In Birdwood Reserve, near where I live, there’s been an epidemic of some sort affecting kawakawa, Piper excelsum. Large plants lose their leaves and the roots seem to die off, because the whole plant falls over, as if wind-thrown. There are still plenty of small plants alive, so it's not wiping them all out:
Kawakawa is very susceptible to spray damage, and certainly some of these dead plants are alongside tracks where weeds have been sprayed lately. I’ve found the slightest spray drift of glyphosate is enough to wither kawakawa leaves and slowly kill the plants.
But some of these plants were growing where I don’t think there would have been any spraying, so I’m wondering if a disease is involved. Indeed one such dead plant is in my garden, where I certainly haven't sprayed, but where nevertheless quite a few plants (not just kawakawa, but also northern rata (Metrosideros robusta) and akeake (Dodonaea viscosa)) have suddenly died. The weather has been normal, except for the snowfall in August.
In cabbage trees (Cordyline) sudden decline is caused by a phytoplasma. This is an extremely small bacterium that's spread by sap-sucking leaf hoppers. So far it's been implicated in deaths of cabbage trees, strawberries, coprosmas, and Phormium (New Zealand flax) (Liefting et al., 2007). I've no reason to suspect any particular cause in this case, but the cabbage tree disease does show how devastating these things can be.
I'd be interested to hear if anyone else has observed similar collapse of kawakawa anywhere.
Lia W. LIEFTING, Ross E. BEEVER, Mark T. ANDERSEN, Gerard R. G. CLOVER Phytoplasma diseases in New Zealand. Bulletin of Insectology 60: 165-166 (2007).