Trees in trouble

Can Australia and New Zealand save their native myrtles? (12 minute read)

Some years ago, I remember an ominous sight in my garden. My fruit trees were smothered with blossom and everything was growing vigorously in the spring sun. But, although there had been no shortage of rain, two of my grapevines were wilting. One was growing in a dry spot, but if a lack of water was the problem, then the other vine, growing at a low point which collected water in wet weather, shouldn’t have been suffering. If wet weather had caused a root rot, then the grapevine in the dry spot shouldn’t have been affected.

The third grapevine in my garden, though, was as vigorous as ever. Seeing that, I knew what the problem was, and that my two wilting vines were as good as dead. My healthy vine was Albany Surprise, a hybrid with both American and European grape parentage. My dying grapes were European varieties. The fact that my hybrid grape was healthy told me I had an insect called phylloxera, which feeds on the roots of grapevines, in my garden. It kills all varieties of European grape, but doesn’t trouble American or hybrid varieties, even though it does feed on them.

What happened in my garden was a microcosm of what happened in Europe in the late 19^th century. Phylloxera entered my garden on a grapevine cutting someone gave me. It entered Europe when people took American varieties of grape to Europe. Soon after the American varieties were introduced, European grapevines began to die. In France, it was a national catastrophe. Growers tried everything to save their vines, but vineyard after vineyard was obliterated.

The only reason we have old European grapes today is because the treasured European varieties were grafted onto American or hybrid vines. Now, all wine grapes and many table grapes are grafted onto phylloxera-resistant rootstock. However, as I learned, table grapes sold to home gardeners are not necessarily grafted, and so remain vulnerable.

The story of phylloxera is a tragic reminder that plant pests and diseases can be devastating when they reach new areas and find plants which have no resistance to them. It’s far from the only example either. There are numerous tales of disastrous outbreaks – potato blight, chestnut blight, Dutch elm disease, spongy moth¹… there are many more too, although more obscure.

Until recently, New Zealand had largely escaped the kinds of outbreaks seen in the Northern Hemisphere. Most of our problems were with introduced mammals such as possums, rather than insect pests or diseases. However, we did have one narrow escape, when an insect arrived from Australia and began attacking mānuka trees in the late 1930s. Farmers were delighted, because mānuka was at that time widely regarded as a weed. They spread infected plant material around the country, and mānuka plants were soon dying in many areas. But, almost as soon as it began, the outbreak subsided. The insect itself came under attack from a type of fungus. By the time mānuka became a valued species once again, for its role in forest restoration, for its ornamental value in gardens and for its honey, few people remembered that it had nearly been lost.

In the last 20 years, however, two damaging diseases have affected native plants. The first is kauri dieback which, as its name indicates, affects our precious kauri trees. It’s a disease which I’m planning to look at another time, so I won’t go into it here. The second is the rust disease I wrote about last week.

When the rust was detected in Australia in 2010, they decided not to refer to it as eucalyptus rust or guava rust, instead using the name myrtle rust. At the time, this was because Australian scientists believed they had a particular variant of the fungus which had first been described on the common myrtle. Later research demonstrated that the fungus in Australia was the same as that in Hawai’i and a number of other countries. But the name myrtle rust stuck in both Australia and New Zealand. Given that it only infects plants belonging to the myrtle family, myrtle rust is a sensible common name. Although I wrote about guava rust last week, this week I’m writing about what has happened since the disease arrived in Australia, so I’ll use the name myrtle rust.

Common myrtle. The dead branch isn’t caused by myrtle rust, although this plant did have a few infected leaves (image credit: Melanie Newfield)

Before myrtle rust arrived in Australia and New Zealand, there were things we could predict about its likely behaviour, but we couldn’t be certain. To understand why this is, it’s helpful to consider the disease triangle. This is a concept developed and used largely by those who study diseases of plants, but in fact it’s relevant to all kinds of infectious disease, and it should probably be learned by vets and medical doctors as well. It also works to explain insect pest outbreaks such as phylloxera.

