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

2Puccinia 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

Family

Apocynaceae

Subfamily

Apocynoideae

Asclepiadoideae

Tribe

Asclepiadeae

Subtribe Genus

Marsdenieae

Oxypetalinae

hortorum 1 (Target weed)angustifolia 1

Species

Araujia

Morrenia odorata 1

brachystephana 1

Hoya carnosa 4

Asclepiadinae Asclepias

Gomphocarpus

curassavica 3

physocarpus 3

Nerieae

Mesechitaeae

Echiteae

Nerium oleander 4

Mandevilla laxa 4

sanderi 4

Parsonsia capsularis 5

heterophylla 3

praeruptis 5

1 = Tested in Argentina and found to be susceptible2 = Seed sent from NZ, plants died before they could be tested3 = Seed sent from NZ , plants tested and immune 4 = Plants sourced and tested in Argentina immune5 = Not testedDid not test outside Apocynaceae family as no attack observed outside of Oxypetalinae subtribe

Periplocoidaeae

Rauvolfioidae

Periploca graeca 5

Vinca major 4

Rust

Oxypetalum caeruleum 2

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

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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

12Hybridisation

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

13Two 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

14EPA 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