The presence of a major resistance gene (Bru1) for brown rust in the sugarcane cultivar R570 (2n about 115) was confirmed by analyzing segregation of rust resistance in a large population of 658 individuals, derived from selfing of clone R570. A subset of this population was analyzed with AFLP and bulked segregant analysis (BSA) to develop a detailed genetic map around the resistance gene. Four hundred and forty three primer pairs were used resulting in the identification of eight AFLP markers surrounding the resistance gene in an interval of 10 cM, with the closest markers located at 1.9 and 2.2 cM on each side of the gene. Efficiency of the AFLP/BSA applied to the complex polyploid genome of sugarcane is discussed, as well as the potential of the newly identified AFLP markers for developing a map-based cloning approach exploiting, synteny conservation with sorghum.

Specific primer pairs were designed to distinguish four genotypes (BRA for Brazil, CUB for Cuba, PER for Peru, and REU for Réunion Island) of Sugarcane yellow leaf virus (SCYLV) by reverse transcription-polymerase chain reaction (RT-PCR). A unique genome fragment was amplified from each genotype, with the exception of genotypes BRA and PER that are phylogenetically relatively close and were designated genotype BRA-PER. These RT-PCR primers were then used to identify the SCYLV genotype(s) present in 18 different sugarcane growing locations in the world, and 245 leaf samples infected by the virus were analyzed. Most samples were infected by only one of the three genotypes, but mixed infections occurred. Genotype BRA-PER was found in all sugarcane growing locations, whereas genotypes CUB and REU were each found in four geographical locations only. Genotypes BRA-PER, CUB, and REU were all three detected in locally bred sugarcane cultivars in Guadeloupe, indicating local transmission of these genotypes. In contrast, only genotypes BRA-PER and CUB were found in locally bred cultivars in Brazil, whereas genotype REU was detected in this country in cultivar R570 imported from Réunion. Similarly, genotypes BRA-PER and REU are both present in Réunion, but genotype BRA-PER has not, as of yet, spread on this island. Presence of several SCYLV genotypes in Brazil, Colombia, Guadeloupe, Mauritius, and Réunion suggests different virus introductions and/or different evolution histories of the virus after its introduction into a new environment.

The spread of Sugarcane yellow leaf virus (SCYLV) in sugarcane plants was studied on Réunion using virus-infected cuttings from four cultivars (R570, R575, R577 and R579). One month after the germination of cuttings in an insect-proof glasshouse, SCYLV was detected by reverse-transcription polymerase chain reaction (RT-PCR) and tissue-blot immunoassay (TBIA) in the leaves, shoots and roots of all cultivars. The distribution of SCYLV in the whole plant did not vary over a 10-to 11-month period of growth. In addition, the spread of SCYLV in sugarcane fields on Réunion was investigated during a survey conducted from 1998 to 2001. Samples were taken in three sugarcane-growing areas, and TBIA was used to detect SCYLV in the three major cultivars (R570, R575 and R579). The percentage of infected stalks varied according to cultivar and growing area, but remained relatively stable for a given cultivar in a given growing area over the 30-month survey period. Cultivar R575 was the most infected cultivar in all three growing areas (mean of 98% infected stalks). The percentage of infected stalks ranged from 16 to 94% in cv. R570 and from 21 to 92% in cv. R579. These results suggested that on Réunion: (i) infected sugarcane stools do not recover from the disease after harvesting; and (ii) the virus is mainly propagated by planting infected cuttings. SCYLV was detected by RT-PCR in the aphid Melanaphis sacchari , a potential vector of this virus. Two months after planting virus-free plants of susceptible cv. R575 in a field surrounded by sugarcane infected with SCYLV, 14% of plants were found infected with the virus. Four months later, 25% of plants were found infected with SCYLV, but no new infections were detected between 6 and 12 months after planting. In the first ratoon crop, 42% of plants were infected with SCYLV after 6 months of growth. Spatial distribution of infected plants suggested that, on Réunion, a small window of time (between 0 and 2 months after planting cuttings) exists during which primary infection can occur. Based on the results obtained in this study, the use of clean planting material for some cultivars and the use of tolerant cultivars should be an efficient means of controlling sugarcane yellow leaf on Réunion.