Another White Smut in Shining Armour?
Tradescantia yellow leaf spot.
The white smut fungus Entyloma ageratinae has been a very successful biological control agent against the weed mist flower (Ageratina riparia) in New Zealand.
Now we have another white smut fungus (Kordyana sp.), which looks like an excellent potential biocontrol agent for tradescantia (Tradescantia fluminensis) to add to the insect attack (see Tradescantia Beetles Gain Momentum). Although a white smut, this fungus causes large distinctive yellow spots to form on the leaves so is commonly referred to as the tradescantia yellow leaf spot.
A list of promising plant pathogens found during surveys of tradescantia in Brazil has been slowly whittled down to this one species by a team of researchers, led by Drs Robert Barreto and Davi Macedo, at the University of Viçosa, Brazil. Some organisms proved to be too benign (e.g. a Ceratobasidium sp.), one species requires further research (the bacterium Burkholderia andropogonis) and other organisms were just too hard to work with in the laboratory/glasshouse (e.g. a rust named Uromyces commelinae).
However, the yellow leaf spot has provided its own set of challenges which have had to be overcome. After much effort, the researchers at Viçosa were able to grow the yellow leaf spot on artificial media, but as the fungus really prefers living plant tissues the fungal structures produced in culture were unable to infect plants. Field observations showed the fungus had no trouble moving from plant to plant on its own so initially, host range tests were designed to take advantage of this natural spread.
62 non-target species, from 31 different plant families, were included in host range tests. Test plants were placed in a shade-house in Viçosa with infected tradescantia plants. Tradescantia plants propagated from material originating in New Zealand were also included as positive controls (i.e. the test would only be valid if the target plants became infected under the conditions provided). New Zealand tradescantia plants showed symptoms one month after they were placed in the vicinity of infected plants in the shade house, but none of the other test plants became infected. “These results demonstrate that the yellow leaf spot is highly host specific,” reported Dr Barreto.
Meanwhile, research continued in the laboratory and a method was developed that allowed the spores to be applied more directly to test plants. Leaves of infected tradescantia plants were collected and attached (with Vaseline) to the underside of a sheet of glass with the leaf surface pointing downwards so fungal spores could fall on to test plants below. This sheet of glass was then placed over the test plants in a dew chamber where ideal conditions for infection were provided for 48 hours. Then plants were transferred to a greenhouse for observation. This direct method was used on 13 species in the Commelinaceae family which had already been tested in the shade-house experiment. Happily, the results were exactly the same, with only Tradescantia fluminensis proving to be susceptible.
An application to the Environmental Protection Authority to import and release the yellow leaf spot is now being prepared. However, even if approval is given there will be some challenges to overcome in achieving a clean colony of the yellow leaf spot. The fungus will not produce fully functional spores on artificial media, and it is not possible to collect and ship spores produced naturally because of their tiny size and fragility. So the yellow leaf spot will have to be imported as actively growing colonies on living tissues from which it will be difficult to exclude other micro-organisms. Fortunately, construction of a pathogen-proof containment facility is now underway in Auckland so we will be able to safely import this material and over time obtain a clean colony which can be released.
This project is funded by the Department of Conservation, National Biocontrol Collective, and the Ministry of Science and Innovation under the Beating Weeds Programme.
Contact: Jane Barton
Jane.barton@ihug.co.nz