Method of improving yield and vigor of plants

A method of improving the yield and vigor of agronomic plants, in particular leguminous plants such as soybeans, involves treating such plants and/or the propagation material of plants with a composition that includes an active agent, such as a fungicide, that has no significant activity against fungal plant pathogens of the treated plant. When the plant is not wheat, a preferred agent of this type is 4,5-dimethyl-N-(2-propenyl)-2-(trimethylsilyl)-3-thiophenecarboxamide (silthiofam). Plants and plant propagation material, such as seeds, that have been treated by the novel method are also described.