Daugrois, J.H. et a/. Proc. ISSCT., Vole 25, 2005 EPIPHYTIC COLONISATION AND INFECTION BY XANTHOMONAS ALBILINEANS OF TWO SUGARCANE CULTIVARS DIFFERING IN RESISTANCE TO LEAF SCALD DISEASE 2 J.H. DAUGROIS'*, P. CHAMPOISEAU~'~ and P. ROTT~ 'CIMD-CA, Station de Roujol, 97170 Petit Bourg, Guadeloupe, French West Indies UMR 385 ENSAM-INRA-CIRAD Biologie et Gknktique des Interactions Plante-Parasite, TA 41/K, Campus International de Baillarguet, 34398 Montpellier Cedex 5, France *Contact author: daugrois@cirad.f+ KEYWORDS: Leaf Scald, Epidemiology, Saccharurn Spp., Phyllosphere. Abstract COLONISATION of the sugarcane leaf canopy by Xanthomonas albilineans appears to be!an important step in the epidemiological cycle of leaf scald disease in Guadeloupe. Previous studies showed that healthy sugarcane plants can be infected by X albilineans after aerial transmission of the pathogen. Variation in colonisation of the leaf canopy of sugarcane cultivars differing in resistance to leaf scald and progress of pathogen populations during consecutive crops is, however, unknown. A trial was set up in Guadeloupe with two sugarcane cultivars differing in resistance to leaf scald, B69566 (susceptible) and B8008 (resistant). Disease-free tissue-culture propagated sugarcane was planted in the field in 1999. Epiphytic populations of X: albilineans were regularly monitored for 3 crops (plant cane and two ratoons) by measuring bacterial populations in water dsoplets sampled on sugarcane leaves at sumise. Infection of sugarcane stalks by X albilineans was determined by isolating the pathogen from the stalk sap after 11-12 months of growth in each crop cycle. In plant cane, the pathogen was first detected on the leaf canopy of both cultivars after three months of plant growth. Once the canopy was colonised by X albilineans, bacterial populations increased more rapidly on cultivar B69566 than on cultivar B8008. However, once the leaf canopy was entirely colonised, epiphytic population size of the pathogen on both cultivars was similar, whatever the crop c cle. Highest epiphytic populations varied between crop cycles and reached 5 2 x lo6, 4 x 10 and 6 x 10' bacteria per ml of water droplet in plant cane, first and second ratoon crops, respectively. Percentage of stalk infection by X albilineans varied according to cultivar and, in cultivar B69566, concurrently with epiphytic populations of the pathogen. The size of epiphytic populations appears therefore critical, but not sufficient, for stalk infection after aerial transmission of the leaf scald pathogen. Introduction Leaf scald, caused by Xanthomonas albilineans, is one of the major diseases of sugarcane (Saccharum spp.) occurring in at least 66 countries of the world (Rott and Davis, 2000). It is a vascular bacterial disease that can cause severe yield losses (Martin and Robinson, 1961). Leaf scald is managed by planting healthy seed-cane &om plants issued from disease-free tissue culture propagation (Feldmann et al., 1994; Flynil and Anderlini, 1990) or from hot-water treated cuttings (Egan and Sturgess, 1980), and by use of resistant cultivars (Walker, 1987). Management of the disease is cosnplicated by latent infections. Stalks can be infected by the pathogen for several months without showing any symptoms. Improvement of diagnostic techniques has allowed detection of the pathogen in numerous symptomless plants (Comstock and Irey, 1992) revealing that X albilineans seems to be randomly distributed among stalk internodes (Pan et al., 1999). Symnptomless sugarcane plants can therefore constitute inoculum sources for field contamination. Additionally, various epidemiological factors may play a role in field contamination. Leaf scald is known to be transmitted mechanically by knives and harvesters and by planting infected setts (Ricaud and Ryan, 1989). However, other means of dissemination were recently pointed out. The pathogen was found in the rhizosphere of infected roots suggesting a possible transmission by root contact (Klett and Rott, 1994).
