Water Stress and Damage Caused byPuccinia jaceaeon TwoCentaureaSpecies

The effect of water stress and disease damage toCentaurea solstitialis(yellow starthistle) andC. calcitrapa(purple starthistle) caused by isolates of the rustPuccinia jaceaewas investigated under greenhouse conditions. Inoculated and noninoculated plants were placed under water stress, defined as reversible wilting; control plants received normal daily watering. Effects of drought stress and infection byP. jaceaewere measured in terms of root biomass, lifespan of leaves with different disease severity levels, and the number of living and dead rosette leaves. Drought stress reduced root dry weights, leaf lifespan, and total number of rosette leaves of both yellow and purple starthistles. Infection by rust also reduced leaf lifespan and total rosette leaves of both species, but it only affected root biomass of yellow starthistle. The only significant interaction between drought stress and rust infection occurred with the leaf lifespan of purple starthistle, but this consisted of drought-stressed leaves having short lifespans at all infection levels. Results withP. chondrillina,a rust fungus that has been used successfully for biological control ofChondrilla juncea(rush skeletonweed) were similar to those ofP. jaceaeon yellow starthistle for root biomass and leaf lifespan. Data from these experiments with drought-stressed plants reinforced the previous conclusions thatP. jaceaehas promise as a biocontrol agent of yellow starthistle. The damage caused byP. jaceaewas similar to that caused byP. chondrillinaon skeletonweed in comparative studies.