Oil emulsions increase efficacy of Phoma herbarum to control dandelion but are phytotoxic

Abstract

Paraffinic-vegetable and vegetable oil emulsions were investigated for their ability to increase efficacy of the fungal pathogen Phoma herbarum as a bioherbicide to control dandelion (Taraxacum officinale) under sub-optimal leaf wetness durations (LWDs) in controlled environments. Mycelia of P. herbarum (20%, v/v) were formulated in 10 oil emulsions; two based on paraffinic-vegetable (soybean) oil mixtures, and eight based on vegetable (canola, coconut and soybean) oil emulsions alone. Emulsions were applied to dandelions using an automatic track sprayer and plants were exposed to sub-optimal LWDs simulating dew periods in field conditions. An aqueous formulation of P. herbarum was included as a standard treatment and formulations without P. herbarum were included as controls. Eight of 10 oil emulsions (two paraffinic-vegetable and six vegetable) significantly increased damage ratings caused by P. herbarum compared to the standard treatment under certain sub-optimal LWDs (P=0.05). However, all oil emulsions caused at least some phytotoxicity when applied alone under most LWDs. Also, the high viscosity of nine oil emulsions caused mixing and application problems. These results indicate that oil emulsions increase damage ratings caused by P. herbarum on dandelion under sub-optimal LWDs, but may not be suitable formulations due to their phytotoxicity and high viscosity.