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

Puccinia araujiae causing disease on moth plant in lab

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?

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

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

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)

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

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

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

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

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

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

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

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

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