As its name suggests, the disease triangle has three parts. The first is the agent of disease, known as the pathogen. In the case of myrtle rust, the pathogen is a type of fungus. The second part is the host plant. Without a host plant, there is no disease. Rust spores² blowing on the wind are not a disease. Myrtle rust can’t be grown on a dish in a lab, but if it could be, then that wouldn’t be a disease either. It’s only the interaction of a pathogen and a host which produces disease. The third part is the environment in which both the pathogen and host occur. This is less intuitive, but important. For example, rust spores need moisture to germinate and infect their host plants, so are often worse in humid conditions. Myrtle rust also has some specific temperature preferences, which means that in New Zealand it’s mainly a problem in the coastal regions of the North Island, for now.

When a pathogen such as myrtle rust arrives in a new area, it may encounter host plants it has encountered before, such as lemon-scented gum, but it encounters them in a new environment. Scientists can make a detailed comparison between the environmental requirements of a fungus and the conditions in a particular place, and use this to make predictions. But it’s an involved process, so while both Australia and New Zealand did this investigation for myrtle rust before it arrived, it can’t be done for every possible new arrival. A pathogen moving to a new area also encounters new hosts, and what will happen with those is even harder to predict. It’s possible to do testing in a laboratory, as is done for biological control. But this kind of testing doesn’t always give definitive answers. It’s one thing to rule out a biological control agent jumping onto native and crop plants. It’s quite another to predict the behaviour of a species such as myrtle rust, which has repeatedly proven its ability to jump onto new hosts in the myrtle family.

Australians, then, watched with dread as myrtle rust began to spread in late 2010. They had studies proving the environment along the eastern coast was perfect for it. They knew it would jump onto new myrtle species. But which species would it infect? And, among those infected, which would just have a few leaf spots, as the ‘ohi’a in Hawai’i did, and which would be devastated?

It didn’t take long for some species to be in real trouble. The native guava and scrub turpentine, both small trees growing in wetter areas from Brisbane to Sydney, went from common to critically endangered within a few years. Although the rust didn’t infect older leaves, new growth was heavily infected, so as older leaves died, they weren’t replaced. Both plants almost completely stopped producing fruit and many wild plants died. While careful cultivation and use of fungicides may keep these species from total extinction, it’s not going to save them in the wild.

Other species began declining too. At one rainforest site in south-eastern Queensland, the canopy was almost entirely made up of nine trees in the myrtle family, with two more trees which grew above this canopy, also in the myrtle family. Neither of the trees which grew above the canopy were affected, but eight of the nine canopy trees suffered serious damage. The ninth species was infected but less damaged. Before myrtle rust, the forest canopy was 94% myrtle species. However, only 63% of the young trees and less than 40% of seedlings belonged to the myrtle family. Unless we can find a way to control myrtle rust, the rainforest will be irrevocably changed.

Since its arrival in Australia in 2010, myrtle rust has infected more than 380 native species. Some of these species have only been infected in cultivation, and their natural habitats are largely unsuitable for myrtle rust. They will survive. But for others, the future looks dire. At least 15 rainforest trees, many of them formerly common, are in danger of becoming extinct in the wild. At least another 30 are declining.

So far, however, one group has barely been touched – the gum trees. This is good news, but it’s unclear entirely why that should be. Is it that the variant of myrtle rust which reached Australia doesn’t affect them? Laboratory testing suggests that is probably part of the story. Is it the environmental conditions they are living in? Perhaps – myrtle rust is at its most damaging in the wettest areas, where gum trees are replaced by other members of the same family. Also, as with all hosts, new growth is most affected. It’s possible that after fires when the gum trees sprout from their burned trunks or there’s a flush of seedlings, myrtle rust may prove a problem.

