This article first appears in: Protecting our native plants, The Gardens Magazine, Issue 145, Winter 2025,
pp. 22-23.
Native plants in Australia face many threats to their survival, with some easier to control than others. One of the very worst is a group of species of Phytophthora (pronounced fy-TOFF-thora), a name made up of two Greek words put together meaning “plant destroyer”. Although these plant pathogens used to be considered fungi, they are water moulds or oomycetes (pronounced oh-o-MY-seets). One species in particular, Phytophthora cinnamomi, has done considerable damage in sclerophyll forests and heathlands in south-western and southern Australia. It’s currently a risk to the survival of more than 200 threatened plant species and more than 30 threatened ecological communities in Australia. It doesn’t only attack native plants, though, but also exotic garden and park plants, as well as agricultural and horticultural crop plants. This species is listed as a key threatening process under NSW and Federal law.
Phytophthora infects healthy plant tissue or recently wounded tissue, not dead or decaying plant matter. As it’s a soil borne pathogen, it can’t be observed. Its symptoms can’t be easily distinguished from the symptoms of some other plant pathogens and pests, or even from nutrient and water stress, so it’s not easy to diagnose. By the time its symptoms can be seen, it’s often too late to save the plant. Its name is very appropriate!
All the species of Phytophthora have a similar life cycle. They are all soil-borne pathogens and can spread through soil water, including irrigation water and floodwater. Some species have above-ground reproductive parts and can be spread by wind, rain or splashes of water as well. They can be dispersed by humans through any activity that moves infected soil, water or plant material, such as construction, plant propagation, harvesting plants, using off-road vehicles, bushwalking and gardening.
Different Phytophthora species can infect different parts of plants. Some species cause root rot, a very common disease, which in turn can lead to dieback. In the wild, dieback can then reduce the amount of habitat and food available for native animals. One devastating species of Phytophthora causes late blight of potato and was partly responsible for the 19th century Irish potato famines. It’s still a major worldwide problem in potato and tomato crops.
One of the most common requests the PlantClinic at the Sydney Botanic Gardens gets is to identify whether Phytophthora is present in the roots of plants with dieback, as Brett Summerell describes. The Botanic Gardens, however, has some self-interest in testing for Phytophthora too. All plants coming into the gardens for planting need to be free of it, as do threatened species plants that are grown up at the Gardens for relocating into the wild to conserve threatened populations. The plants sold to the public at the Gardens also need to be pest-free.
The traditional testing procedure used by the Gardens to detect Phytophthora was very time-consuming. It involved trying to culture Phytophthora in plant material that was exposed to the soil to be tested. After a week or so, cultured material was transferred onto a lab agar plate to grow further. Finally, what was found on the plate was visually identified using a microscope. All this took over a month, was inefficient and not always accurate. Even though this process was enhanced over time by using DNA-based identification, it was still very time-consuming, laborious and still not accurate enough. Botanic Gardens research found that up to 30 per cent of plants produced for urban greening projects were infected with Phytophthora and this also added urgency to the search for a much better testing procedure.
Given how serious a threat Phytophthora is, finding a quicker, cheaper and more effective testing procedure has been a priority for the Gardens. Recently, they’ve partnered with a commercial outfit, EcoPath Solutions, and developed a greatly improved testing procedure. The new process allows batches of potted plants, soil samples, water samples and potting mix components to be tested at one time. Items to be tested are put in a sealed chamber and watered by a computer-controlled irrigation system to maximise production of Phytophthora spores, which are then collected and incubated to provide enough biomass for testing. The actual test is then done by analysing DNA from the spores directly or by using a procedure exactly like a COVID-19 rapid antigen test (RAT), except the RAT is for Phytophthora. This new testing routine can be done in about a week and with greater accuracy. The plan is to set up one of these testing stations at each of the three gardens and, in future, to encourage the nursery and restoration industries to take it up.
Checking samples for Phytophthora in labs is important but so is finding where it occurs in the ground out in the wild. We’ve all heard about drug detection dogs and truffle-sniffing dogs. It turns out that there are also Phytophthora detection dogs. Spaniels are being used in trials in NSW and WA to determine whether they can sniff out Phytophthora. So far, it seems that they can find plants infected with a given pathogen they’re trained to sniff out and possibly even distinguish between some different Phytophthora species. Using DNA-based methods to detect the pathogen over large areas is prohibitively expensive but these clever spaniels may become a useful first-pass tool to detect it in the field.
Many of us are no doubt familiar too with the boot cleaning stations that have been set up in some areas where soil-borne pathogens occur in an attempt to prevent people from spreading the pathogens on their boots. These stations usually have brushes to remove soil and disinfectant to kill pathogens. The effectiveness of this approach apparently had not been experimentally tested until Edward Liew, from the Australian Institute of Botanical Science at the Sydney Botanic Garden, carried out tests in 2021. He looked at two types of disinfectant and two types of brush with two soil types, each either wet or dry, all in various combinations. The results were a little surprising. The overall conclusion was that all hygiene measures were better than doing nothing but some combinations were only effective with sandy or dry soils. None of the measures worked completely effectively when boots had wet, loamy soil on them. As some hygiene practices are not effective under some conditions, this has implications for management of Phytophthora. One important practical outcome of these tests was that the best portable hygiene kit for hikers to use, where no other equipment is provided, is a stiff brush to remove soil from boots and a spray bottle of 70 per cent methylated spirits.
While the distribution of Phytophthora is better known from WA to Victoria, it is also present in NSW. There are some useful maps of its possible distribution in NSW and native species susceptible to it here. The Botanic Gardens does offer some general information about it and how to deal with it here. The PlantClinic at the Royal Botanic Garden will test samples for a fee. Once Phytophthora has arrived somewhere, it’s very difficult to treat. The best advice seems to be vigilant with preventative measures, as there is no cure.