The effects of Puccinia lagenophorae on Senecio vulgaris in competition with Euphorbia peplus

(1) Monocultures and mixtures of Senecio vulgaris (groundsel) and Euphorbia peplus (petty spurge) were grown in 4-litre tubs at a range of population densities and proportions in mixtures and in the presence or absence of the S. vulgaris-specific pathogen Puccinia lagenophorae (a rust). (2) Rust-infection greatly reduced S. vulgaris dry weights, especially of the roots, both in monocultures and mixtures but the yields of both control and rusted S. vulgaris were little changed by competition from E. peplus at any planting density. E. peplus yields were greatly reduced by competition from S. vulgaris but less so in the presence of rust, especially at lower planting densities. (3) The magnitude of rust-induced reductions in the yields of S. vulgaris decreased as the proportion of E. peplus in mixtures increased. However, increases in E. peplus yields when S. vulgaris was rusted were unaffected by species proportions. (4) The inequality (skewness) of the dry weight and plant height hierarchies of S. vulgaris increased with population density and rust infection but were not changed by competition from E. peplus. However, because most E. peplus plants were severely suppressed in mixtures with S. vulgaris, hierarchies were much more unequal than in monocultures. The weight and height of the largest E. peplus plants were greater, and the proportion of very small individuals less, in mixtures with rusted S. vulgaris than with the controls. Hence, if the effects of competition were determined for plants of differing status in the population hierarchy, then it was evident that only the stronger E. peplus individuals were able to grow better when S. vulgaris was rusted. There was no interaction between infection and status on the performance of S. vulgaris in mixtures. (5) S. vulgaris, whether control or rusted, strongly outcompeted E. peplus even though the root dry weights of the latter were often substantially greater. The more rapid early growth and greater height of S. vulgaris appeared to be the primary factors determining the outcome of competition between these species under our conditions. Rust infection had little effect on the competitive ability of S. vulgaris because it reduced plant height only slightly; any reduction in the density of the canopy of rusted S. vulgaris appeared to be significant only at low planting densities. (6) The effects of rust infection on the competitive ability of its host in mixture with E. peplus are compared with the results of previous investigations involving the S. vulgaris-P. lagenophorae system and the possible implications of these results for future studies of the role of pathogens on interspecific competition are discussed.