Leu, Friend or Foe for Broom Biocontrol?
In May last year (Issue 64) we reported on the recent discovery of “Candidatus Liberibacter europaeus” (Leu), a new and possibly pathogenic bacteria affecting broom (Cytisus scoparius). This bacterium can’t be cultured and can only be identified using modern molecular techniques. It is thought to have been introduced along with the broom psyllid (Arytainilla spartiophila) in 1993. At that time, these organisms were unknown to science, although disease symptoms had been apparent in crops such as potatoes and citrus for many years, e.g. “zebra chip” potatoes in the USA. The discovery of these organisms, and the ability to identify them using molecular techniques, has led to an explosion of work worldwide as scientists try to protect valuable crops. Landcare Research scientists have been collaborating with Plant & Food Research and Lincoln University to learn as much as possible about how Leu affects broom plants and how it is transmitted. It is not clear whether damage seen on broom when significant psyllid populations are present is due to the psyllids (as previously assumed), Leu, or both. “A survey has shown that Leu is widespread on broom plants in New Zealand but only occurs in places where the broom psyllid has established,” said Simon Fowler who has been working on the project. Additionally, psyllids taken from the original collection sites in the UK were positive for Leu and the DNA matched the New Zealand Leu samples. “This leads us to believe that there is a strong likelihood that Leu did hitch-hike a lift to New Zealand on the broom psyllids brought out from the UK to control broom. However, at that time the insects were not able to be screened for this disease since we did not know it existed nor had the tools to find it,” said Simon.
“It is a bit of a double-edged sword,” explains Simon. “On the one hand we are pleased that the broom could be suffering due to the presence of Leu, but on the other, we have concerns that other psyllid species (either native or exotic) might probe the broom and become infected, which would potentially lead to other plant species becoming exposed to Leu. We have discovered that a range of psyllid species can be found on broom and kōwhai (Sophora microphylla), which is the closest native relative of broom in New Zealand. For example, despite being highly host specific, we have found broom psyllids sitting on kōwhai and surprisingly, kōwhai psyllids (Psylla apicalis) sitting on broom,” said Simon. There is no evidence to suggest that the broom psyllids are attacking kōwhai or that the kōwhai psyllids are attacking broom, but they may occasionally be probing these plants to test them out. “We detected Leu in two individual kōwhai psyllids that were collected from kōwhai plants, which, despite being only a small proportion of the total number of psyllids tested, is still cause for concern,” he added. It isn’t known whether these infected kōwhai psyllids would be able to transfer the pathogen to kōwhai. Investigations in collaboration with Plant & Food Research have been unable to detect Leu in kōwhai. “It is comforting to confirm that Leu isn’t prevalent in kōwhai psyllids, and even more comforting that we haven’t been able to detect Leu in any of the kōwhai tissue samples tested,” said Simon. “These are just preliminary results and more sampling is required to confirm our findings, but kōwhai has had over 20 years’ exposure to the broom psyllids so we might have expected to detect Leu and see symptoms by now if the psyllids had transferred it to kōwhai trees,” Simon said. A PhD student at Lincoln University has just started to investigate whether other psyllids in New Zealand are carrying novel indigenous “Ca. Liberibacter” species. So far only six species of Leu have been discovered worldwide, but there is a lot at stake because of their devastating effect on horticultural crops.
“A longer term question for us is whether we should continue to use sap-sucking insects like psyllids as biocontrol agents, and whether psyllids are likely to transfer “Ca. Liberibacter” from broom to other plants,” said Simon. To help answer the latter question and better understand how the psyllids vector “Ca. Liberibacter”, a series of experiments have been established at the Landcare Research campus in Lincoln. The aim of these experiments is to determine whether Leu is a harmless endophyte (symptomless) or not. Studies undertaken by Plant & Food Research have shown that a similar bacterium (“Candidatus Liberibacter solanacearum”) can be symptomless in some plant species but not in others. The Italian scientists that originally discovered Leu in pear trees consider it to be a harmless endophyte. If this is the case, the implications for biocontrol concern whether we are able to predict which plant species are likely to be symptomless and which are not. If psyllids do not depend on the presence of the “Ca. Liberibacter” to cause damage to weeds, it will be possible to line-rear imported psyllids prior to their release to eliminate any trace, but this will be costly and time-consuming. “So far the experiments have not given us any defi nite clues, but then it is not known how long it takes for the disease to establish in the plants or how long it takes for the symptoms (if any) to become apparent,” concluded Simon. In any case, the experiments will need to run for some time, since the psyllids reproduce slowly with only one generation a year and to ensure that the test plants are infected.
This research was funded by the Ministry of Business, Innovation and Employment as part of Landcare Research’s Beating Weeds Programme.