(1) Determine the amounts and intervals of sprinkler irrigation treatments associated with the onset, development, and severity of SDS. (la) Experimental site. Soils at the Rossville unit of the Kansas River Valley Experimental Fields are predominantly in the Eudora series, with texture varying from silt loam to sandy loam. Irrigation is with a linear travel sprinkler system that is capable of variable rates of water application. The field has equipment for planting and harvesting of replicated plot research. The field location chosen for this research has a wellestablished history of SDS. (1b) Varieties. Two round-up ready varieties, one susceptible and one resistant to SDS will be chosen for this experiment after consultation with Dr. Bill Schapaugh (K-State Soybean Breeder). (lc) Irrigation regimes and experimental design. Irrigations will be conducted at late vegetative stage (V4), at the vegetative/flowering transition (RI), and the beginning pod stage (R3). Irrigation amounts will be divided into "low", "medium", and "high". The amount of water applied will depend upon the soil conditions (KanSched2 irrigation program), with the goal of saturation at the "high" watering amount. The "low" irrigation level will have no irrigation prior to R3. Normal irrigation regimes will be implemented at R3 for the low, medium, and high treatments. Thus, the experimental design will be a split-split plot, where irrigation timing will be the whole-plot, amount of irrigation will be the split-plot, and varieties will be the split-split plot. Four replications will be conducted at the split-split plot level for each variety x irrigation regime x irrigation timing combination. (Id) Disease severity estimation of SDS and yield. Visual ratings of the percent leaf area expressing foliar symptoms of SDS will be taken weekly after the onset of symptoms. Soybean growth stages will be recorded for each rating and continue through R6. Yield will be measured at harvest for each split-split plot replicate. This data will be related to the disease onset period within the season in order to establish estimates and predictions for yield losses to the disease.
(2) Determine the influence of planting date upon sudden death syndrome in Kansas- (2a) Experimental site. The experimental site for the planting date study will be the same as indicated for the irrigation experiment described above. The field chosen will have a well established history of sudden death syndrome. (2b) Varieties. Two varieties, one susceptible to SDS and one with moderate resistance, will be used. These will be chosen in consultation with the soybean breeder in order to make sure they have close maturities. (2c) Planting dates and experimental design. Based upon the recommended planting date ranges for soybeans as described in the Kansas Crop Planting Guide (KAES/CES), planting dates will range from 05 May to 16 June. During this period, four planting dates will be used: 05 May, 19 May, 02 June, and 16 June. The experiment will be set up in a randomized complete block design with four replications. (2d) Disease severity estimation of SDS and yield. These variables will be measured as described in Procedure #1.
(3a) Determine the relationship between root mass/area and SDS disease severity. For soybean, the influence of root mass/area upon disease severity of SDS is unknown. Therefore, this objective seeks to examine the baseline relationship between these variable and severity of the disease. In addition to disease severity, the amount of pathogen infecting the root will be related to root mass/area based on a CFU/g (mass) or CFU/cm 2 (area) calculation. F. virguiliforme inoculum will be produced on a millet carrier, dried, and added to rows at planting at a level of 10 g/ft using a cone planter in order to ensure high inoculum and disease pressure. At least four soybean varieties (two susceptible and two resistant) will be tested in order to discover consistent relationships between varying levels of root mass/area, host resistance, disease severity, and pathogen root colonization. All tissue below the crown, including the primary and secondary roots will be harvested at three time points during the season (V4, RI and R3), washed, and measured for area using Assess 2.0 (APS software) and then dried overnight and weighed on a scale for mass measurements. The numbers of nodules and nematode cysts will also be counted prior to grinding root samples for F. virguiliforme CFUs.
(3b) Determine the relationship between soil compaction and SDS disease severity. Backqround: There are two forms of soil compaction: surface and subsurface. While surface compaction can be partly alleviated with normal tillage operations, subsurface compaction below the normal tillage depth will remain. Fracturing or cutting subsurface compacted soil (i.e. "subsoiling") results in better root health and penetration and concomitant yield improvements for many crops, since it is well-known that soil compaction greatly influences root development. A diagnostic tool to measure the extent and depth of subsurface compaction is a "penetrometer", or soil compaction tester. Procedure: This study aims to establish a baseline relationship between soil compaction and SDS severity. To do this, a split-plot experiment will be established at the Rossville site where one half of the plot will be planted normally with the addition of inoculum at planting (see Procedure #3a). The second half of the plot will be planted (with inoculum added, as above) and a tractor will be driven repeatedly between the rows in order to mimic soil compression. The experiment will include four replications of susceptible and resistant soybean varieties. A FieldScout SC-900 digital soil compaction meter will be obtained from Spectrum Technologies, Inc. in order to acquire soil compaction measurements.