EFFECT OF SCLEROTINIA-SCLEROTIORUM ON 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 ev aluated in a growth chamber study using addition series methods, Total seeding densities ranged from 2000 to 60000 seeds/m(2). Mean bluebunc h 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 influenc e than spotted knapweed density on both bluebunch wheatgrass and spott ed knapweed weights (2.11 and 0.51, respectively) when not under the i nfluence 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 bluebun ch wheatgrass density. Spotted knapweed weight per plant also was redu ced by the addition of S. sclerotiorum (1.4 to 1.2 mg) but there was n ot a corresponding increase in bluebunch wheatgrass weight. S. sclerot iorum decreased competition between spotted knapweed and bluebunch whe atgrass. This study provides evidence that establishment of bluebunch wheatgrass on spotted knapweed infested rangeland may be improved by c ombining S. sclerotiorum with high grass seeding rates.