Smut for Tradescantia Soon
Tradescantia infected with the yellow leaf spot fungus. Image: Robert Barreto
The three beetles released for the biological control of tradescantia (Tradescantia fluminensis) continue to go from strength to strength in New Zealand. However, there has been an instance already of beetles released in a riparian area in Northland, thought to be high enough up to be well out of harm’s way, being affected by an extreme flood. As well as these infrequent catastrophic events, there are plenty of tradescantia-infested areas that are regularly flooded in which it will be difficult for the beetles to be effective, and so we are still considering introducing a yellow leaf spot fungus that may be better suited to such soggy situations. Tradescantia typically bounces back from remaining fragments quickly after a flood, but beetle populations take much longer to recover. However, fungi tend to thrive in damp conditions and infection may build and spread in only weeks or months.
We have referred to this smut-like fungus previously as Kordyana sp., but recently it was officially named as Kordyana brasiliensis. Spores germinate on the upper surface of tradescantia leaves, as long as some moisture is present, and about 10 days later yellow leaf spots appear on the upper leaf surface and these develop, expand and turn brown as the infected leaves die. In another 15 days the spot centres turn white underneath, and if humidity is high enough, new basidiospores form there. Basidiospores are very small, and are easily spread through air turbulence, and the whole cycle starts again as soon as they land on a new tradescantia plant.
We obtained permission to release the yellow leaf spot fungus in New Zealand in 2013. However, because of funding constraints, we have been releasing and monitoring the beetles first. The jury is still out on whether we need the fungus but, because of the flooding issues, there is a good chance it will be needed here. “In Brazil we most commonly saw the yellow leaf spot fungus close to waterways where the beetles were less common, probably because of flooding,” said Simon Fowler, who has led the project. Meanwhile, the Australians have been investigating the potential of the yellow leaf spot to control tradescantia there.
In Australia a weed needs to be declared a biocontrol target by the Invasive Plants and Animals Committee (IPAC) before applications to release agents against it can be submitted. Tradescantia was approved as a target by IPAC in December 2015. The smut fungus had already been applied to a range of plants closely related to the target weed in Brazil and found to be highly host specific. “However, those tests were performed for New Zealand so some plants of significance in Australia were not included,” explained Louise Morin (CSIRO), “and so further tests were required by our authorities”.
The tradescantia yellow leaf spot was imported into the CSIRO containment facility at Black Mountain, Canberra, in July 2014, and a culture established on Australian tradescantia plants. Plant species to be included in host range testing were chosen according to recent molecular phylogenies of the family Commelinaceae, to which tradescantia belongs. The fungus was applied to seven species that had previously been tested in Brazil (including the target weed) and 22 additional species or cultivars of relevance to Australia (ornamental plants, weeds and native taxa). “Results confirmed that the yellow leaf spot fungus is highly host specific and able to damage all of the tradescantia accessions we tested,” revealed Louise. She is now working on an application to release this fungus in Australia. “We hope to submit it by the end of August this year.”
It is reassuring the Australian tests confirmed that tradescantia leaf spot is highly host specific, and handy that we can now obtain it from just ‘over the ditch’. This will potentially be easier than getting it from South America, especially since this smut fungus does not do well in artificial culture and needs to be transported quickly inside living plant tissues. All in all, the tradescantia yellow leaf spot looks like a very promising biocontrol agent for both Australia and New Zealand.
The tradescantia project in New Zealand has been funded by the National Biocontrol Collective and in Australia by the Department of the Environment. We acknowledge the assistance of Robert Barreto and Davi Macedo, at the University of Viçosa, Brazil, who have helped to find, test and supply the yellow leaf spot fungus.