By Heather Chalmers
Providing tools to assist the potato industry to manage and control Tomato Potato Psyllid is the focus of several research projects being undertaken by Plant & Food Research.
Plant & Food science group leader, annual crops group, Dr Gail Timmerman-Vaughan said it was five years into a six year MBIE-funded programme with three research aims.
Firstly, this was to understand the psyllid’s sensory response to sound, sight and smell. This could be used to devise tactics to control and manage psyllids, either at the level of monitoring, or by disrupting mating.
Another focus was to explore population genetics and relate this to disease development and Liberibacter presence.
“The potential for the psyllid to develop insecticide resistance is also being investigated as insecticide is such an important arsenal in its control,” she told a Potato Research Forum at Lincoln.
The third focus was potato breeding and understanding the plant response to TPP and Liberibacter.
A bacterium, Candidatus Liberibacter solanacearum (CLso), is carried and passed on by TPP. In tubers, symptoms are called zebra chip disease, which results in darkening of the potato chip when fried. The Liberibacter species was first identified in 2008, simultaneously by two research groups in New Zealand and the United States.
Plant & Food research associate Lee-Anne Manning said her work involved understanding aspects of TPP behaviour which could potentially be manipulated or disrupted.
“Kye Chung Park is leading our part of the research, and Jung Ah did much of the visual testing. I did most of the chemistry.”
“By looking at psyllids using an electron microscope we know they can see, smell and make noise. They have relatively large compound eyes indicating good eyesight.
“A close-up of antennae shows the sensilla main olfactory organ, and closer still the pores on the sensilla. The pores are a good morphological indicator for olfactory function, so this gives us strong evidence that they use smell to communicate.
“Single sensillum recording is a way we can record responses of TPP to odours. We identified odour compounds produced by host plants of TPP such as potato.”
“Of the 40 odour compounds tested, eight responded strongly, others less so.”
Electro retinnogram is where some electrodes are placed in the eye and record a signal when the insect is exposed to different wavelengths of light. This method showed stronger response to UV, blue, green, yellow and weaker response to red, orange and infrared.
Another visual test done was bioassay where they were able to expose TPP to different wavelengths and track individual insects. Again UV, yellow and green showed a change in behaviour, said Lee-Anne Manning. “When the yellow light starts they move more purposefully to the light. When the light goes off they go back to random movements.”
Future work will be to identify more active chemical compounds as well as wavelengths which appear to change TPP behaviour. In field or greenhouse trapping trials, traps will be used which combine visual, olfactory and auditory stimuli.
In related research, Tom Sullivan said that while looking at TPP under a binocular microscope he noticed some odd behaviour. “Their wings were vibrating quite fast, which was not flight related,” but instead produced sound vibrations in the plant.
He used a laser vibrometer to listen to psyllid calls between males and females. In trials, the psyllids can’t see each other or physically interact, but they can hear each other. “We believe they use sound to locate each other on the plant.
“As the male call starts this behaviour, we tested a synthetic male call to see if it could be used to manipulate TPP behaviour, which it did. Now we understand that the female call made in response to the male call is used by males to locate females on the plant.
“It is possible that a synthetic female call could be better at manipulating this behaviour and could potentially be used to lure and kill male psyllids, or for mating disruption,” said Tom Sullivan.
Psyllid DNA variation
Lincoln scientist Jessica Dohmen-Vereijssen, speaking on behalf of research leader Rebekah Frampton, said that in investigating genetic variation of the TPP population in the different potato growing regions of New Zealand, almost 400 psyllids were captured for mitochondrial DNA.
Psyllids were collected in early, mid and late season to determine if there was variability in the population within a season.
“This showed that there was more diversity in New Zealand than was expected from a single incursion.”
The DNA was compared with publicly available sequences from California and Texas, and the majority of New Zealand samples were identical to the Californian DNA sequence.
“As it was difficult for us to get our hands on the Californian psyllid, or its DNA, we went over there and collected psyllids ourselves. These haven’t been analysed yet.”
TPP was collected from Hawke’s Bay, Tasman, Pukekohe, Ohakune, Southbridge and Motukarara. Ten psyllid colonies were set up from these collections, each based on one female.
The next experiments will test the psyllid’s sensitivity to insecticides, as well as Liberibacter uptake and transmission.
“This is to see whether the genetic differences found in New Zealand TPP translate into something that is biologically meaningful,” said Dr Dohmen-Vereijssen.
Plant response to zebra chip
Lincoln-based scientist Dr Margaret Carpenter is researching potato plant responses to zebra chip disease through changes in gene expression.
The trial used Moonlight potatoes grown in a glasshouse. In one treatment, potatoes were infected using “hot” psyllids, which were infected with the Liberibacter bacterium causing zebra chip. Another treatment used “cold” psyllids which were uninfected and a control was also uninfected.
Potato plants were in mesh bags in which psyllids were released. After two days an insecticide was applied to kill the psyllids. “We are looking at the impact of the bacterium on the plant, rather than the psyllid.
“PCR testing for liberibacter confirmed that samples from plants exposed to ‘hot’ psyllids were infected, while samples from ‘cold’ psyllid and control plants were not, which is what we were aiming for,” Dr Carpenter said.
After seven weeks the leaf, stem and tuber were sampled for changes in gene expression. Changes were more evident in the tuber than the leaf or stem.
“There were similar numbers of increased and decreased gene expression because of the disease.
“Zebra chip disease increased glucose and fructose concentrations in diseased plants and there is a reduction in starch. There is a lot of fructose and glucose around, but the plant doesn’t seem to do anything with it.
“Big changes are involved in plants affected by zebra chip disease. Some of it is plant driven and some pathogen driven and we can’t distinguish between this.
“There are huge changes going on at a molecular level in diseased plants. This is just the tip of the iceberg and there is still a lot to understand about it,” Dr Carpenter said.