Do not risk poor health in your orchard

An orchard showing signs of stunting

Figure 1: Stunting in an orchard caused by a severe strain of citrus exocortis viroid, as a result of untested budwood propogation.

Symptoms of bark scaling

Figure 2: Symptoms of bark scaling with severe strain of exocortis viroid.

Nerida Donovan, NSW DPI

'Biosecurity' is more than a catchy buzz word. It can make or break an industry.

Citrus industries overseas are crumbling under the force of devastating citrus diseases such as Huanglongbing (HLB). We are lucky not to be battling HLB here in Australia, but we do have many diseases that have the potential to wreak havoc if plants are propagated using infected material.

Figure 1: Stunting in an orchard caused by a severe strain of citrus exocortis viroid, as a result of untested budwood propogation.

Many virus and viroid diseases can cause stunting, yield loss and even death in some scion and rootstock combinations, yet other varieties could be symptomless carriers. For most graft-transmissible diseases, symptoms will not be seen in nursery trees – the signs will appear a few years later in the orchard.

By that time, the disease is likely to have spread to surrounding trees through root grafting or on cutting tools.

Nothing can be done to save infected trees; you need to pull the trees out and replant. The only way to be sure of what's in your budwood and rootstock seed is to get it from a tested source, as supplied by Auscitrus.

Tim Herrmann and a committee of experienced growers and nurserymen manage Auscitrus, a not for profit industry organisation whose primary focus is to protect the Australian citrus industry from disease.

Graft-transmissible diseases are spread through infected propagation material and via sap on cutting tools. The Auscitrus source trees are managed using strict biosecurity protocols and are routinely tested for a range of graft-transmissible diseases.

Independent testing is provided by the NSW Department of Primary Industries citrus pathology team led by Dr Nerida Donovan, which is based at Elizabeth Macarthur Agricultural Institute (EMAI).

Detecting these diseases can be difficult because the disease particles could be present below detectable levels and the particles might not be spread evenly through the tree. The NSW DPI citrus pathology team is working on an industry-funded project managed by Horticulture Innovation Australia (HIA) developing improved detection methods for graft-transmissible diseases.

Figure 2: Symptoms of bark scaling with severe strain of exocortis viroid.

There are more benefits to using Auscitrus material than avoiding graft-transmissible diseases. Auscitrus checks budwood source trees for trueness to type and maintains budwood source trees in a secure facility where all pest and disease issues are managed to the highest standard.

There are many blocks throughout Australia that have low productivity due to the questionable source of budwood, such as a neighbour's trees. Sourcing your own budwood can save 30 cents, but a poorly producing tree over a production lifecycle can cost you over $2000 from reduced yield.

All Auscitrus budwood shipments can be traced beyond the source trees, back to the original mother tree held in the National Citrus Repository. New variety imports and local selections are maintained as 'foundation trees' in insect-proof repository houses at Auscitrus and EMAI, and are tested regularly to ensure their high health status.

The maintenance and testing of public varieties in the repository is funded by an HIA grant to Auscitrus who contract NSW DPI to perform the testing. Private variety owners pay a fee to hold trees in the repository; at least one of each scion variety in each location.

Next time you are buying nursery trees ask where the budwood and rootstock seed came from. Planting untested trees is not worth the risk.