Host plant selection principles
- Rust fungi are biotrophic (require living plant material to grow) and are mostly highly host-specific
- There are no New Zealand native species in the sub-family Asclepiadoideae
- The nearest related native genus is Parsonsia, in the sub-family Apocynoideae
- There are three native Parsonsia species, but no other native species within the family
- There are a range of species with horticultural value within the subfamilies Asclepiadoideae and Apocynoideae.
- The closest relative to moth plant is the minor ornamental tweedia (Oxypetalum caeruleum)
- The best known relatives are the swan plants, Gomphocarpus fruticosus and Asclepias curassavica, which have social value as host plants for butterflies
- Other less related species with horticultural value include hoya, nerium and mandevilla
Given the narrow host range of rust fungi, and the low representation of this family in the native and exotic New Zealand flora, relatively few test plants are required to define the host range of this rust
Preliminary host range testing
Experiments continue. Below are the results obtained to date. For each of the tested species, the number of plants that developed rust pustules over the total of inoculated plant is given:
| No. developed rust pustules | No. tested | |
| Araujia angustifolia | 6 | 6 |
| Morrenia odorata | 6 | 7 |
| M. brachystephana | 6 | 6 |
| Asclepias curassavica | 0 | 12 |
| Gomphocarpus physocarpus | 0 | 14 |
| Hoya carnosa | 0 | 3 |
| Nerium oleander | 0 | 5 |
| Mandevilla laxa | 0 | 1 |
| Parsonsia heterophylla | 0 | 5 |
| Vinca major | 0 | 4 |
Note: In all experiments plants of A. hortorum were included as positive controls.
Plants of Araujia megapotamica and A. sericifera have not been available for testing so far. On request, only macroscopic symptoms have been examined so far.
Preliminary conclusions
- Results indicate that the host range of this rust is restricted to species in the sub-tribe Oxypetalinae.
- One species of Parsonsia, P. heterophylla, was tested to represent all three members of this genus. This is the only genus of plants native to New Zealand which is closely related to the target weed. P. heterophylla was found not to be susceptible to the rust, so it is unlikely that the rust would damage any native plants in New Zealand.
- A final report on testing will be available by November 30, and will replace this interim report.