Daugrois, J.H. et a/. Proc. ISSCT., Vol. 25, 2005 Xaflthomonas albilineans was also found in guttation droplets (Sordi and Tolteshi, 1986; Autrey et al., 1995), on the leaf surface of symptomatic and symptomless plants, and in aerosols above a diseased field (Klett and Rott, 1994). Sugarcane leaf canopy may also support high densities of epiphytic populations of X albilineans prior to plant infection and symptom expression in the field, revealing the importance of the epiphytic phase of leaf scald in plant contamination (Daugrois et al., 2003). Leaf scald outbrealcs in Florida, Louisiana and Texas were also suspected to be linked to strains of X. albilineans that survive better epiphytically or have a greater propensity toward aerial tsansmission (Davis et al., 1997). In Guadeloupe, high population densities of the pathogen are often observed on the leaf surface of sugarcane, and these populations were shown to play an important step in the epideiniological cycle of leaf scald disease. If necrotic symptoms attributed to aerial plant contamination by the leaf scald pathogen may vaiy according to cultivar resistance (Daugrois et al., 2003; Comstock, 2001), no data are available regarding colonisation of the leaf canopy of sugarcane crrltivass differing in resistance to leaf scald. The objective of this study was to evaluate the establishment and development of epiphytic populations of X. albilineans following aerial depositiol~ on two sugarcane cultivars differing in resistance to leaf scald during several cropping cycles, and to relate epiphytic colonisation to subsequent systemic infections of the sugarcane plants. Materials and methods Plant material The experiment was conducted with disease-fiee tissue cultured plantlets. Plants were propagated in vitro and transferred to the greenhouse as previously described (Feldmann et al., 1994). The trial was set up by transferring four-week-old greenhouse plants to the field. The trial was divided in four plots organised in a chequered pattern. Two plots contained cultivar B69566 (susceptible to leaf scald) and two plots contained cuitivar B8008 (resistant to leaf scald) (Rott et al., 1995), and each plot was formed by quadrats containing 32 plants each (4 rows of 8 plants). Plots A and C contained 9 and 15 quadrats of cultivar B69566, respectively. Plots B and D contained 9 and 15 quadrats of cultivar B8008, respectively, and uneven quadrat number of plots was due to field design. The field trial was set up in May 1999 and harvested in April 2000, April 200 1 and April 2002 in plant cane, first ratoon and second ratoon crops, respectively. I! Epiphytic population densities of X. nlbilinenns After transfer of piants to the field, the dew and/or rainwater available on leaves was sampled early in the morning for detection of X albilineans. Thirty two droplets were sampled in each quadrate in plant I cane (one droplet per plant) and 16 droplets were sampled in first and second ratoon crops (one droplet every two plants); and pooled in a sterile 2 I& microtube. Samplings were performed every two to four weeks in plant cane and once a month in first and second ratoon crops, as long as water droplets were available on leaves. Each pooled sample and its 100- fold dilution in sterile distilled water were plated on XAS medium (Davis et al., 1994) using the Spiral system (Interscience, 78860 Saint-Non-La-Breteche, France) to determine population size ofx albilineans. Bacterial populations were recorded as Log [(cfu/ml)+l], where cfu stands for colony formitig units. Stalk infection by X. albilineans A total of 125 to 225 stalks were randomly sampled per cultivar at the end of each crop cycle. Clear cut sections were made with a pruning shear in the middle of the lower and top third parts of each stalk. The pruning shear was alcohol-disinfected and flamed between each sample. Cut sections were printed for five seconds on XAS medium, and plates were recorded for the presence of X albilineans colonies after five days of growth at 28 C. Bacterial identification was randomly controlled by seroagglutination with specific X. albilineans antibodies (Rott et al., 1994). 1 Symptom observation Necrotic leaf symptoms were recorded when water was sampled on leaves for determination of epiphytic populations of X; albilineans, as described by Daugrois et al. (2003). Briefly, symptoms were recorded according to a 0-3 scale: 0 = no symptom, 1 = one short (1-10 cm) necrotic lesion per stalk, 2 = several short necrotic lesions or one large (> 15 cm) necrotic lesion per stalk, 3 = two or more large necrotic lesions per stalk.
