Risk assessment re. introducing a rust
fungus (Puccinia araujiae) for the biological
control of Moth Plant (Araujia hortorum)
Jane Barton
Plant pathologist
1
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)
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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
Subfamily
Tribe
Subtribe
Genus
Araujia
Species
hortorum 1 (Target weed)
angustifolia 1
Oxypetalinae
Morrenia odorata 1
brachystephana 1
Asclepiadeae
Apocynaceae
Asclepiadoideae
Asclepiadinae
Oxypetalum
Asclepias
caeruleum 2
curassavica 3
Gomphocarpus physocarpus 3
Apocynoideae
Marsdenieae
Hoya
carnosa 4
Nerieae
Nerium
oleander 4
Mesechitaeae
Mandevilla
laxa 4
sanderi 4
Echiteae
Periplocoidaeae
Rauvolfioidae
Parsonsia
capsularis 5
heterophylla 3
praeruptis 5
Periploca
graeca 5
Vinca
major 4
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
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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
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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
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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