Late blight of the potato and tomato is considered to be the most destructive commercial crop disease in the world. There is therefore considerable interest in finding novel methods to control late blight. Japanese Knotweed is an invasive, non-native species that currently inhabits several parts of the UK, North America, and Western Europe. Because Japanese Knotweed contains the compounds resveratrol and emodin, which are well-documented bioactives against a wide range of diseases, it is possible that extracts of this plant may be effective against late blight. The research described in this thesis aimed to determine whether late blight could be prevented by using extracts of Japanese Knotweed as an organic fungicide.
Rhizome from the Japanese Knotweed plant was collected and extracted with a series of solvents of an increasing polarity. A zoospore-release bioassay was designed to assess the effectiveness of the Japanese Knotweed extracts on Phytophthora infestans, the causal agent of late blight. This assay was used as a fast screening method to assess the effectiveness of the different extracts. The ethyl acetate extract in particular showed some inhibitory activity against the P. infestans using the zoospore release assay. The known active compounds found in Japanese Knotweed, emodin and resveratrol, were also tested by this assay. Resveratrol showed limited inhibition against the release of zoospores, whereas emodin did not. Other parts of the plant were also extracted, and largely showed negative results, or produced little extract.
As a method to find the source of the activity in the extract, the active rhizome extract was passed through a silica gel column. Each of the major fractions obtained from the column was analysed for chemical composition by NMR and UV-Vis spectroscopy, and then passed through the in vitro bioassay. A comparison of spectra showed that the active fraction did indeed contain
resveratrol-like components.
In vivo tests were then carried out to assess if the extract would work as a plant protectant. For an extract to qualify as a biopesticide in this country, it could only be extracted using ethanol or water (according to correspondence with the PSD at the time of testing). Therefore, only ethanolic extracts were tested in the in vivo assays. The extract was initially tested on detached plant leaves, which showed inhibition against P. infestans infection. Bioassays were conducted on small tomato plants and whole potato plants. These led on to a final field trial. The extract was compared to a commercial fungicide, and another extract taken from Hedera Helix showing potential as a plant protectant. Although the extract did not perform as well as the commercial fungicide, it did give a show significant inhibition to infection when compared to a control plot, and therefore may have some potential as a biopesticide for the organic farming market.