What Do Zebra Chips Have to Do with Broom Biocontrol?
Zebra chips. Image - Plant and Food Research
Let’s start with “zebra chip” potatoes. In 2009 a team of scientists, led by Lia Liefting of the Ministry for Primary Industries (MPI), examined tissue from hot-house tomato (Solanum lycopersicum) and capsicum (Capsicum annuum) plants that had been attacked by the tomato potato psyllid (Bactericera cockerelli) and had disease symptoms. In the USA and other countries, the tomato potato psyllid was known to be associated with diseases that led to “zebra chip” (literally light and dark stripes in chips), but no causative pathogen had been identified. Exhaustive testing for a range of possible pathogens, including fungi, bacteria, viruses, viroids and phytoplasmas, drew a blank. Transmission electron microscopy was used by MPI to look at sections of leaf tissue and revealed a possible culprit. A bacterium-like organism, restricted to the phloem of the plants, was discovered that proved to be unculturable, i.e. it refused to grow on medium in a Petri dish like other bacteria. DNA was extracted from the plant tissue containing the unculturable bacterium, and molecular tests determined that this was a novel species closely related to Liberibacter species that are vectored between plants by psyllids (sap-sucking insects in the family Psyllidae). This was the first time that the link had been made between the tomato potato psyllid, a pest that arrived in New Zealand in 2006, and a disease-causing organism. It was named ‘Candidatus Liberibacter solanacearum’ (Lso) – the “candidatus” term referring to its unculturable nature, as bacterial taxonomy requires organisms to be cultured.
The tomato potato psyllid has caused major damage to tomato and potato crops, particularly in the northern parts of New Zealand. Plant and Food Research (PFR) scientists are actively researching ways to manage these serious problems. They have discovered that the pest psyllid has spread Lso into several other plant species. These include the weed boxthorn (Lycium ferocissimum) and native poroporo (Solanum aviculare). Intriguingly, Lso does not so far appear to behave like a pathogen in poroporo, i.e. it seems to be a symptomless “endophyte” – an organism that can live within a plant without causing any harm. Nevertheless, both the exotic and indigenous plant species act as reservoirs for Lso from which tomato and potato crops can potentially be infected via their shared psyllid vector.
There are now six species of Liberibacter known worldwide, all vectored by different psyllid species, and they are associated with some serious plant diseases. ‘Ca. L. asiaticus’, ‘Ca. L. americanus’ and ‘Ca. L. africanus’ are associated with citrus greening in Asia, the Americas and Africa. This disease has been known for nearly 100 years but was only associated with Liberibacter with the advent of molecular genetic techniques. Early in 2011, an Italian group reported the pear psyllid pest (Cacopsylla pyri) hosts and vectors ‘Ca. Liberibacter europaeus’ (Leu henceforth). The Italians believe that Leu in pears also behaves as a symptomless endophyte, rather than a pathogen, because despite high counts of the bacterium in the pear plant tissue, no specific disease symptoms could be observed in the infected plants, at least in the short term.
Discovering Lso was a major scientific breakthrough by the staff at MPI together with PFR. This work has big implications for the horticultural industry and globally this is “hot science” at the moment. But what does this have to do with broom biocontrol? In 2011, during routine sampling of potato crops in Canterbury for Lso, PFR scientists saw what they thought were typical Liberibacter symptoms on nearby Scotch broom (Cytisus scoparius) plants. The symptoms included wilting, stunted growth of shoots, shortened internodes, leaf dwarfing and leaf curling. A large population of broom psyllid (Arytainilla spartiophila) was also noticed on the plants. The PFR scientists tested the broom plants and the psyllids for Liberibacter species, including Leu (the Liberibacter species discovered by the Italians in pears), although Leu was unreported from New Zealand. To their surprise, DNA extracted from both organisms revealed signatures characteristic of Leu.
Landcare Research, PFR and MPI immediately collaborated on a “delimiting survey”, which indicated that Leu was widespread. No incursion response from MPI was therefore justified. The survey also showed that broom was only positive for Leu in areas where the broom psyllid was present, consistent with the hypothesis of broom psyllid being the vector and introduction route for Leu into New Zealand.
What does this discovery mean for New Zealand? Simon Fowler explains: “While the broom psyllid is highly host specific, Leu is not, and incidental probing by the psyllid (to ‘taste’ if they have the right host plant) might represent a risk of transfer to non-target plants.” Such spillover effects would be minor unless there was another psyllid species that could then pick up Leu and transfer it widely to the population of non-target plants. As a precaution, Landcare Research decided to survey the nearest native relative to broom, kōwhai (Sophora microphylla), and the host-specifi c kōwhai psyllid (Psylla apicalis). Leu could not be found in either kōwhai or its psyllid, even in areas where the broom psyllid was common on nearby broom and both were positive for Leu. “Given the length of time that Leu has been in New Zealand, it would most likely have had time to become evident in the kōwhai population if transfer was at all probable,” said Simon. Simon added, “... of course we don’t know whether Leu, even if it got into kōwhai, would act like a pathogen or behave like a symptomless endophyte.” Furthermore, Landcare Research scientists are unsure whether the Liberibacter symptoms claimed to be seen in broom in New Zealand are a result of the bacterium, or the direct result of attack by the psyllid and the broom twig miner (Leucoptera spartifoliella), which was also very common at the original site sampled in 2011. “However, even if Leu was harmless in broom that doesn’t mean it would be harmless in other plants. We can see from Lso causing serious diseases in tomatoes and potatoes, but not in poroporo, that the disease nature of these bacteria is unpredictable,” said Simon. There are still a great many unknowns and much research to undertake.
A priority is to understand what pathogens other psyllids in New Zealand are carrying. New Zealand has a rich native fauna of psyllids, as well as many self-introduced species that mostly attack Acacia and Eucalyptus species from their original home in Australia. There may be many more Liberibacter species yet to be discovered! Other key research questions include whether psyllids should be completely avoided in future as potential biocontrol agents because of possible associated organisms, and whether the action of the disease-causing organisms would mean that if it was possible to “clean up” the psyllids they would have reduced effectiveness as biocontrol agents. Landcare Research and PFR scientists are collaborating to work on these sorts of questions, and are looking at developing a new set of risk assessment protocols that would include screening of biocontrol agents for Liberibacter species.
For further information on tomato potato psyllid in New Zealand see Controlling the tomato/potato psyllid in the home garden.
Landcare Research’s involvement in this project is funded by the Ministry of Business, Innovation and Employment as part of the Beating Weeds Programme.