Effect ofSclerotinia sclerotiorumon the Interference between Bluebunch Wheatgrass (Agropyron spicatum) and Spotted Knapweed (Centaurea maculosa)

Effectiveness of the common soil fungus Sclerotinia sclerotiorum as a biological control for spotted knapweed and its effect on competitive interactions between spotted knapweed and bluebunch wheatgrass were evaluated in a growth chamber study using addition series methods. Total seeding densities ranged from 2000 to 60 000 seeds/m 2 . Mean bluebunch wheatgrass plant weight was 3.5 times greater than spotted knapweed weight per plant, respectively. Coefficient ratios estimating species interaction showed bluebunch wheatgrass density had a greater influence than spotted knapweed density on both bluebunch wheatgrass and spotted knapweed weights (2.11 and 0.51, respectively) when not under the influence of S. sclerotiorum . Niche differentiation ratios indicated a lack of resource partitioning between species (1.11). S. sclerotiorum reduced spotted knapweed density by 68 to 80% without reducing bluebunch wheatgrass density. Spotted knapweed weight per plant also was reduced by the addition of 5. sclerotiorum (1.4 to 1.2 mg) but there was not a corresponding increase in bluebunch wheatgrass weight. S. sclerotiorum decreased competition between spotted knapweed and bluebunch wheatgrass. This study provides evidence that establishment of bluebunch wheatgrass on spotted knapweed infested rangeland may be improved by combining S. sclerotiorum with high grass seeding rates.