Impact of downy mildew (Plasmopara halstedii) infection on the development and metabolism of sunflower.
A spontaneous outbreak of downy mildew was observed in a sunflower field in Tübingen, Germany. Changes in development and metabolism of systemically infected plants were determined over a whole season. Plant growth, i.e. av. height, leaf area and biomass production in general, was reduced by between 70 and 90% compared with healthy plants, while the total number of leaves increased by c. 30%. Infection also led to alterations in secondary metabolism. Contents of sesquiterpene lactone were lower in leaf samples of infected plants than in healthy ones, but this was considered to be due to a lower density of the sequestering glandular trichomes rather than to a reduced synthesis within each gland. However, phenylpropanoid biosynthesis was accelerated by infection, leading to accumulation of the coumarin scopoletin. Investigation of this phytoalexin reaction in sunflowers artificially inoculated with the pathogen showed that the reaction was restricted to an area up to only c. 6 mm from the inoculation site, and that max. accumulation occurred 120 h after inoculation.