Interactive impacts of a herbivore and a pathogen on two resistance types of Barbarea vulgaris (Brassicaceae)

Heimes C, Thiele J, van Mölken T, Hauser TP

Abstract

It is well known that pathogens and arthropodherbivores attacking the same host plant may affecteach other. Little is known, however, about their combinedimpact on plant fitness, which may differ from simple additiveexpectations. In a 2-year common garden field experiment,we tested whether the pathogen Albugo sp. (whiteblister rust) and the herbivorous flea beetle Phyllotretanemorum affected each other's performance on two resistancetypes (G-type and P-type) of the crucifer Barbareavulgaris ssp. arcuata, and whether biomass, reproductionand survival of the plants were affected by interactiveimpacts of the antagonists. Most of the insect-resistantG-plants were severely affected by white rust, whichreduced biomass and reproductive potential compared tothe controls. However, when also exposed to flea beetles,biomass loss was mitigated in G-plants, even though apparentdisease symptoms were not reduced. Most of the insectsusceptibleP-plants were resistant to white rust; however,the number of flea beetle mines tended to increase in plantsalso exposed to Albugo, and biomass at the last harvest wasslightly lower in the combined treatment. Thus, interactive impacts of the herbivore and pathogen differed between thetwo resistance types, with an antagonistic combined impactin G-plants, which lasted surprisingly long, and a slightsynergistic impact in P-plants.