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.

A simple and inexpensive technique has been developed to allow the simultaneous screening of young hybrid seedlings for susceptibility to Alternaria Brown Spot (ABS) caused by Alternaria alternata and Citrus Scab (CS) caused by Elsinoë fawcettii. It is employed within six months of seed sowing, prior to hybrids being field planted, and has significantly improved breeding efficiency. By using the same culture techniques to multiply each pathogen, greater flexibility and ease of inoculation has been achieved. Plates of each pathogen are scraped/macerated to remove spores and other colony-forming units, strained through a coarse filter, combined and immediately sprayed onto vigorously growing seedlings. These seedlings are then incubated at ~25°C and high humidity for 4-7 days before being returned to the greenhouse. Hybrids susceptible to ABS develop symptoms within two weeks and are immediately discarded. A second round of culling is performed after four weeks when CS symptoms have expressed. The process is repeated three times to minimise 'disease escapes' and has virtually eliminated these diseases from our field plantings of new hybrids. Nucellar seedlings of genotypes known to be resistant and/or susceptible to each pathogen were used to confirm that mixing the two pathogens did not cause a change in symptom development for either disease.