Résumé
Two trials were set up in a humid area in Guadeloupe with disease-free tissue cultured plants of sugarcane. Trial I was located in a banana growing area distant from sugarcane fields, and trial 2 was adjacent to commercial sugarcane fields. In the first trial, a Xanthomonas albilineans strain belonging to serovar 3 (strain XaS3) was detected in water sampled at sunrise on the leaves, 13 weeks after planting sugarcane in the field. This strain randomly invaded the sugarcane canopy. Seven weeks later, a new strain belonging to serovar 1 (strain XaS1) appeared on leaves. The populations of XaS1 progressively increased on the leaf surface with a maximum of 2.4 x 104 cfu/ml of water, whereas populations of XaS3 progressively decreased. Leaf scald symptoms were first noted 26 weeks after sugarcane planting. However, only XaS1 was isolated from symptomatic leaves and a few sugarcane stalks. When sugarcane plants were inoculated in the greenhouse by the decapitation method with the two leaf scald strains, only XaS1 induced disease symptoms. In trial 2, a strain of X albilineans belonging to serovar 1 was detected in water available on leaf surfaces 11 weeks after planting sugarcane in the field. No other strain was found. X. albilineans populations increased rapidly, reaching more than 1 X 106 Cfu/ml of water sampled on leaves at sunrise. The first disease symptoms were observed on the susceptible cultivar B69566 14 weeks after sugarcane planting. Symptoms appeared later on the resistant cultivar B8008 and were less severe, but bacterial leaf surface populations on both cultivars were similar. When sugarcane was harvested after 12 months of growth, 41 out of 226 (18%) stalks of susceptible cultivar B69566 were colonized by X. albilineans, whereas only 2 out of 114 (1.7%) stalks of resistant cultivar B8008 were colonized. Although the leaf scald pathogen is thought to be mainly transmitted by infected cuttings, aerial transmission of X. albilineans also occurs and colonization of sugarcane phyllosphere by virulent strains of the pathogen appears to be an important step in the epidemiological cycle of leaf scald disease in Guadeloupe. (Texte intégral)