Daugrois, J.H. et a/. Proc. ISSCT., Vol. 25, 2005 Results Bacterial populations on sugarcane leaf surface In plant cane in 1999, X; albilineans was first detected in water droplets in one quadrat of cultivar B69566, 10 weeks after the transfer of sugarcane plants to the field. The leaf scald pathogen was detected two weeks later on cultivar B8008. After first detection, bacterial population densities increased and spread rapidly within the field on both cultivars during three months, but remained lower on cultivar B8008 than on cultivar B69566. After this period and at the end of the wet season (December 1999), the mean bacterial populations reached equal densities (5 x lo6 cfu/ml) in water droplets sampled from the sugarcane leaf surface of both cultivars (Figure 1). During the dry season (end of January to June 2000) and until harvest of the plant cane crop in April 2000, no water was available on leaf surfaces and no bacteria could be sampled. Jan 99 Ju199 Jan 00 Jul 00 Jan 01 Jul 01 Jan 02 daily rainfall cv. 869566 Fig, I-Population densities of albilineans in water droplets sampled during the 1999-2001 crop seasons from the leaf surface of two sugarcane cultivars differing in resistance to leaf scald. After three months of sugarcane growth in the first ratoon crop, X; albilineans was isolated again frotii leaf surfaces, so011 after the retu1-n of significant rainfalls in August 2000 (Figure 1). At this time, the mean population densities of the pathogen were low on both cultivars (1.5 and 20 cfu/ml of water). Populations then increased again more rapidly on cultivar B69566 than on cultivar B8008. At the end of the wet season (November 2001), the mean population densities in water sampled froin leaf surfaces were similar (10' cfu/ml) on both cultivars, but lower in size when coinpared to the previous year. As in plant cane, X albilineans could not be detected during the dry season (January to June 200 1) and until the first ratoon crop was harvested in April 200 1. At the beginning of the second ratoon crop and until September 2001, sampling was impeded because of erratic rainfalls and lack of water on leaves. When water was again available on leaves (October 2001), first samplings revealed high population densities of X; albilineans (5 x 10' cfu/ml) on both cultivars (Figure 1).
Daugrois, J.H. et al. Proc. ISSCT., Vol. 25, 2005 Symptom observation Leaf scald symptoms were first observed on cultivar B69566 in August 1999, five weeks after detection of X albilineans in water droplets on tile Leaves. Six weeks later, 21 weeks after the transfer of healthy plants to the field, 3 1 % of the stalks of cultivar B69566 showed symptoms, whereas no syinptolns were observed on stalks of cultivar B8008 (Table I). Table I-Percentage of sugarcane stalks showing leaf symptoms due to aerial contamination of leaf surface. Recording of symptom severity follows a 0-3 scale and total number of observed stalks is indicated in brackets. Symptoms were recorded 21 and 44 weeks after transferring plants io the field. * Score 0 = no symptom, score 1 = one short (1-10 cm) necrotic lesion per stalk, score 2 = several short necrotic lesions or one large (> 15 cm) necrotic lesion per stalk, score 3 = two or more large necrotic lesions per stalk. First syn~ptoms were observed on stallts of cultivar B8008 one week later, i.e. 10 weeks after first detection of X, albilineans on leaves of this cultivar. At the end of the plant cane crop, syinptoms were observed on 17% and 4% of stalks of cultivars B69566 and B8008, respectively. Leaf scald syinptoins resulting froin systemic coloilisation of sugarcane plants by X albilinenns were obsemved in the following ratoon crops only for cultivar B69566. Three per cent of stalks of this cultivar showed side sl~ooting and ilecrotic leaf lesions ill second ratoon crop. Stalk infection by X albilineans Stalk infection was estimated by randoin sampling of stalks at the end of each crop. In plant cane, X; albilineans was found in 18% and 1.7% of stalks of cultivars B69566 and B8008, respectively. Bacteria were mainly detected in the lower third of the stalks but 15% of infected stallcs of cultivar B69566 were colonised both in the upper and