Interaction BetweenColletotrichum coccodesand Thidiazuron for Control of Velvetleaf (Abutilon theophrasti)

Velvetleaf (Abutilon theophrastiMedik. # ABUTH) is difficult to control with existing weed control strategies. Some measure of control can be obtained with a fungus,Colletotrichum coccodes(Wallr.) Hughes, used as a bioherbicide, but when the bioherbicide application was combined with the plant growth regulator thidiazuron (N-phenyl-N′-1,2,3-thidiazol-5-yl-urea), weed control was substantially improved. Thidiazuron alone interfered with normal development of velvetleaf, causing stunting and initiation of axillary bud growth. Tank mix applications ofC. coccodesand thidiazuron acted synergistically to increase velvetleaf mortality. WhenC. coccodeswas applied 10 days after thidiazuron as a split application, weed control was less than for tank mix applications. However, two applications ofC. coccodesplus thidiazuron were more effective than equivalent single tank mix applications. Under laboratory conditions, high concentrations of thidiazuron (2× field application rates) inhibited growth ofC. coccodes, but at field application rates thidiazuron did not reduce disease development. Combinations of thidiazuron andC. coccodesmay provide effective control of velvetleaf in the field.