In the end, it took 6 years from when myrtle rust was detected in Australia to when it was found in New Zealand. Once it arrived, though, it behaved as it had in Australia – jumping onto native plants in the myrtle family. Swamp maire, already rare because so many swamps in New Zealand had been drained, was pushed to the brink of extinction. Rātā moehau, with only 13 trees remaining in the wild, was already at the brink. Cultivated plants have been infected with myrtle rust, putting the species in even greater peril. Three common shrubs, including ramarama, a popular plant used as foliage in flower arrangements, were also badly affected. All three joined swamp maire and rātā moehau on the list of New Zealand’s most endangered species.

So far, myrtle rust has been reported infecting 13 of our native species. As well as the species I’ve mentioned, it has infected pōhutukawa and a number of types of rātā – it’s not yet clear how much damage it will do to these. However, it hasn’t been reported on any species of mānuka or kānuka. It’s likely that these have been spared.

What can be done about a species such as myrtle rust? There is a large body of knowledge about managing fungi which infect plants, including rust diseases, but most of this knowledge comes from crops. Much of it won’t help protect wild plants. Fungicide sprays, for example, can be used to keep cultivated plants alive and therefore save species from extinction. However, there are major drawbacks to applying them in natural forest, including the risk of fungi becoming resistant, the practicalities of spraying trees in remote areas and the impact on native fungi, which are important for the survival of other native plants.

Since the arrival of myrtle rust in Australia and New Zealand, there has been a huge amount of research. Some of that research has focused on understanding the rust better, because it’s hard to control a disease without knowing about it. Some of that research has focused on the immediate steps needed to save the most endangered plants, such as fungicide treatments or how to store seed and preserve other plant material for propagation if seed storage isn’t possible. But some of the research has looked at longer-term solutions, such as biological control.

Just as the disease triangle can help understand how plant diseases occur, it can also help to manage diseases. While we most often think of controlling the pathogen, there are approaches which focus on the host and environment as well. For example, researchers are looking carefully at the host plants. For once-common species, they are doing genetic analysis to make sure that they are preserving as much of the natural variation in the species as possible. They are also monitoring carefully to see whether some plants are more resistant than others – perhaps by selecting the most resistant plants, species can be re-established where they have been lost.

Researchers have also looked carefully at where in New Zealand myrtle rust grows best and matched that against the distribution of susceptible plants. Some species, such as southern rātā, mostly grow outside the areas most suitable for myrtle rust, so even though they can be infected they are not in serious danger. For species such as ramarama and swamp maire there are currently few places they grow which aren’t suitable for myrtle rust. However, there areas they can grow where myrtle rust won’t, so these areas could be used for restoration – as long as they aren’t areas which will become more suitable under climate change. Some parts of Australia are likely to become too warm for myrtle rust in the years to come, but, unfortunately, myrtle rust is only likely to get worse in New Zealand.

I’ve dug through much of the research which has been done since myrtle rust arrived in New Zealand, and it’s impressive. In the last few years, we have learned so much. But we don’t have a solution. To be fair, there probably isn’t a single solution – it’s going to take a range of approaches, depending on the location and the species at risk. Still, my point stands. Neither New Zealand nor Australia knows how we are going to save our native myrtles.

But there’s another threat which I have to mention. In New Zealand, much of the credit for the progress which has been made must go to the Ngā Rākau Taketake research programme, which was begun in 2018 and linked with one of New Zealand’s National Science Challenges. And, like all the National Science Challenges, the funding for that programme ended this year. So far, there’s no plan for what might happen next. I can only hope that further research is funded before it is too late.

1. Spongy moth used have another name, but this old name was a racial slur so there’s been a recent effort to change it. Spongy moth is a translation of the French name for the moth, which has sponge-like egg masses.

2. In case you missed it last week, spores are like seeds but for fungi. They are tiny and powdery and in the case of rust fungi, are often reddish-brown so look a little like rust. However myrtle rust has yellow spores.