the lower parts. Only 0.7% of infected stalks were colonised by the pathogen in the upper third and not in the lower part. In first ratoon crop, 3.5% and 1.4% stalks of cultivars B69566 and B8008, respectively, were infected. The pathogen was found in the lower part of 90% of colonised samples. In second ratoon crop, percentage ofinfected stalks reached 13% for cultivar B69566 but reinained low (3%) for cultivar B8008. As in previous crops, X albilineans was ~nainly detected in the lower part of stallts. Whatever the crop, cultivar B8008 was never found infected simultaneously in the upper and lower parts of the stalk. I11 contrast, when considering all three crops together, the pathogen was found simultaneously in the upper and lower parts of 20% of infected stalks of cultivar B69566. Relationship between percentage of stalk infection and population densities of X. albilineans on leaf surface In cultivar B69566, the percentage of stalks colonised by the pathogen at harvest increased with the population size ofx, albilineans on leaf surface at mid-crop stage (end of rainy season) (Figure 2). The percentage of stallcs colo~~ised by the pathogen at harvest of cultivar B8008 was low whatever the population size ofx albilineans on leaf surface at mid-crop stage. Discussion Aerial contamination of the leaf canopy of sugarcane by X albilineans occurs in Guadeloupe prior to plant infection (Daugrois et al., 2003). High densities of pathogen populations can be detected on leaf surfaces, and subsequent migration of bacteria leading to systemic invasion of sugarcane stallts mnay occur. In this paper, we analysed the epiphytic evolution of X, albililzeans and associated plant infection in two sugarcane cultivars differing in resistance to leaf scald.
Daugrois, J.H. et a/. Proc. ISSCT., Volr 25, 2005 Leaf surface densities of X, albilineans at mid-crop (Log[(cfu/ml)+l]) Fig. 2-Relationship between leaf surface populations of X. albilineans at mid-crop and number of infected stalks at harvest (plant cane, first and second ratoon crops) of two sugarcane cultivars differing in resistance to leaf scald. PC = plant cane, R1 = first ratoon crop, R2 = second ratoon crop. The leaf scald pathogen colonised the leaf surface of both the susceptible (B69566) and the resistant (B8008) sugarcane cultivars. Colonisation was, however, different between the two cultivars in plant cane and first ratoon crop. In plant cane, population densities of X. albilineans in water sampled on leaves of the resistant cultivar remained lower for three months than those of the susceptible cultivar. This difference in colonisation of the leaf canopy may be associated to cultivar propensity to enhance or minimise bacterial ingression and egression process on leaf surface, a process that is usually involved in development of epiphytic populations of infectious bacteria (Beattie and Lindow, 1999). On the other hand, when colonisation of the sugarcane canopy by the pathogen was achieved at the end of the rainy season, the size of pathogen populations was similar on the two sugarcane cultivars. Despite similar sizes of bacterial populations on the leaf surface, incidence of symptoms following leaf infection by X. albilineans was higher for susceptible cultivar 1369566 than for resistant cultivar B8008. Such a variation in cultivar response to foliar symptom expression in sugarcane affected by leaf scald was also reported by Cornstock (2001) in Florida. Additionally, symptoms appeared later in cultivar B8008 than in cultivar B69566. Differences in colonisation of leaf canopy, symptom expression and disease incidence suggest that differential ingression and progression of X albilineans exist on and in the sugarcane plant according to cultivar and resistance level toward the pathogen. Analyses of stalk infection also support this hypothesis. Indeed, after plant surface contamination, the percentage of infected stalks was higher'for susceptible cultivar B69566 (18%) than for resistant cultivar B8008 (1.7%). Additionally, distribution of the pathogen within the sugarcane stalk was more extensive in cultivar B69566 than in cultivar B8008. In both cultivars, the pathogen was mostly found in the lower part of stalks, indicating that X. albilineans penetrated the stalk mainly through leaves that were present during
