Citrus tristeza virus replication and movement in seedlings of 71 rootstock genotypes.

Abstract

Citrus tristeza virus (CTV) replication and movement were studied in 1-year-old seedlings of 71 rootstock genotypesi, by inoculation with buds of an Imperial mandarin carrying multiple endemic strains of CTV including those causing seedling-yellows and quick-decline, but free of orange-stem-pitting strains. A virus-free Rough lemon bud was inserted 30-40 mm above the infected bud on each of the 965 nursery trees to study virus movement. A further subset of 226 trees of the same rootstock genotypes were budded with virus-free Imperial mandarin to serve as a control. Virus replication was detected (using direct tissue blot immunoassay) in most seedlings within six months of budding, with levels of infection indicating significant differences between nucellar selections, hybrid families, and within hybrid families. Genotypes lacking Poncirus in their pedigree were rapidly colonised by the virus, while those with Poncirus parentage were often either resistant or slow to replicate CTV. Large differences in the percentage of infected seedlings from Citrus x Poncirus hybrid families indicate that transmission of resistance is complex and not independent of the seed parent. CTV moved rapidly even in resistant genotypes with 100% of virus-free Rough lemon buds acquiring the virus within three months of budding. Tolerance to the diseases caused by CTV is an essential requirement of rootstocks used in Australia and this work has helped to describe initial virus replication in existing and potentially new commercial rootstocks.