathogens 237 picture was somewhat different for breaks could be reduced by appropri- CGD, since in Sangmelima (Came- ate biological control programmes, roon) severe greening symptoms especially in Cameroon and Ethiopia were seen with a TDl25 index of only where low percentage of parasitism 30 days. This is suggestive that a heat was noticed. The high proportion of tolerant strain of CGD may be pres- secondary parasites in Rwanda and ent. Malawi indicates that integrated pest Among endemic Rutaceae flora, managment combining biological and Clausena anisata is one of the more chemical protection should be im- important host plant of the African plemented. This kind of approach has citrus psylla. proven effective for a similar situation It is anticipated that psylla out- in southern Africa.

LITERATURE CITED

1 Bove, J. M., E. C. Calavan, S. P. Capoor, R. E. Cortez, and R. E. Schwarz 1974. Influence of temperature on symptoms of California stubborn, South Africa greening, Indian citrus decline and Philippines leaf mottling, p. 12-15. In Proc. 6th Cod. IOCV. Univ. Calif., Div. Agr. Sci., Richmond. 2. Catling, H. D., and G. C. Green 1972. The influence of weather on the survival and population fluctuations of Trioza erytreae (Del Guercio), a vector of greening, p. 58-67 In Proc. 6th Conf. IOCV, Univ. Calif., Div. Agr. Sci., Richmond. 3. Hollis, D. 1984. Afrotropical jumping plan lice of the family Homoptera : . Entomol- ogy Ser. Vol. 49. Bull. British Museum Natural History. 102 pp. 4. Hollis, D. 1987. A new citrus-feeding psyllid from the Comoro Islands with a review of the Diaphorina amoena species group (Homoptera). Systematic Entomol. 12: 307-310. 5. Schwarz, R. E. 1976. Citrus greening in Ethiopia. FAOIUNDP ETH 41002 Tech. Rep. Inst. Agr. Res., Addis Abeba. 6. Schwarz, R. E., and G. C. Green 1972. Heat requirements for symptom suppression and inactivation of the greening pathogen, p. 44-51. In Proc. 5th Conf. IOCV, Univ. Florida Press. Gainesville. 7. Tsedeke, Abate 1986. Survey results on citrus psyllas and aphids Doc. IAR 10 pp. 8. Van Bruggen, A. H. C., and Almaz Yilma 1985. Virus and virus-like diseases of citrus in Ethiopia. FA0 Plant Prot. Bull. p. 2-12.