Pōhutukawa hit by myrtle rust infections, researcher says

Myrtle rust infections have been found on pōhutukawa trees in Auckland at unprecedented levels this summer, according to scientist Dr Maj Padamsee, of Waipapa Taumata Rau, University of Auckland and Manaaki Whenua - Landcare Research.

For the first  time, the disease has been reported on Rangitoto Island, home to the largest pōhutukawa forest.

Myrtle rust, caused by the fungal pathogen Austropuccinia psidii, infects plants in the myrtle (Myrtaceae) family. In Aotearoa, Myrtaceae include some of our most iconic trees, pōhutukawa, mānuka, kānuka, swamp maire, ramarama, and rohutu.

Summertime creates ideal conditions for this disease to thrive. In previous summers, the highly susceptible ramarama and rohutu, both species in the genus Lophomyrtus, were hardest hit. But serious infections have been reported on a broader range of Myrtaceae with each summer that passes following the arrival of the pathogen in 2017.

"Last season, we saw the disease take a toll on swamp maire," says Padamsee, leader of the Beyond Myrtle Rust research programme. "Anecdotal evidence from our researchers suggests that pōhutukawa have been hit much harder this season than in past seasons."

Reports of infected pōhutukawa in Auckland increased on iNaturalist, a citizen science platform. In addition, a recent survey of pōhutukawa infections completed by University of Auckland students working with researchers at Manaaki Whenua - Landcare Research showed that some trees are experiencing a particularly high level of infection.

"If this level of infection keeps happening every myrtle rust season, then we could potentially lose some of these trees," says Dr Padamsee.

The survey also noted infections on buds, flowers and developing seed capsules. "The real worry is that the infection might interfere with seed production," says Maj. "We’ve seen from ramarama and rohutu that once the bud and seedpods are infected, they don’t go through their reproductive cycle."

Myrtle rust is a slow acting disease. It may be that we don’t see the full extent of myrtle rust impacts for several years. But if the uptick in infection severity in New Zealand, not to mention the extinctions of endemic Myrtaceae being seen in Australia, is anything to go by, this is only the beginning.

"My daughter has learned songs about the pōhutukawaa tree at school," says Dr Padamsee. "I want her to be able to see these trees when she grows up."