Treasured trees face silent killers
The garden's such a jungle. Just when we get used to one nasty disease, along comes another to spoil the fun.
The latest one hitting the headlines is a killer fungus, Chalara fraxinea, ash dieback, attacking English ash, Fraxinus species.
Although the disease has been known in Europe for 20 years, after it was first identified in Poland in 1992, it has now been found in several locations in Britain, where it threatens to wipe out the indigenous ecology.
Just last week the virulent killer was also found in ash trees in Northern Ireland, where it threatens what remains of their native trees.
Now, you might say, "so, why should we care?". While we grow ash here, it's an exotic import, and you might say we should be more concerned about our indigenous forests. And quite rightly so. But it's the message it tells that's important for us.
And, like it or not, we also have lots of ash trees in this country - which, if nothing else, would prove expensive to remove if the disease arrived here and killed them. Years ago, when dutch elm disease arrived in Auckland, the main cost of the incursion was that of removing thousands of dead and dying trees that posed a potential risk from falling.
But, cost aside, the loss of trees from disease of any kind in their native habitat is alarming and of concern, as we should know from the cabbage tree killer and, even more alarming, the disease now killing our iconic kauri.
If you haven't caught up with the news, kauri are falling victim to an insidious fungal-like killer called Phytophthora taxon Agathis, or PTA, as it's been dubbed. It's insidious because it's so small and usually shows up in the tree when it's too late to help, not that there's much that can be done to save the giants of the forest succumbing to this creepy and creeping killer.
But phytophthora is nothing new. It was Phytophthora infestans that caused the potato blight and infamous Irish famine of the 1840s. What's new is the type specific to and killing our native and endemic (found only in New Zealand) kauri, Agathis australis.
Alarmingly, the kauri killer presents a greater threat than that killing cabbage trees, which is caused by a phytoplasma (micro-organism) transmitted by sucking insects, most likely passionvine hopper. Phytophthora can be present in the soil for years, and kauri take so much longer to grow (and be replaced) compared to cabbage trees.
And, just like the fungus killing the English ash, scientists say PTA's recent advent indicates that it was probably introduced to, rather than originating in, New Zealand.
Similarly, dutch elm disease is thought to have been introduced to Europe, then America, where it affected the indigenous elm species of each country. Its origins are thought to have been in Asia, where the indigenous species, Ulmus parvifolia, is largely resistant, although not immune, to the disease. The explanation given is that Ulmus parvifolia probably developed resistance because it co-evolved in the presence of the disease.
It's Darwinism at work - survival of the fittest.
It was the same with the potato famine. The Irish potato crop was wiped out because there was no resistance in the stock. Plants were genetically all the same, with the same susceptibility, because they were grown from tubers of the same plant. Much later, when the disease was understood, resistant varieties were found among wild plants in South America, where potatoes originated.
What's alarming about the ash disease is that when it was first noticed in Britain earlier this year, in a plant consignment from the Netherlands, it was assumed to have been unwittingly imported from Europe with the nursery stock. However, more recent outbreaks scattered through woodlands in other parts of England suggest spores may have been windblown from Europe.
Where the disease started is anyone's guess, but applying the co-evolution theory and looking for the location of any naturally resistant species may hold the key to its origins and its potential control.
The same may be true of the kauri outbreak. Initially, blame pointed to kauri trees on Great Barrier Island which, in the 1970s, succumbed to an unidentified disease, now attributed to PTA. However, earlier introduction of the pathogen could have come in the 1940s with trees that are close relatives of kauri, planted in Northland's Waipoua kauri forest. Did they bring PTA with them?
Possibly no-one will ever know just where it came from. And, given that our kauri is one of a kind, albeit with close relatives throughout the Pacific, and that PTA is so host-specific to kauri, finding disease-resistant survivors may be improbable.
As northern Maori elders, scientists and conservationists all lament, finding a way forward is a race against time. Strategists report that the micro-sized enemy is but a mere 500 metres from and potentially closing in on the giant kauri Tane Mahuta, father of our forests.
Tane Mahuta is estimated to be more than 1000 years old, was already mature when many Maori arrived, was a big tree when Magna Carta was signed in the 13th century, and was even bigger when Abel Tasman skirted our shores. Standing before Tane Mahuta and looking up (to the canopy, and the heavens, if you like) is a truly humbling experience.
Maori say the kauri forest is "tired" and global warming, predicted to bring direr weather to the kauri forests of the north of the North Island, is no help. Scientists say kauri will be particularly vulnerable to such conditions because the trees tend to grow on ridgelines, where the soil is dry, and have shallow root systems, which are vulnerable to changes to soil moisture.
And it's the roots where the unseen enemy gains entry to its benign host.
Tellingly, research on trees in the disease hotspot of Auckland's Waitakere Ranges showed that 70 per cent of affected trees were next to walking tracks. Walkers' shoes scuffing the exposed roots on tracks were implicated in not only damaging the vulnerable roots but also potentially inoculating them with the pathogen, which can survive in just one gram of soil.
Cleaning stations have been installed on some tracks and boardwalks built in sensitive areas, such as that on the track to Tane Mahuta. In other areas, tracks have now been closed.
Hope may lie with the rare Australian wollemi pine, Wollemia nobilis, one of the closest relatives of the kauri and in the same Araucariaceae family. Described, like kauri, as a "living dinosaur", the wollemi was discovered in a remote canyon in the Blue Mountains in 1994, with fewer than 100 specimens in the wild. Since then, some trees have also been found to be infected with Phytophthora cinnamomi, a commonly occurring phytophthora affecting a wide range of plants, including many common garden species. It is believed the disease was introduced to the Wollemi area on human footwear.
Since the wollemi were discovered, their exact location has been kept a secret and access is controlled. To protect the trees, the public can only see wollemi in cultivation.
Perhaps closing access to the kauri forests is the only hope for the trees' survival. Their uncertain future has been described as a potential national tragedy, and is certainly grim. Teams of people are working collaboratively on the challenge, which may yet beat them.
For more reading, go to: kauridieback.co.nz; rbgsyd.nsw.gov.au/plant-info/wollemi-pine.
Check out the kauri in Anzac Park off Rutherford St and the Ulmus parvifolia in Buxton Square, outside Clearmount House.
JOBS TO DO
Remove suckers from the base of grafted trees, removing those below the grafted union which are growing from the rootstock.
Prune camellias after flowering to remove spent blooms and developing seed pods and open the bushes up for aeration, and to allow sun on the shoots and encourage bud formation for next spring.
Mulch rhododendrons, azaleas and camellias to retain soil moisture and protect roots from the hot summer sun. Feed them plenty of nitrogen-rich bulky fertiliser to boost new shoot growth.
Look for subtropicals such as avocados, tamarillos and papaya in the garden centres to plant in the garden, now that the risk of frosts is (hopefully) past. Tie long shoots on climbing roses to prevent wind breakage. Keep harvesting broad beans and peas to ensure ongoing pod production.