Department of Plant Pathology, Iowa State University, Ames, IA 50011, USA

Background and objectives
Stresses imposed on crops by agricultural chemicals such as herbicides, in conjunction with physical factors, can lead to increased root diseases in crops. Herbicide stress weakens and predisposes plants to rapid fungal colonization. The use of herbicide-tolerant crops, such as Roundup Ready soybean, could reduce damage from root diseases, which are seen in conventional crop cultivars following herbicide applications. The present study is part of a research programme aimed at evaluating the impact of herbicide-tolerant soybeans and soil factors on the development of soybean root diseases. We report on the influence of herbicides and soil texture on the development of sudden death syndrome (SDS) in Roundup Ready soybean, caused by the soilborne fungus Fusarium solani f. ;sp. glycines.

Materials and Methods
Experiments were conducted in a growth chamber (20C, 16-h photoperiod) to compare the development of SDS in soybean planted in five soils of different texture, formulated from a natural soil (sand 52%, silt 25%, and clay 23%) and river sand. The five soil types were obtained by mixing natural and river sand in the following volume ratios of 1:0, 1:1, 1:2, 2:1, and 0:1 (natural soil:river sand). SDS development was monitored over a period of 4 ;weeks after planting.

Field studies were carried out at two locations in Iowa, Story and Scott Counties. In Story County, plots were artificially infested with F. ;solani. Glyphosate was applied to Roundup Ready soybean plants (Pioneer variety 9344) at two growth stages (V2-V4, and R1-R2) at the normal recommended field rate. The experiment was conducted in a randomized complete design with three replications. In Scott County, plots were established in a farmer's field with history of SDS. Roundup Ready soybean (Pioneer variety 9344) and standard soybean (cultivar BSR101) cultivars were planted in plots which were 4-m long and four rows wide with a between-row spacing of 1 m. One pre-emergence (dimethenamid) and four post-emergence (acifluorfen, imazethapyr, glyphosate, and lactofen) herbicides were used. A control (with no herbicide applied) was included in the study. The pre-emergence herbicide was applied 3 ;days after planting, and all post-emergence were applied at V4-V6 growth stage. The experiment was conducted in a randomized complete block design with three replications and a factorial arrangement of treatments in a strip-plot. Cultivars were assigned to whole plots, and herbicide treatments to subplots. SDS was assessed visually on 7 August and 29 September 1997, as average percentage of leaf area in the centre two rows of each plot exhibiting SDS symptoms.

Results and conclusion
In the growth chamber, incidence and severity of SDS were greatest in river sand and decreased in magnitude as the sand content in potting medium decreased. The severity and incidence of SDS was lowest in natural soil. In the Story County experiment, the level of SDS observed was very low and not significant across all plots. The mean SDS severity was 1.5% in control plants (not sprayed with glyphosate), and 2.5% and 3.5% in plants sprayed at V2-V3 and R1-R2, respectively. In the Scott County experiment, the response of the Roundup Ready cultivar (Pioneer variety 9344) to SDS was similar to that of the regular soybean cultivar (BSR101) when sprayed with the selected herbicides. Across both cultivars, there was no significant increase in SDS severity in plots sprayed with herbicides when compared with control plots.