Résumé
In a previous study (Alegria et al., 2002), 46 virus isolates obtained from sugarcane leaves showing mosaic symptoms and collected in Cameroon and Congo were identified as hitherto undescribed Sugarcane mosaic virus (SCMV) genotypes. Additionally, the 26 isolates from Cameroon constituted a relatively homogeneous phylogenetic group (group V), whereas the 20 isolates from Congo belonged to two other groups (groups VI and VII). Several observations made in sugarcane fields in Cameroon and Congo, and in greenhouse trials in Montpellier/France, suggested that variations in pathogenicity occur within SCMV isolates from Africa. Severe symptoms of mosaic were observed on sugarcane cultivar R570 in Congo, whereas no symptoms of this disease have been reported so far in Cameroon on the same cultivar; similar observations were made in greenhouse trials after inoculation of plants of cultivar R570 with SCMV isolates from the two countries. A study was therefore undertaken to compare the pathogenicity of six African SCMV isolates (two from Cameroon and four from Congo) by inoculation of two sugarcane cultivars (B8008 and R570). These isolates were selected among the three different phylogenetic groups identified in Cameroon and Congo. Three experiments were set up at different dates in a controlled environment (growth chamber, 12h daylight at 26-28°C and 12h obscurity at 24-26°C). Symptoms and disease progress were studied for two months after mechanical inoculation of sugarcane leaves with carborundum. The presence/absence of SCMV was checked at the end of the experiments by tissue blot immunoassay with all the new leaves that appeared after inoculation. A very good relationship was observed between leaf symptoms and plant colonization by the virus, indicating that once present in leaf tissues, the six SCMV isolates induced mosaic symptoms and SCMV isolates were systemically distributed in infected plants. All virus isolates induced symptoms on cultivar B8008; however, symptom expression and severity varied with the isolate, the date of observation and the experiment. The reaction of cultivar R570 varied significantly according to the experiment. In the first two experiments, only isolates from Congo belonging to group VI were able to produce symptoms on cultivar R570, whereas all isolates produced symptoms on this cultivar in the third experiment. It appears that, under certain conditions, cultivar R570 is susceptible to SCMV isolates from Cameroon and Congo. In contrast, under other conditions that still have to be identified, this cultivar is only susceptible to isolates of phylogenetic group VI. Factors that could account for such unexpected results will be discussed. (Texte intégral)