Weed-suppressive soils consist of naturally occurring microorganisms that suppress weedy plant species by inducing disease symptoms and inhibiting growth and development. Modifying current management practices to enhance weed suppressiveness of soils can contribute to a reduction in chemical weed control and promote sustainable agroecosystems. Soils were found to be suppressive to Abutilon theophrasti after 8 years of continuous maize- A. theophrasti research within a 4 ha field at the Agricultural Research and Development Center (ARDC) in Eastern Nebraska. Experiments were established to determine if soil from this field (soilA) was biologically suppressive to A. theophrasti. Four hypotheses were tested: 1. A. theophrasti growth will be reduced in soilA compared to similar soil types, 2. Sterilization will cause soilA to be conducive to A. theophrasti, 3. Mixing soilA with conducive soil will transfer suppressiveness to the conducive soil, 4. Soil suppressiveness is correlated to the population levels (colony forming units) of soil pathogenic fungi. To identify the specific organism responsible for suppression, root sections of symptomatic seedlings from both the field and greenhouse were cultured. Pathogenicity of F. lateritium was verified by inoculating A. theophrasti roots in the greenhouse and assessing mortality and growth reduction compared to inoculation with pure water.

Results/Conclusions

A. theophrasti mortality was greatest (84±0.07%) and biomass was smallest (0.30±0.31 g plant^-1) in soilA compared to four similar soils collected at ARDC. A. theophrasti plants grown in sterilized soilA yielded 0.15±0.03 g/plant compared to 0.03±0.03 g/plant for the unsterilized soilA. The average height and biomass of A. theophrasti plants decreased with increasing amounts of soilA mixed with a conducive greenhouse soil (soilG) at 7 different ratios (0, 3, 6, 9, 12, 15, and 20 % v/v soilA/soilG). Of the three soilborne pathogenic fungi (Rhizoctonia solani, Pythium species and Fusarium species) tested, there were 10 times as many colony forming units (cfu) of Fusarium species (1.6 x 10⁵ cfu g^-1 soil) in soilA compared to conducive soils. Fusarium lateritium was identified in more than 80% of root cuttings from symptomatic field and greenhouse plants. Plant growth after two weeks was reduced 22% when A. theophrasti roots were inoculated with F. lateritium and grown in conducive soil. Results indicate that F. lateritium may contribute to the weed suppressive ability of agricultural soils. Further research is needed to assess the effects of F. lateritium on A. theophrasti population biology in the field.