Mild strain cross protection against orange stem pitting strains of citrus tristeza virus (CTV) (CT03002)

This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

Citrus tristeza virus (CTV) was an aphid-borne virus that caused the most economically important viral disease of citrus. There were numerous strains of the virus (mild to severe) causing several diseases and exhibiting different symptoms in different citrus species.

One disease caused by CTV was orange stem pitting (OSP) which made infected trees unproductive. In Australia, the disease had only been found to occur in Queensland. Orange production was a comparatively small sector of the Queensland citrus industry but this disease would have had a devastating impact on national orange production if it spread to citrus growing areas in other states.

In most citrus producing areas, losses due to CTV were minimised through the use of tolerant rootstocks such as Poncirus trifoliata, citrange and rough lemon. In countries like Australia where CTV was widespread, the only available means of controlling the virus in susceptible scions was by mild strain cross protection (MSCP). Cross protection involved ‘pre-immunising’ trees with a mild strain of the virus to protect against more severe strains. MSCP had been used to effectively protect grapefruit trees against stem pitting in Australian orchards for the past 40 years.

There was little information on the number and diversity of strains that cause OSP in Queensland. Also, a suitable pre-immunising isolate had not been identified to protect sweet oranges against OSP. There was evidence that the mild CTV isolate used to protect grapefruits against stem pitting was only likely to be effective against one of the two OSP strains known to occur in Queensland.

In 2001, a field trial was established in Queensland to test a number of potential pre-immunising isolates. Each tree was assessed for stem pitting symptoms and sampled for biological and/or molecular analyses. Surveys were also been conducted in OSP affected areas to find new OSP or potential pre-immunising isolates.

Results confirmed a high disease pressure in the area of the field trial, including the two known OSP inducing isolates. There was also evidence that other OSP-inducing isolates were present in the field but were sufficiently different from known isolates to remain undetected by the existing assays. This meant that the researcher had no rapid method to detect OSP-inducing CTV, nor a full understanding of the diversity of OSP strains that occur in Queensland.

All field trees inoculated with mild strains were offered greater protection against OSP than the non-inoculated control trees. It was premature to recommend any of the pre-immunising isolates tested in this field trial for use in a cross protection strategy however this project had pinpointed some isolates with potential and some isolates that were unlikely to be of practical use in a field situation against OSP.

This work provided evidence that the genetic diversity of CTV in the field was greater than previously thought. More work was needed to characterise the isolates collected in this study. A more extensive and structured survey of Queensland orchards was also recommended to identify other OSP-inducing isolates.