Daugrois, J.H. et at. Proc. ISSCT., Vol. 25, 2005 the wet season (the first half of the crop). These leaves were supposed to support highest population densities of the pathogen. Indeed, there was a good relationship between pathogen population densities six.months before detemination of stalk infection, when the epiphytic Ieaf inoculum was high, and the number of infected stalks of cultivar B69566 at harvest. In contrast, stalk infection of cultivar B8008 was always low and not related to epiphytic population densities of the pathogen. If most stalk infections occurred at the bottom part of plants, the pathogen was found in 5% of infected stalks only in the upper part of the stalk, indicating that infection lnay also occur later in the season when surface leaf moisture and pathogen inoculum decreased concurrently with the number of necrotic symptoms on the leaves. Such random stalk infections were also described in leaf scald affected sugarcane in Louisiana (Pan et al., 1999). We demonstrated herein that epipl~ytic leaf populations of X albilineans may vary according to sugarcane cultivar. On the other hand, a major difference between the two cultivars used in this study was the severity of necrotic symptolns following leaf surface contamination by the leaf scald pathogen and the number of stalks that were infected after leaf canopy colonisation. Cultivar resistance to leaf scald, and especially internal stalk colonisation, therefore appears to be a key factor in the cycle of leaf scald disease. However, specific climatic conditions may ful-ther affect epiphytic populations of the pathogen (Henis and Bashan, 1986), and enhance plant contamination of susceptible cultivass. REFERENCES Autrey, L.J.C., Saumtally, S., Dookun, A,, Sullivan, S. and Dhayan, S. (1995). Aerial transmission of the leaf scald pathogen, Xanthomonas albilineans. Proc. Int. Soc. Sugar Cane Technol., 21(2): 508-526. Beattie, G.A. and Lindow, S.E. (1999). Bacterial colonisation of leaves: A spectrum of strategies. Phytopathol., 89: 353-359. Comstock, J.C. and Trey, M.S. (1992). Detection of the sugarcane leaf scald pathogen, Xanthomonas albilineans, using tissue blot imrnunoassay, ELISA, and isolation techniques. Plant Dis., 76: 1033-1035. Comstock, J. C. (2001). Foliar symptoms of sugarcane leaf scald. Sugar J., 64(3): 23-32. Daugrois, J.H., Dumont, V., Champoiseau, P., Costet, L., Boisne-Noc, R. and Rott, P. (2003). Aerial contamination of sugarcane in Guadeloupe by two strains of Xanthomonas albilineans. Eur. J. Plant Pathol., 109: 445458. Davis, M.J., Rott, P., Baudin, P. and Dean, J.L. (1994). Evaluation of selective media and ilnmunoassays for detection of Xanthomonas albilineans, causal agent of sugarcane Ieaf scald disease. Plant Dis., 78: 78432. Davis, M.J., Rott, P., Warmuth, C.J., Chatenet, M. and Baudin, P. (1997). Intraspecific genomic variation within Xanthomonas albilineans, the sugarcane leaf scald pathogen. ~h~to~athol., 87: 316-324. Egan, B.T. and Sturgess, O.W. (1980). Colnmercial control of leaf scald disease by thesinotherapy and a clean seed program. Proc. Int. Soc. Sugar Cane Technol., 17(2): 1602-1606. Feldmann, P., Spotille, J., Grkdoire, P. and Rott, P. (1994). Micropropagation of sugar cane. In: Teisson, C. ed. In Vitro Culture of Tropical Plants. La Librairie du Cirad, Montpellier, 15-17. Flynn, J.L. and Anderlini, T.A. (1990). Disease incidence and yield performance of tissue culture generated seedcane over the crop cycle in Louisiana. J. Am. Soc. Sugar Cane Technol., 10: 113. Henis, Y. and Basltan, Y. (1 986). Epiphytic survival of bacterial Ieaf pathogens. In: Foltkema, N.J. and Van Den Heuvel, J, ed. Microbiology of the phyllosphere. Cambridge University Press, Cambridge, 252-268. KIett, P, and Rott, P. (1994). Inoculu~n sources for the spread of leaf scald disease of sugarcane caused by Xanthomonas albilineans in Guadeloupe. J. Phytopathol., 142: 283-291. Martin, J.P. and Robinson, P.E. (1961). Leaf scald. In: Maltin, J.P., Abbott, E.V. and Hughes, C.G. ed. Sugar-Cane Diseases of the World, Vol. I. Elsevier Publishing Co., Amsterdam, 79-1 07.
