History in New Zealand
The mist flower fungus native to Jamaica and Mexico. It was used in Hawai'i in a successful biological control programme against mist flower in the 1980s. Manaaki Whenua – Landcare Research imported it from Hawai’i and released it at nine sites with serious mist flower infestations in the top part of North Island (Northland to Waikato) late in 1998.
How would I find/recognise it and what is its lifecycle?
You are most likely to see plants infected with mist flower fungus during spring and early summer because the optimum conditions for infection are warm temperatures (16°C to 20°C) and high humidity. In ideal warm and moist conditions, infected plants will develop angular reddish-brown lesions with yellow margins on the upper surfaces of leaves in about 12–14 days.
If you turn these leaves over, the undersides of each lesion may have a powdery white appearance because large numbers of white spores have been produced there. These characteristic white spores are why the fungus is sometimes called a white smut fungus. These spores are spread by wind and water (streams, rain splash or on wet animals). As the disease progresses, the lesions on the upper surfaces of the leaves join together and the entire leaf can become dark brown.
There is another fungus in the genus Phoma sp. that causes similar disease symptoms on mist flower in New Zealand, but it does not produce white spores. Unless the characteristic white spores of mist flower fungus (hence the sometimes used alternative name, mist flower white smut fungus) are present, only an experienced plant pathologist will be able to tell symptoms caused by the two fungi apart.
How does it damage mist flower?
The mist flower fungus causes leaves to die and fall from the plant prematurely. These attacks weaken infected plants and – after several years – will cause them to die, leading to a decline in mist flower cover.
Will it attack other plants?
The mist flower fungus will only attack mist flower (Ageratina riparia). In laboratory tests, the closely related Mexican devil weed (Ageratina adenophora) developed slight disease symptoms, but the fungus was unable to complete its life cycle on this host. In the field in New Zealand, Hawai'i and South Africa, no disease symptoms have been seen on Mexican devil weed or any other plant species, even when the plants were growing beside infected mist flower.
How effective is it?
Mist flower fungus established and caused rapid damage at all nine sites where it was first released at in 1998 and then spread rapidly across mist flower infestations over the top of North Island. Within 2 years it had reached Great Barrier Island. Within 5 years it was present at mist flower sites all over the North Island without having been officially released near them, including Napier to the east, New Plymouth to the west, and Wellington in the south.
The proportion of live mist flower leaves infected by the fungus reached peaks of over 90%, in monitored plots and levelled out at nearly 60% of live leaves after a few years. However, even in heavily damaged plants, the very young leaves, growing tips and stems were largely unaffected, leaving a resource for a second biocontrol agent– the mist flower gall fly – to attack. Because mist flower fungus thrives in damp conditions and warm (but not hot) weather, levels of attack on mist flower are especially impressive in spring, at times leaving affecting all mature leaves.
Monitoring from 1998 to 2003 showed that repeated spring/summer attacks by the fungus (before the mist flower gall fly became widespread) were associated with reductions in the average percentage cover of mist flower in dense infestations from 81% to 1.5%.
In 2008, the mist flower fungus was infecting 55% of live leaves, despite the average percentage cover of mist flower being reduced to <1% in New Zealand. However, by this stage, the mist flower gall fly (introduced in 2001) was also widespread in the monitoring plots. Recent observations suggest that these levels of attack by the mist flower fungus (and the mist flower gall fly) are ongoing with mist flower present as widespread, scattered plants.
How can I get the most out of it?
It is not necessary to spread mist flower fungus in the North Island. However, if you do come across any infestations of mist flower that are uninfected (e.g. on outlying islands, or in the South Island), then it may be worth acting to establish a population.
How do I select a release site?
Mist flower fungus thrives in damp conditions and warm (but not hot) weather.
Read the Guidelines for selecting release sites for biocontrol agents.
How do I collect it for release at other sites?
To move the fungus, soak/agitate infected leaves for a few minutes in a small amount of water (e.g. 2 cm in the bottom of a bucket), and then spray the spore suspension onto uninfected plants. This spore suspension should keep for up to 30 hours if refrigerated.
You may also be able to infect mist flowers by:
- dragging a bundle of infected stems through wet, uninfected plants
- wiping a damp sponge over the underside of infected leaves, and then wiping both sides of uninfected leaves with the sponge on the same day. If you try this method, be sure to keep the sponge cool and wet during transit.
If transfer has been successful, characteristic white smut pustules should appear on the underside of mist flower plants after about four weeks.
How do I manage the release sites?
Avoid any activities that will interfere with the mist flower fungus, such as herbicide application. If you need to undertake control measures, then avoid the release site.
