1. Risk assessment re. introducing a rust fungus (Puccinia araujiae) for the biological control of Moth Plant (Araujia hortorum)
Jane Barton
Plant pathologist
18 yrs. experience in using pathogens (disease-causing fungi) for biological control of weeds

2. Puccinia araujiae causing disease on moth plant in lab

3. Two most important risks re. introducing moth plant rust to NZ
Will it cause negative impacts on any desirable plant species (native or exotic and valued)
Could it displace or damage any native fungal species?

4. Background: Rusts as biocontrol agents
Worldwide 28 spp. of fungi have been released as classical biological control agents for weeds
Of these, 18 spp. (64.3%) have been rusts (Basidiomycota: Pucciniales)
Rusts particularly good biological control agents due to
High host specificity
Dry, air-borne spores (spread readily)
High virulence
There are about 7,800 spp. of rusts known (probably more exist) and at least 234 species of rusts already occur in NZ

5. Host specificity
Most plants are resistant to most pathogens because they have a set of defences against them e.g.
Chemicals toxic to fungi
Strong cell walls that act as a physical barrier
No two plant species have exactly the same set of defences

6. Host specificity
To cause disease a pathogen must have a set of aggressive features that can match and overcome all the defences of a particular plant e.g.
Specific chemicals that break down specific toxins
Enzymes that break down cell walls
They also need an environment that is favourable for infection (e.g. moist conditions)

7. Host range testing
Universally accepted method developed by Wapshere 1974
Main criterion for inclusion on test list = Taxonomic relationship to weed
Plants that are closely related have similar sets of defence mechanisms
Pathogen that can overcome defences of a given plant may need only one small genetic change to attack a close relative but would need many changes to attack non-relative
Thus, plant test lists start with closest relatives of target weed and progress outwards until host range understood

8. 1 = Tested in Argentina and found to be susceptible
Family
Subfamily
Tribe
Subtribe
Genus
Species
Araujia
hortorum 1 (Target weed)
angustifolia 1
Oxypetalinae
Morrenia
odorata 1
brachystephana 1
Asclepiadeae
Apocynaceae
Asclepiadoideae
Oxypetalum
caeruleum 2
Asclepiadinae
Asclepias
curassavica 3
Gomphocarpus
physocarpus 3
Marsdenieae
Hoya
carnosa 4
Nerium
oleander 4
Nerieae
laxa 4
sanderi 4
Apocynoideae
Mesechitaeae
Mandevilla
Echiteae
Parsonsia
capsularis 5
heterophylla 3
praeruptis 5
Periplocoidaeae
Periploca
graeca 5
Vinca
major 4
Rauvolfioidae
Rust
1 = Tested in Argentina and found to be susceptible
2 = Seed sent from NZ, plants died before they could be tested
3 = Seed sent from NZ , plants tested and immune
4 = Plants sourced and tested in Argentina immune
5 = Not tested
Did not test outside Apocynaceae family as no attack observed outside of Oxypetalinae subtribe

9. Expected impacts on Oxypetalum caeruleum (Tweedia)
Moth plant rust can probably complete its life cycle and cause disease on Tweedia
This is not a “spill-over-effect”. Tweedia doesn’t need to be near moth plant to become infected
Tweedia is a garden ornamental that attracts butterflies which feed on its nectar
It can be protected in gardens by rust-killing fungicides e.g. Yates fungus fighter (Myclobutanil)
If the rust isn’t a good biocontrol agent, it won’t do much harm to Tweedia: if it is harming Tweedia, probably working well as an agent

10. Safety of pathogens released as biocontrol agents internationally
To-date 28 Pathogens have been released in 38 biocontrol projects worldwide
The only negative impacts recorded in the field were minor damage to 6 non-target species (all of which was predicted in pre-release testing)
The only evidence of a pathogen changing its host preference since release was a narrowing of host range
Chances of increase in host range no more likely for introduced rust species than for a native one (in case of moth plant, less likely as less host relatives in NZ)
Thus, pathogens are a safe and useful tool for biological weed control

11. Could moth plant rust displace any native species of fungi?
Waipara et al. (2006) examined the pathogens associated with moth plant in NZ
Most disease symptoms observed were minor
16 species of fungi were identified
Only 2 were probably the primary cause of the disease symptoms observed: Colletotrichum gloeosporioides and a Microsphaeropsis sp.
These are mild pathogens of moth plant, indicating only a tenuous host relationship
As no host-specific native pathogens were found on moth plant, none could be displaced by Puccinia araujiae

12. Hybridisation
Hybrid the offspring of cross between two very closely related taxa
One possible risk of introducing Puccinia araujiae is that it might hybridise with a native Puccinia species
Two potential negative impacts of this:
hybrid rust might back-cross with parent and “pollute” genetic integrity of native rust
hybrid rust might have a different host range to its parents and thus pose risk to non-target plants
Hybridisation requires very close contact between the two parent species. That can only happen if they share a host plant. There are no other Puccinia species that occur on moth plant or tweedia in NZ
There is no risk of hybrid rusts forming on moth plant

13. Two most important risks re. introducing moth plant rust to NZ
Will it cause negative impacts on any desirable plant species?
Do not expect damage to any native plants
Will probably cause negative impacts on one exotic plant species (Tweedia)
Could it displace or otherwise interfere with any native species of fungi? No

14. EPA question re. genetic variation in moth plant in NZ
Some pathogens specific to subset of plants within a species
Researchers tested moth plant from 4 populations in Argentina and 3 populations in NZ
Rust infected some moth plant material from all sites
While less reps. than ideal, and at least 1 plant from most sites did not develop disease, these results were consistent with their results on other susceptible Araujia and Morrenia species
The researchers concluded that the moth plant rust was specific at the subtribe level, not at the species or sub-population level

15. EPA question re. Development of resistance
Pathogens are not like chemical herbicides
Continuous arms-race between pathogen and host so that as host evolves, so does the pathogen
Through thousands of years, rust symptoms on moth plant may become more or less severe
Resistant moth plant individuals may or may not arise through time
Meanwhile, there is no evidence to suggest that any moth plant present in NZ right now is resistant to the rust