Phytophthora is Greek for 'plant destroyer', and it's hard to think of a more apt name; phytophthora dieback is currently running rampant in every corner of the globe, infecting and killing thousands of species of tree, as well as other plants. Australia is one of the countries most grievously affected by this silent menace, with entire forests becoming infected and native tree populations taking a huge hit. But what is it? Are your trees vulnerable? And what can you possibly do to stop its spread?

Defining Dieback

It is important to separate phytophthora dieback from other kinds of dieback; the word dieback is often used as a generic term for large and sustained drops in tree populations, but can be caused by any number of factors, from invasive weeds to excessive soil salinity. Phytophthora dieback is a distinct phenomenon with a specific cause, but this does not make it any less dangerous or difficult to prevent.

Phytophthora dieback takes its name from the organism that causes it, a microscopic water mould called phytophthora cinnamomi. These tiny, fungus-like colonies inhabit the soil and move from tree to tree and plant to plant by 'swimming' in the groundwater. Once they come across the roots of a plant they are able to infect, they attach themselves and grow into the root itself, absorbing cellulose and other carbohydrates and effectively destroying the structure of the roots from within. Eventually an untreated root will die and rot, denying the rest of the plant the nutrients it absorbed, and spreading bacterial and fungal infection from the rotting sections into the rest of the plant, causing a swift death.

Tree species vulnerable to phytophthora dieback

The following list is by no means exhaustive; phytophthora cinnamomi is an invasive species in Australia, and the full extent of the trees and plants it can infect has not yet been fully catalogued. However, some of the more common trees susceptible to dieback are:

• Jarrah: phytophthora dieback has been referred to as Jarrah dieback in the past, and this hardy native tree is particularly vulnerable, especially in plantations.
• Blackbutt: the other major species of eucalyptus at risk.
• Hakea: several subspecies of these small, shrub-like trees are vulnerable, including the Fan and Hood Leaved Hakeas.
• Woody Pear: the related Sandplain Woody Pear is also at risk.
• Snottygobbles: closely related Spreading Snottygobbles are vulnerable

It should be noted that Western Australian species are, for whatever reason, particularly vulnerable, and the majority of dieback cases are concentrated there. If you are raising trees in WA, take particular care.

How can I prevent/cure a phytophthora infection?

There is currently no cure for phytophthora dieback, and honestly it doesn't look like we're even close to one. The Department of Parks and Wildlife is currently researching the use of phosphite to boost a tree's natural defences against phytophthora infection, but this is not yet approved for public use, and has had limited success in trials.

As such, prevention is by far the most effective tool when it comes to keeping your trees uninfected:

• DO NOT take wild soil from anywhere, not even if it's only from a few miles away, and not even if your soil is deeply unsuitable for growing trees. Soil cross-contamination is how the disease has spread so far and so quickly, and without it the organisms are forced to migrate manually through groundwater, significantly slowing the spread of infection. Importing soil from overseas already carries with it enormous legal penalties, so avoid that too.
• Taking clippings, seeds or even firewood from wild forests is an excellent way to bring phytophthora into your own backyard; follow local bylaws regarding firewood collection to the letter.
• If possible, use steam-pasteurised soil and mulch, either professionally pasteurised or done yourself.
• Keep all pruners, shears and secateurs scrupulously clean to prevent infection spread.
• An infected tree may not show any symptoms before is suddenly and inexplicably dies, but if you notice any unusual yellowing of leaves, or darkening of the roots, take precautions (isolate the tree if at all possible). Infected trees should not be destroyed out of hand, as they have a chance to recover—a clearly dying tree, however, should be destroyed and disposed of very carefully.
• Observe and obey any travel restrictions or quarantine zones imposed by local authorities, and keep to footpaths when out in the bush. Do not travel in forests or other heavily wooded areas for several days after heavy rain.

For more information, contact Dowling Tree Services.