Fiona Constable

A survey of key Australian pome fruit growing districts for 11 quarantinable pathogens was conducted using reverse transcriptase polymerase chain reaction (RT-PCR) and PCR methods. The targeted exotic organisms included two bacteria, two phytoplasmas, four viroids and three viruses. Except for Pear blister canker viroid, which has been previously reported in Australia, no other exotic pathogen was detected. RT-PCR assays were also used to detect four endemic viruses, Apple stem pitting virus, Apple stem grooving virus, Apple chlorotic leafspot virus and Apple mosaic virus. The results showed that each virus is widespread in Australia and that many pome fruit trees are infected with a combination of two or more virus species. The results also indicated that genetic variation occurs between strains of Apple stem pitting virus and strains of Apple stem grooving virus

Pear blister canker viroid (PBCVd) was detected in pear (Pyrus sp.), nashi (Pyrus serotina) and quince (Cydonia oblonga) trees from various pome fruit growing regions of Australia using dot-blot hybridisation and RT-PCR techniques. Characteristic symptoms of PBCVd infection were not observed on the infected trees. This is the first report of PBCVd infecting nashi varieties. The 12 Australian PBCVd isolates showed 92–99% nucleotide sequence identity with previously published sequences of PBCVd from other countries. The Australian isolates fell into two distinct groups based on sequence similarity and secondary structures. The possible origin of PBCVd in Australia is discussed.

The complete coat protein gene sequence is described for three isolates of Strawberry necrotic shock virus (SNSV) isolated from strawberry (Fragaria vesca) from Australia. Sequences for these isolates were found to have close identity to SNSV isolates from North America. This is the first report of SNSV from Australia.

Heteroduplex mobility assay (HMA) was used to determine if genetic variability existed amongst isolates of Australian grapevine yellows (AGY) and the Buckland Valley grapevine yellows (BVGY) phytoplasmas. Genetic variability was detected in two isolates of AGY phytoplasma from the same grapevine when they were compared with an AGY phytoplasma standard using HMA of the elongation factor Tu (tuf) gene. HMA of the tuf gene also showed that an isolate of papaya die back (PDB) phytoplasma, which is closely related to AGY phytoplasma, was different from the AGY phytoplasma standard. The two AGY variant phytoplasmas and PDB phytoplasma were indistinguishable from each other when they were compared using HMA of the tuf gene. No variability was observed amongst isolates of BVGY phytoplasma using HMA of the tuf gene.

Surveys were conducted in four Chardonnay vineyards for 3 to 6 years and one Shiraz vineyard for 3 years to determine the yearly percentage of grapevines affected by Australian grapevine yellows disease (AGYd), restricted growth disease (RGd) and late season leaf curl disease (LSLCd). In each of the Chardonnay vineyards in each year, all three diseases were characterised by remission of disease in some grapevines, recurrence of disease in other grapevines and new observations of disease in previously unaffected grapevines. The pattern of temporal incidence of each disease was different between vineyards for the survey period. Although Koch's postulates have not been fulfilled, phytoplasmas are considered to be the most likely cause of AGYd. While some grapevines exhibited a combination of AGYd and RGd or AGYd and LSLCd, both RGd and LSLCd can occur independently of AGYd. Statistical analyses using log-linear models also indicated that RGd and LSLCd were not always associated with AGYd. Thus, it is possible that phytoplasmas are not the cause of RGd or LSLCd and their association is coincidental. Expression of AGYd in Shiraz grapevines occurred later in the season compared to Chardonnay. Very little recurrence of AGYd was observed in the Shiraz grapevines indicating that the variety Shiraz responds differently to phytoplasma infections, assuming that AGYd in Shiraz is a phytoplasma caused disease. RGd and LSLCd were not observed on any grapevines in the Shiraz vineyard.

Three blocks of Chardonnay in one vineyard in the Buckland Valley of Victoria, Australia, were surveyed over 4 years for grapevine yellows disease (GYd). Buckland Valley grapevine yellows phytoplasma (BVGYp) was the only phytoplasma detected by polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) analysis in GYd affected grapevines. GYd affected many grapevines and was characterized by remission of disease, some recurrence and occurrences in previously unaffected grapevines. A regional survey of the Buckland Valley indicated that both GYd and BVGYp occurred in the same restricted grape growing area. Within this area, BVGYp was detected in two vineyards that had been established using planting material from different sources. One could therefore speculate that BVGYp was present in these grapevines as a result of aerial transmission and was not present in the original planting material.