Christmas, summer and pōhutukawa are a holy trinity for many of us. In northern Aotearoa, pōhutukawa’s flowers, gnarled branches and cooling shade make it more temple than tree. This is a report of its likely decline.
The problem is myrtle rust, a fungal disease that could cause the mass death of pōhutukawa and other tree species belonging to the myrtle family, says Beccy Ganley, a plant pathologist with Plant and Food Research and science leader of Ngā Rākau Taketake, Saving our Iconic Trees from Kauri Dieback and Myrtle Rust. “I struggle to find pōhutukawa with no symptoms of infection. Previously there was the occasional lesion, but now we’re seeing the tips covered in rust. It’s got worse every summer.” The fungus thrived in last summer’s humidity, she adds.
The myrtle rust that blew here on westerly winds in 2017 is known as the “pandemic” strain. It has been sickening trees in Australia since arriving there in 2010, having spread from its native Brazil. Here, wind has carried it everywhere that’s warm and damp enough for its survival, including the entire North Island and the top of the South.
Our most susceptible trees are ramarama, rōhutu, and, most vulnerable of all, maire tawake, also known as swamp maire. It grows to mighty heights in always-wet areas and bears tiny white pōhutukawa-like flowers. Its fleshy kernels are the only seeds that researchers within Ngā Rākau Taketake haven’t figured out how to store.
Rust may kill all these susceptible trees in a few years, says Ganley. “There are already localised extinctions of ramarama.” She has watched the disease progress in Australia. “At least two myrtle species there are now functionally extinct [too sick to reproduce]. There are many other localised extinctions.”
Pōhutukawa is less susceptible, but that’s no guarantee of survival. “When similarly susceptible trees in Australia first got infected it didn’t seem to cause major problems. Then five years later, 300-year-old trees started dying. The Australians tell me the pōhutukawa symptoms remind them of those species.”
Ganley likens the disease’s slow progress to cancer. “It affects only the soft, new material – shoots, flowers, fruit. The tree can limp along for ages, but its canopy slowly thins. It is functionally dead when it’s not producing flowers or fruit. If you see lots of twigs at the canopy tips in winter, you can be 99% sure it’s myrtle rust.”
Efforts to contain the disease ended in 2018, but Ganley says there’s plenty to salvage. Getting rid of lilly pilly hedges would help.
“Hedges are a constant source of new growth, and lilly pilly is an incubator of myrtle rust that tosses spores everywhere.”
Other defences are possible – such as breeding resistant trees and developing a spray based on a natural resistance mechanism of cells called RNA interference – but will take years and need funding.
Ganley is calling for national co-ordination for seed banking, potentially growing refuges of uninfected seedlings of maire tawake in myrtle rust-free southern regions, and advising mana whenua and councils on the outlook for their trees. With pōhutukawa, that includes the loss of coastal erosion protection, shade and tourism opportunities, and the risk of falling branches.
A funding bid led by the Ministry for Primary Industries for a co-ordinated myrtle rust strategy was rejected by the government in 2019. The Department of Conservation last banked myrtle seeds three years ago and doesn’t know how long they remain viable. It will need significant funding to collect more. This research ends in June and there’s no guaranteed renewal of funding.
But the loss of our national Christmas tree? “I think most New Zealanders wouldn’t accept that,” says Ganley.