Daugrois, J.H. et a/. Proc. ISSCT., Vol. 25, 2005 Pan, Y.B., Grisham, M.P., Burner, D.M. and Wei, Q. (1999). Distribution of the leaf scald pathogen in infected sugarcane stalks. J. Am. Soc. Sugar Cane Technol., 19: 8-15. Ricaud, C. and Ryan, C.C. (1989). Leaf scald. In: Ricaud, C., Egan, B.T., Gillaspie, Jr., A.G. and Hughes, C.G. ed. Diseases of Sugarcane: Major Diseases. Elsevier Publishing Co., Amsterdam, 39-58. Rott, P. and Davis, M.J. (2000). Leaf scald. In: Rott, P., Bailey, R.A., Comstock, J.C., Croft, B.J. and Saumtally, A.S. ed. A Guide to Sugarcane Disease. La librairie du Cirad, Montpelliel; 38-44. Rott, P., Davis, M.J. and Baudin, P. (1994). Serological variability in Xanthomonas albilineans, causal agent of leaf scald disease of sugarcane. Plant Pathol., 43: 344-349. Rott, P., Soupa, D., Brunet, Y., Feldmann, P. and Letaurmy, P. (1995). Leaf scald (Xanthornonas albilineans) incidence and its effect on yield in seven sugarcane cultivars in Guadelo~~pe. Plant Pathol., 44: 1075-1084. Sordi, R.A. and Tokeshi, H. (1986). Presence of Xanthomonas albilineans in guttation droplets of sugarcane and sweet corn leaves showing leaf scald disease symptoms. STAB, 4(6)Jul/Aug: 60-63. Walker, D.I.T. (1987). Breeding for resistance. In: Heinz, D.J. ed. Sugarcane Improvement through Breeding. Elsevier Science Publishers B.V., Amsterdam, 455-502. I I COLONISATION EPIPHYTE ET INFECTION PAR MNTHOMONAS ALBILINEANS DE DEUX CULTIVARS DE CANNE A SUCRE VARIANT POUR LEUR RESISTANCE A L'ECHAUDURE DES FEUILLES J.H. DAUGROIS, P. CHAMPOISEAU et P. ROTT MOTS CLES: ~~id~miolo~ie, Saccharurn spp., Phyllosph6re. RBsumi? LA colonisation de la phyllosphbre de la canne h sucre par Xanthomonas albilineans est une &tape importante du cycle infectieux de la maladie de lycchaudure des feuilles en Guadeloupe. Des Ctudes recentes ont perrnis de montrer que des plants sains de canne a sucre peuvent 6tre infect& par X. albilineans suite ri une transinission de l'agent pathogbne par voie acrienne. En revanche, l'impact du niveau de resistance des vasict&s de canile a sucre sur la colonisation de la phyllosphkre et sur la dynainique des populations epiphytes de X albilineans, au cours de cycles de rdcolte successifs reste inconnu. Une experimentation a CtC mise en place au champ en Guadeloupe avec deux cultivars de came A sucre ayant des niveaux diffdrents de rcsistance A l'echaudure des feuilles, B69566 (sensible) et B8008 (resistant). Des plants sains de canne A sucre issus de culture in vitro ont Bt6 transf6res au champ en 1999. Les populations epiphytes de X albilineans ont Bte quantifiees ~Cgulikreinent dans les gouttes de ros6e prelevdes au lever du j~ur A la surface des feuilleq de canne h sucre, et pendant trois cycles de rdcolte (canne plantee, premikre et deuxibme repousses). La presence de I'agent pathogene dans les tiges de canne ri sucre a Bt6 d6temin6e par isolement des batteries ii chaque cycle de rdcolte, aprbs 11 21 12 inois de croissance des plantes. En came plantee, l'agent pathogbne a Bt6 detect6 la premibe fois B la surface des feuilles du cultivar B69566 trois mois apses le transfest des plantes au champ. Apses le debut de colonisation de la canopee par X. albilineans, les populations de l'agent pathogkne ont augment6 plus rapidement sur les feuilles du cultivar B69566 que sur celles du cultivar B8008. Neanmoins, en fin de colonisation de la phyllosphere, les pogulations 6 iphytes dkterminbs dans les gouttes de rosde Maient similaires pour les deux cultivars : 5 x 10, 4 x 102 et 6 x 10' batteries par ml en canne plantke, en premibre et en deuxihme repousse, respectivernent. Le pourcentage de tiges infect6es par X albilineans a varie en fonction du cultivar et, pour le cultivar B69566, en fonction des populations Cpipllytes de l'agent pathogene. Des populations 6piphytes ClevCes semblent indispensables, mais non suffisantes, pour l'infection des tiges de canne h sucre aprhs transmission par voie akrienne de l'agent causal de 1'Cchaudure des feuilles.
