Infection of sugarcane by SCYLV associated with high populations of Melanaphis sacchari in Guadeloupe

A sugarcane trial (18 rows of 65m each) was established with 1,900 disease free tissue culture propagated plants of cultivar SP71-6163. In Brazil, this cultivar is highly susceptible to yellow leaf caused by the Sugarcane yellow leaf virus (SCYLV). Two rows of SCYLV-infected plants of cultivar FR90714 were also planted next to one side of the SP71-6163 plot. Colonization of disease-free plants by aphids was monitored for 22 weeks after sugarcane planting in the field. The number of SCYLV-infected plants was estimated by tissue blot immunoassay (TBIA) with the bottom part of the midrib of the top visible dewlap leaf Two aphid species, Melanaphis sacchari and Sipha flava, were identified in the trial. Only populations of M. sacchari, a vector of SCYLV, were, however, taken into account during the survey. M. sacchari was first observed on two plants out of the 1,900 four weeks after sugarcane planting in the field. Aphid population increased slowly until 10 weeks when 301 (17%) plants were colonized with a mean of 51 aphids per colonized plant. Then, the number of aphids in the field decreased, most likely due to natural control by ladybugs which were also present on the sugarcane leaves. Aphid population increased again between 17 and 22 weeks after planting and, on week 22, 2,822 M. sacchari aphids were found on 41 colonized plants out of 47 (87%) randomly observed. One month earlier, only 222 plants out of the 1,900 (12%) were colonized by M. sacchari. One top visible dewlap leaf was sampled every 10 plants of SP71-6163 on each sugarcane row every 1-7 weeks (182 to 200 leaves per sampling date). The first SCYLV-infected leaves were found on two plants, 16 weeks after planting. Four weeks later, when high aphid populations were observed, 8% of samples were positive. After 25 weeks, one leaf was sampled from each plant in the trial and 113 leaves out of 1879 (6%) were infected by SCYLV. Infected plants were randomly distributed in the field, but the first row adjacent to infected plants of cultivar FR90714 showed a higher percentage of infection (17%). Percentage of infected leaves remained at about 6% until the end of sampling, 38 weeks after planting. Sugarcane stalks were harvested after 44 weeks of growth in the field and, after five weeks of growth in the first ratoon crop, 170 out of 817 sampled leaves (21%) were infected by SCYLV. At the same time, aphids were observed on 112 out of 309 plants (36%) and a mean population of 26 aphids per colonized plant was determined. Winged adult aphids were also observed on 28 plants out of 309 (9%). In our trial, sugarcane infection by SCYLV seemed, therefore, to be associated with high aphid populations at the beginning of each crop. Rapid increase of yellow leaf incidence may therefore occur in Guadeloupe when diseased cane and aphid vectors of SCYLV are present. (Texte intégral)