The team also had to ensure that the extract and the root-symbiotic microorganisms also inhibit relevant soil-borne pathogens in asparagus and basil – in this case Fusarium species. ‘In laboratory tests, we found that it had a good inhibitory effect,’ says Gillmeister. So it was time for a field trial. A practical partner was found in the Winkelmann asparagus farm in North Rhine-Westphalia, and the company Agraforum supported the project from an economic perspective.

Practical field testing takes time

In the first field trial, the researchers immersed the root balls of annual asparagus plants in suspensions of rhubarb root extract or microorganisms, as well as a combination of both, prior to planting. The microbial suspension contained both fungi of the genus Trichoderma and bacteria of the genus Bacillus – certain isolates of both genera are known to colonise the root zone of plants and exert beneficial effects there. The team proceeded in a similar manner in the second field trial. Here, however, the researchers used young plants, which were also dipped in the suspensions before planting and later watered with them again.

‘Over the course of the project, we regularly took root and soil samples to check whether the microorganisms had established themselves well in the root zone,’ explains Gillmeister. In fact, all relevant strains were detectable in sufficient density even over the winter. Sufficient colonisation of the rhizosphere and the soil influenced by the roots with beneficial microorganisms is a prerequisite for them to interact successfully with the plant and protect it from fungal pests.

Due to the short project duration of two years, the team has not yet been able to detect any differences in growth or the occurrence of disease symptoms when comparing the treated plants with the control plants. ‘The asparagus plants from the first field trial were not harvested until this year, after the end of the project,’ the researcher regrets. However, the asparagus farm will continue to monitor the trial plots. ‘The next two years will show whether the treatments were successful,’ Gillmeister is confident. If the treatments prove successful, the proposed method will provide a good basis for further research.

Mixed results, but interesting potential

Initial positive results have already been achieved in parallel pot trials. In asparagus, but also in basil, no toxic damage to the leaves was observed when the plants were treated with rhubarb extract. ‘This is also a fundamental prerequisite for further work,’ says Gillmeister. The most promising results were obtained in pot trials with young asparagus plants: When an asparagus plant is attacked by soil-borne harmful fungi, this is evident from leached or lysed roots. ‘This was observed in the untreated control plants,’ reports Gillmeister. In contrast, the plants that were treated individually or in combination showed significantly less root damage.

However, no disease-suppressing effect was found in the pot experiments with basil, neither through the microorganisms, nor through the extract, nor through a combination of both. ‘Instead, we observed increased plant growth as a side effect,’ says Gillmeister, ‘according to the economic experts at Agraforum GmbH, this is an approach to strengthening plants that could be pursued further.’

The bottom line is that there is sufficient evidence, particularly with regard to the use of root-symbiotic microorganisms, to pursue this approach in a more in-depth project. ‘Two years is simply too short a period to obtain meaningful results, especially with a perennial crop such as asparagus,’ concludes Gillmeister.

Author: Björn Lohmann