Photo: Supplied / Manaaki Whenua / Landcare Research
New research using remote sensing technology could help in the fight against myrtle rust.
Myrtle rust was first detected in New Zealand in 2017 and over time infected trees can die because they cannot regenerate.
Pōhutakawa, mānuka, rātā and commercially grown species such as eucalyptus and feijoas are at risk.
Crown Research Institute Scion scientists have successfully detected myrtle rust in nursery plants days before the plants show signs of infection.
Early detection would let nurseries start control treatment and stop the disease in its tracks.
Principal scientist Mike Watt said high precision equipment was being used to detect myrtle rust infection in leaves of rose apple deliberately inoculated with the myrtle rust pathogen.
The technology senses differences in leaf temperature and light wave lengths in infected apple trees.
"The canopy temperature we found in infected plants was actually lower because the pathogen punches the leaves, which means the leaves lose more water, they transpire more, which cools the leaves down.
"And the other feature was that the canopy temperature was more variable, in spots on the leaf, so it wasn't uniform across the leaf compared to the control plants which weren't inoculated.
"So just using those two variables, we were able to classify all of the plants with 100 percent certainty before the before we could actually see the see the disease on the plant."
Watt said in terms of the hyperspectral imagery, they could look at changed in the wavelength of light reflected from infected leaves.
"One blue green index is basically an indicator of chlorophyll levels. And it showed that chlorophyll was being reduced in the infected plants. It could actually allow us to detect the disease, think it was three days before we saw any symptoms. So once again, pre-visual detection was possible,' he said.
Photo: Supplied / Manaaki Whenua / Landcare Research
It could be a couple of years before the technology was available for commercial nurseries because testing was now needed on species like eucalyptus and pōhutakawa, he said.
Because the thermal and hyperspectral equipment used by the Scion researchers can be mounted on drones, the team hopes to one day develop field-based methods to detect myrtle rust infections rapidly and remotely.
"There are certainly systems operating in Europe that use thermal and hyperspectral data at large scales to detect diseases such as xylella fastidiosa in olive orchards in Spain. And those are very successful at pre-visual detection. So I don't see any reason why we couldn't scale up to that level, get to operate those sensors from a fixed wing aircraft normally," he said.
Forest pathologist and research group leader Stuart Fraser leads the ecology and environment team investigating myrtle rust. He describes the latest research as "incredibly promising."
"As part of a wider programme of research, we've been monitoring myrtle rust's impact and seasonal progression across the North Island for several years," he said.
"It's important that we throw everything at it and use a range of available technologies to accelerate research and management, so we can reduce myrtle rust damage to our most vulnerable plants and landscapes."
Currently, myrtle rust has an uneven distribution across the North Island, and on the top part of the South Island. It has also been reported from Christchurch. It was most likely to be seen during warm, wet conditions.
The disease causes bright yellow-orange powdery pustules on young leaves, shoots, fruits and flowers in the myrtle family, causing deformation of the leaves, and twig dieback.
Repeated severe infection can cause decline or death of large trees.
