New research using remote sensing technology could hold the key to combating the spread of the plant disease myrtle rust.
Scientists at Crown Research Institute Scion have found a way to detect myrtle rust days before plants show signs of infection, providing hope that nurseries in the future can start control treatment sooner and stop disease outbreaks.
Led by data scientist Elizaveta Graevskaya, the Scion team used high precision equipment to detect myrtle rust infection in leaves of rose apple deliberately exposed to the myrtle rust pathogen.
Using thermal imaging the team detected decreases in leaf temperature in infected plants at least a day before symptoms could be seen. Transpiration measurements showed the temperature drop was caused by higher rates of water evaporation from the leaves as the fungal infection punctured individual cells, which cooled the leaves down.
The researchers also used a hyperspectral sensor to look at changes in the wavelength of light reflected from infected leaves.
The ratio of blue/green light reflected by infected leaves was noticeably different from healthy leaves up to three days before symptoms were visible. This change suggests that myrtle rust reduces the amount of chlorophyll in infected leaves. Chlorophyll is the green pigment that gives leaves their colour and absorbs energy from the sun during photosynthesis.
The results have prompted the team to expand their research to include other vulnerable species from the myrtle family, starting with eucalyptus in spring.
Research lead author and Scion principal scientist Mike Watt presented the findings as part of a webinar series delivered by Beyond Myrtle Rust – a collaborative research programme that Scion contributes to.
“This sophisticated technology has allowed us to detect myrtle rust infection before it can be seen visually in leaves,” Watt said.
“We hope this research can be used to develop a robust detection methodology that will benefit commercial nurseries in the future.”
Scion pathologists have been involved in research to understand the impact of the disease since it was first detected in mainland New Zealand in 2017.
While myrtle rust will be impossible to eradicate, Scion forest geneticist Heidi Dungey said this latest research is a major step towards arming nurseries with the tools they need to make better disease management decisions.
The technology also has potential to be used in the field, and at a much larger scale.
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.
Myrtle rust poses a threat to plants such as pōhutakawa, mānuka and rātā, as well as commercially grown species like eucalyptus.
Currently, myrtle rust has an uneven distribution across the North Island, and on the northernmost part of the South Island. It has also been reported in Christchurch.
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.
The research findings have been published in the international journal Phytopathology.