Weed-suppressive soils contain naturally occurring microorganisms that suppress a weed by inhibiting its growth, development, and reproductive potential. Increased knowledge of microbe–weed interactions in such soils could lead to the identification of management practices that create or enhance soil suppressiveness to weeds. Velvetleaf death and growth suppression was observed in a research field (fieldA) that was planted with high populations of velvetleaf, which may have developed via microbial mediated plant–soil feedback. Greenhouse studies were conducted with soil collected from fieldA (soilA) to determine if it was biologically suppressive to velvetleaf. In one study, mortality of velvetleaf grown for 8 wk in soilA was greatest (86%) and biomass was smallest (0.3 g plant−1) in comparison to soils collected from surrounding fields with similar structure and nutrient content, indicating that suppressiveness of soilA was not likely caused by physical or chemical factors. When soilA was autoclaved in another study, mortality of velvetleaf plants in the heat-treated soil was reduced to 4% compared to 55% for the untreated soil, thus suggesting that suppressiveness of soilA is biological in nature. A third set of experiments showed that suppressiveness to velvetleaf could be transferred to an autoclaved soil by amending the autoclaved soil with untreated soilA; this provided additional evidence for a biological basis for the effects of soilA. The suppressive condition in these greenhouse experiments was associated with high soil populations of fusaria. Fusarium lateritium was the most frequently isolated fungus from roots of diseased velvetleaf plants collected from fieldA, and also was the most virulent when inoculated onto velvetleaf seedlings. Results of this research indicate that velvetleaf suppression can occur naturally in the field and that F. lateritium is an important cause of velvetleaf mortality in fieldA.
Nomenclature: Velvetleaf, Abutilon theophrasti Medic., ABUTH; corn, Zea mays L.; soybean, Glycine max (L.) Merr