EFFECT OF CONTROL TREATMENTS ON THE ROOT RESERVES OF CALIFORNIAN THISTLE (CIRSIUM ARVENSE)

In autumn 1994, 3 month old Californian thistle plants grown from seed were transplanted at regular spacings into a ryegrass/white clover sward. Seven treatments including three topping treatments, the herbicides clopyralid and MCPB, and a mycoherbicide Sclerotinia sclerotiorum, were applied to thistle foliage the following summer. Treatment effects on root mass and bud production were measured in December 1995. Clopyralid and S. sclerotiorum reduced dry root weight relative to control 95% and 60% respectively and also reduced the number of root buds and shoots. MCPB had little effect on root weight but reduced root bud numbers by 60%. Thistles topped twice also showed reductions in root weight and shoot numbers.