Biological control of Senecio vulgaris in carrots (Daucus carota) with the rust fungus Puccinia lagenophorae

Abstract

Senecio vulgaris is a troublesome weed in horticulture that is tolerant or resistant to a range of herbicides, and is therefore a candidate for biological control. The rust fungus Puccinia lagenophorae is a potential control agent, but being biotrophic, it is not suitable for use as a mycoherbicide. We tested the effects of induced rust epidemics on S. vulgaris, and investigated the resulting shading dynamics within this system. We sowed carrots at 267 seeds m−2 on five farms in western Switzerland in plots either with or without S. vulgaris seeds (also at 267 seeds m−2, within crop rows), and with or without strong sources of rust spore (four inoculum sources placed every 2 m along crop beds). Plots with S. vulgaris and no inoculum source produced low total carrot yields (18% of yield from S. vulgaris-free plots), whereas plots with S. vulgaris plus rust inoculum sources produced 48% of the yield from S. vulgaris-free plots. There were, however, no significant differences in several commercially important yield parameters such as the number of first grade carrots between S. vulgaris-free plots and those with S. vulgaris plus inoculum sources. Carrots grown with S. vulgaris were smaller and had skewed size distributions compared with the weed-free controls, but adding rust inoculum reduced this effect. This reduced size variability and reduced weed competition have important economic benefits to farmers. Carrots had longer leaves when grown with S. vulgaris, suggesting that carrots and S. vulgaris competed for light. Percent ground cover and height of S. vulgaris relative to carrots was reduced by the presence of rust, and these effects were most obvious one month after placing inoculum sources in the field. Our results are encouraging, even with artificially high weed competition, but further research is needed before this paradigm can be proposed for practical applications.