Risk to New Zealand Myrtles
Myrtle rust poses a major threat to New Zealand’s native biodiversity and ecosystems given that many native species rely on healthy trees for their survival.
New Zealand has 37 native Myrtle species that could be affected my Myrtle rust. This includes pōhutukawa, rātā, mānuka and kānuka.
There are also many exotic species such as feijoas, bottlebrushes and lilly pilly.
Species used for hedging (such as Syzygium) create lots of dense young growth that comes from repeated pruning. This may contribute towards creating an ideal environment for the fungus to rapidly grow and spread.
Recently, there have been fairly high levels of infection found in Metrosideros species (pōhutukawa, rātā and a range of hybrids).
If there are high impacts on keystone forest species, like pōhutukawa and rātā, other native flora and fauna in those ecosystems are also likely to suffer significant impacts.
Myrtle rust attacks soft, new growth, including leaves, fruits and flowers. The potential that it can destroy the food relied upon by some species of native birds, lizards and insects is significant.
We believe that this past year, thanks to drought conditions and a mild winter, has been the perfect breeding ground for Myrtle rust to spread exponentially. Myrtle rust reproduces rapidly, in as little as two to four weeks and so we saw 2-3 cycles of it in November/December.
The long term effect that Myrtle Rust has on tree species is that they are not able to regenerate and due to repeated infections over time can eventually die. Continued infection weakens their immune systems, which can expose them to other diseases, as the plant needs to put resources into fighting the disease instead of growing.
Myrtle rust also makes plants more susceptible to secondary infections, in the same way that people with an immune deficiency may contract illnesses more easily than normal. Any secondary infections from pests, like possums or insects, could be the tipping point for these affected plants. Also, environmental conditions like drought can, have a devastating effect on an infected tree.
Moving forwards from here, in conjunction with a treatment plan, we need to look at how we can contribute towards reducing the amount of Myrtle rust.
One way that we can do this it to look at reducing impact through our plant selection. We can no longer, in good conscience, recommend or plant Syzygium plants.
Through our own observation and discussions with others in the horticulture industry we have found Syzygium to be the prime host for Myrtle rust, amongst a range of other pest species such as Wax Scale and Psyliids.
If we look at replacing these with more resistant species, we should (hopefully) see a big reduction in the presence of Myrtle rust. If you are interested in looking at alternative hedging options, please get in touch with us.
From the Team at Second Nature Gardens