Promising Pathogen for Cruel Climber
Moth plant rust with hyperparasite (Cladosporium) inset. Image - Freda Anderson
The invasive climber Araujia hortorum is pollinated by insects, especially moths, and so is usually referred to as moth plant in New Zealand. However, a secretion inside the flowers sometimes traps and kills these winged visitors, so it is also known as cruel plant. It isn’t just the insect-murdering reputation of this vine that makes it unpopular in New Zealand. Moth plant grows quickly, smothering desirable vegetation, and its milky sap is a poisonous skin irritant.
Towards the end of 2011 we were granted permission to release the first biocontrol agent for moth plant, a beetle (Colapsis argentinensis). Unfortunately releases of the beetle have not yet begun due to increased restrictions in South America around the exportation of native biodiversity. Confirming and meeting the new requirements is proving to be a protracted business. The earliest that beetle releases could begin would be spring 2015 and, paperwork permitting, a second agent might also be ready for release at that time.
A rust (Puccinia araujiae) was identified by Rolf Delhey and Mirta Kiehr, from Universidad Nacional del Sur in Bahía Blanca, Argentina, as the most promising pathogen found during surveys in the native range. More recently Freda Anderson at CERZOS, also in Bahía Blanca, has built on their work by confirming their findings about its life cycle. “The rust is microcyclic and autoecious, which means it only has two types of spores (some have as many as five) and it completes its life cycle on moth plant, without the need for any other host,” confirmed Freda. However, unfortunately the rust is commonly attacked in the field by another fungus, a hyperparasite. It has proven quite challenging to get rid of this unwanted parasite, which can quickly overgrow and kill rust colonies.
Fortunately Freda is no stranger to difficult lab work and is now well on the way to obtaining a clean colony of the rust and is confident of success. The rust only grows on living plants, so Freda took plant material with rust pustules on it from the field and surface-sterilised the tissue to get rid of other organisms. She then cut small (2.5 mm) discs of infected tissue and put these onto agar plates. Once telia (resting spores) had germinated on the plates the cleanest ones were selected and suspended over moth plant for 24 hours in an environment favourable for infection. This allowed healthy plants to be inoculated with basidiospores, and in due course new pustules would develop. Meanwhile, Freda kept the agar plates, and if the hyperparasite developed on them then plants infected with material from those plates were assumed to be contaminated and were destroyed. The remaining plants provided material to repeat the process, gradually reducing the hyperparasite infection. “But wouldn’t you know it, the hyperparasite was still there in my fourth generation!” reported Freda. “So I started all over again and I think I now have several new fourth-generation plants free of the hyperparasite.”
Meanwhile, Freda also completed the work begun by Rolf and Mirta to establish the host range of the rust. “We knew this pathogen would not be specific to just moth plant because it had been reported from several hosts in the field,” reported Jane Barton (contractor to Landcare Research). However, the reported hosts were close relatives (belonging to the same subtribe as moth plant, Oxypetalinae) not present in New Zealand. Not many plants in the same family as moth plant (Apocynaceae) are present and valued in New Zealand. Notable exceptions are native jasmine (Parsonsia spp.) and the favourite food of monarch caterpillars: blood flower (Asclepias curassavica) and swan plant (Gomphocarpus spp.). Luckily, testing showed these are not at risk. One plant that was not able to be tested is the minor ornamental tweedia (Oxypetalum caeruleum). Since tweedia is in the Oxypetalinae subtribe it is likely to be a host to the rust. The trade-off between protecting this plant and bringing a serious weed potentially under control will need to be weighed up as part of the EPA process. An EPA application is currently being prepared, which will be submitted by Northland Regional Council, on behalf of the National Biocontrol Collective, before Christmas.
This project is funded by the National Biocontrol Collective.
Contact
Jane Barton
Jane.Barton@ihug.co.nz