1Centre International en Recherche Agronomique pour le Développement, CIRAD-CA, Station de Roujol, 97170 Petit Bourg, Guadeloupe, FWI; 2 CIRAD-CA, Programme Canne à Sucre, BP5035, 34032 Montpellier Cedex 1, France

Background and objectives
Leaf scald caused by Xanthomonas albilineans is one of the most important diseases of sugarcane. Present in more than 60 locations in the world, this bacterial disease can cause severe yield losses. Control measures include cultivation of resistant cultivars, planting of healthy material and prevention against introduction of new strains of the pathogen. Most sugarcane cultivars are polyploid, interspecific hybrids between the domestic species Saccharum officinarum and a wild relative Saccharum spontaneum. This later species seems to be a basic source for resistance to sugarcane leaf scald disease, and only a few clones of S. officinarum or other species such as S. robustum are known to be resistant to the disease. Assessment of resistance to leaf scald based solely on symptom expression can be unreliable because plants can be infected without showing symptoms. Additionally, physiognomy of several wild species makes symptom evaluation difficult. Resistance to leaf scald is associated with limited colonization of the sugarcane stalk by Xanthomonas albilineans [1]. The objective of this study was to analyse bacterial population densities in stalks of various clones of wild relatives of sugarcane after artificial inoculation of plants with the pathogen.

Materials and methods
A total of 58 wild species clones [4 S. sinense (Ssi), 6 S. barberi (Sb), 12 S. officinarum (So), 24 S. spontaneum (Ssp) and 12 S. robustum (Sr)], and 9 commercial Saccharum interspecific hybrids (Shy) were used. Cuttings were planted according to a randomized block design with 3 replications. Five-month old plants were inoculated in the field by the decapitation technique with X. albilineans strain GPE5SR[2]. A 2 cm-stalk sample was taken 6 months after inoculation from the third internode below and from the third internode above the inoculation area. It was centrifuged for 20 min at 1500 g and aliquots of the extracted sap were streaked on selective medium containing streptomycin and rifampicin [1]. Bacterial colonies were counted after 7 day incubation at 28°C. Thirty stalks (10 per block) were used to determine mean pathogen population densities expressed as log cfu/g of fresh weight.

Results and conclusions
Pathogen population densities varied according to the clone and the species but also to the sampled stalk location. Mean values ranged from 0.7 for Ssp to 5.8 for So in the lower part of the stalk, and from 0.8 for Ssp to 4.6 for Sr in the upper part of the stalk. Additionally, the populations of X. albilineans in the internodes below the inoculation area were generally higher than those above. This difference was especially observed in plants with a high sucrose production (So, Ssi, Sb and hybrids) and a clone x stalk location interaction was found. A graphical presentation of experimental results, showing population densities in the upper internode vs. population densities in the lower internode, was therefore used to identify different colonization groups. Two groups were clearly distinguished : a group of 22 clones that were less colonized in the lower stalk location than the resistant cultivar NCo310 (20 of which belonged to Ssp, one each to Sb and Sr), and a group of 13 clones that were more colonized in the 2 stalk locations than the susceptible cultivar B69379 (5 of which belonged to Sr, 7 to So and 1 to Shy). Additionally, 3 Sr were highly colonized in the upper part of the stalk. The other clones were colonized at similar levels compared to the resistant or the susceptible cultivar, but were not grouped according to a specific species. Limited colonization of the stalk by X. albilineans appears to be related to the Ssp genome. Numerous clones of other species such as Sr and So, which do not contain genes derived from Ssp, were highly colonized by the pathogen. Using Ssp as a source for resistance to leaf scald, based on disease symptoms and degree of colonization, appears therefore very promising. However, considering genetic complexity of the sugarcane genome, it will be necessary to investigate inheritance of this resistance in order to effectively use this species in breeding programs for production of sugarcane resistant cultivars.

1. Rott P et. al., 1997. Phytopathology 87, 1202-13.