EFFECTS OF HERBICIDES ON SCLEROTINIA CROWN AND STEM ROT OF ALFALFA

This study was conducted to determine whether herbicides and adjuvants registered for postemergence use in alfalfa have an effect on Sclerot inia crown and stem rot (SCSR). In a controlled environment, disease s everity index (DSI) of alfalfa seedlings was reduced by pronamide and 2,4-DB compared with the untreated control, whereas bromoxynil and 13% sethoxydim + petroleum-based adjuvant (PEA) increased DSI. In the fie ld, disease severity in all herbicide treatments was similar to that i n untreated alfalfa. In a second controlled-environment study, pronami de and 2,4-DB reduced DSI compared with the no herbicide control when seedlings were inoculated 1 day after herbicide application, but this protective effect was not observed when seedlings were inoculated 8 da ys or longer after herbicide application. The results demonstrate that several herbicides are capable of suppressing or enhancing SCSR sever ity in a controlled environment if seedling inoculation occurs soon af ter herbicide application; however, the residual effect of these herbi cides on SCSR appeared to be much shorter than the 4- to 6-week infect ion period occurring in the field.