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 saccbari, 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 cuttin