(Myrtle) rust never sleeps

I've spent most of my adult life working as a landscape gardener, and being a gardener means coping with weeds and plant pests and diseases.

Over the past half-a-century I've observed a steady stream of new weed plants, insects, fungi and viruses come and establish themselves here in New Zealand. A constantly changing ecosystem is the new normal.

On a farm up the Whanganui River in the 1960s, I remember the arrival of the "manuka blight", a black fungus on the bark and leaves of manuka shrubs.

Sheep farmers at the time naively hoped it would eradicate manuka, the "weed" that needed constant scrub-cutting, but no - manuka survived.

Being in the family myrtaceae, along with 100 other genera, manuka leptospermum scoparium is susceptible to the myrtle rust that has just blown across from Australia and has now established in nearby Taranaki.

The family gets its name from the common myrtle, myrtus communis, a hedging shrub with quasi-religious uses going back to ancient Greek and Old Testament times.

The only European member of the family, the common myrtle, has small white flowers with prominent stamens.

In Roman times, the common myrtle was associated with Venus, the goddess of love, and it was thought to stimulate genital mucus. To this is day it is used in European bridal bouquets.

Myrtle rust, puccinia psidii - or guava rust as it was known to begin with - is a fungus thought to have originated on guavas on the border between Brazil and Uruguay early in the 20th century before slowly spreading through South America attacking both native and exotic myrtle genera.

The rust was next observed in the Caribbean where it severely damaged the Jamaican allspice industry.

Allspice's West Indian cousin, pimenta racemosa, which yields bay oil, has escaped infestation. Another myrtaceus spice is the clove syzygium aromaticum, the dried flowers of which provide the cloves of commerce.

Originally from the Moluccas, near New Guinea, by the 19th century clove plantations had been established in Zanzibar where in the 1960s many were killed by a fungus disease simply called "sudden death".

It is not known if this was related to the myrtle rust.

In 1977 a new variety of the "guava rust" appeared in Florida on melaleuca quinquenervia, an Australian paper-bark which had naturalised in the Everglades.

In 2005 it materialised in Hawaii where it was called the "olia rust" and it quickly colonised all the Hawaiian islands. By 2005 it was in Japan and in 2011 on Hainan Island, China, and in 2013 it reached New Caledonia and South Africa.

In 2010, it had reached Australia, the "centre of diversity" of the myrtaceae family.

More than 45 per cent of Australian myrtaceae genera now have one or more species that hosts the myrtle rust (350 species infected). By 2015, myrtle rust was in Tasmania and this year it arrived in Aotearoa.

The rust attacks soft actively growing new leaves and very young stems and starts as small purple spots on the underside of the leaf followed by bright yellow spores or pustules on these purple spots.

Death is caused by repeat foliage die-back.

One of the common features of the myrtaceae family is that the leaves and fruit have glands containing essential oils.

One theory is that these essential oils are being exploited by this new puccinia fungus.
There are a range of fungicides that have been effective in a plant nursery context but even then they need rotating.

Once the fungus escapes into the wild, it is no longer controllable. It appears that some myrtle genera are more vulnerable than others.

In Australia, the rhodomyrtus species, the native guava and the bush turpentine have been decimated in the wild.

In New Zealand, the Australian ornamental trees and shrubs and the popular South American fruiting shrubs feijoa and guava are all at risk, as are the native myrtle genera kunzea, leptospermum, lophomyrtus, meterosideros and syzygium.

Manuka, kanuka, maire, ramarama, pohutukawa and the ratas all find themselves in the firing line of this a global fungal pandemic. Let's keep our fingers crossed that they pull through.

When Fred Frederikse is not building, he is a self-directed student of geography and traveller.