Daugrois, J.H. ef a/, Proc. ISSCT., Vol. 25, 2005 COLONIZACION EPIFITICA E INFECCION POR XANTHOMONAS ALBILINEANS DE DOS CULTIVARES DE CARA DE AZUCAR QUE DIFIEREN EN RESISTENCIA J.H. DAUGROIS' *, P. CHAMPOISEAU'~~, y P. ROTT~. 1 CIRAD-CA, Station de Roujol, 97170 Petit Bourg, Guadalupe, Indins fiancesas occidentales 2 UMP, 385 ENSAM-INR.1-CIRAD Biologie et Gkne'tique des Interactions Plante-Parasite, TA 41/K, Campus International de Baillarguet, 34398 Montpellier Cedex 5, Francia * Autor a contactar: daugrois@cirad.fr PALABRAS CLAVES: Escaldadura de la Hoja, Epidemiologia, Sacclzarum Spp., Filosfera. Resumen LA COLONIZACION del follaje de la cafia de azucar por Xanthonzonas albilineans aparece como un paso importante en el ciclo epidemiologico de la escaldadura de la hoja en Guadalupe. En el pasado estudios demostrason que plaitas sanas de cafia de azi~car pueden ser infectadas por medio de transinisibn acrea del patbgeno. La variaci6n en la colonizaci6n del follaje de 10s cultivares de la caiia de azucar que difierei~ en su resistencia a la escaldadura de la hoja y el progreso de las poblaciones del patogeno duralite cosechas sucesivas es, sin embargo, todavia desconocida. Con tal objetivo, se estableci6 un experiment0 en Guadalupe con dos cultivares de caiia de azucar que diferian en su resistencia a la escaldadura de la hoja, B69566 (susceptible) y B8008 (resistente). Material libre de la enfermedad se propag6 a traves de cultivo de tejido y se sembr6 en el campo en 1999. Las poblaciones epifiticas dex albilineans fueron supellrisadas regularmente durante tres cortes (plantilla y dos socas), deter~ninando las poblaciones bacterianas en las gotitas de agua toniadas en las hojas de cafia de azucar a la salida del sol. La itifecci6n de 10s tallos de la cafia de az6car por X albilinear~s f~~e determinada aislando el patogeno de la savia de tallos de 11-12 lneses de edad, en cada corte. En la plantilla, inicialmente el patogeno fue detectado en el follaje en ambos cultivares a 10s tres meses de edad del cultivo. Una vez que el follaje fue colonizado por X albilineans, Ias poblaciones bacterianas aumentaron lnhs rhpidamente en el cultivar B 69566 que en el cultivar B 8008. Sin embargo, uiia vez que el follaje fue coloiiizado completa~nente, la cantidad de la poblacidn epifitica del pat6geno en ambos cultivares fue similar, sin importar el torte. Las poblacio~ies epifiticas mayores variaron entre 10s cortes y alcanzaron 5 x lo6, 4 x lo4 y 6 x 1 o5 bacterias por ml de gotita de agua en plantilla, primera y segunda soca, respectivamente. El porcentaje de tallos infectados por X. albilineans vari6 segun el cultivar y, en el cultivar B 69566, de acuerdo con las poblaciones epifiticas del pat6geno. El tainafio de poblaciones epifiticas aparece como critic0 per0 no lo suficiente para causar la infection en el tallo, despuds de que ha ocurrido la transinision adrea del pat6geiio causal de la escaldadura de la hoja.