(1a) Determine the influence of planting date upon sudden death syndrome (SDS) in Kansas. (i) 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. The field has equipment for planting and harvesting replicated plot research. The field location chosen for this study has a well-established history of SDS. (ii) 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. (iii) 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 the first year of the experiment, four planting dates were used: 05 May, 19 May, 02 June, and 16 June. A similar set of dates will be used during the 2016 season. The experiment will be established in a randomized complete block design with four replications. (iv) 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 to establish estimates and predictions for yield losses to the disease.

(1b) Examine the interaction between ILeVO seed treatment and planting date for SDS. (i) Background. ILeVO is a seed treatment product produced by Bayer Crop Science that has activity against sudden death syndrome and root nematodes including soybean cyst nematode, reniform nematode, and root-knot nematode. Reported yield improvements from Bayer range between 2.6 to 14.3 bushels per acre. (ii) Procedure. The same varieties used for the planting date study (above) will be used for this study. ILeVO seed treatments will be incorporated into the planting date study design (above) to compare the effect of ILeVO seed treatments and their interactions with planting date. Disease severity estimations and yield will be conducted as described in Objective #1a above. Populations of the SDS pathogen and soybean cyst nematode will be measured in each plot. Also, populations of the charcoal rot pathogen will be measured from R7 roots and post-harvest soil samples. Since abiotic soil properties influence disease severity and pathogen populations, soil samples will be collected from all plots at planting time in order to determine abiotic characteristics such as pH, P, K, NO3, NH4, organic matter, and soil texture.
(2) 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 (weight) or CFU/cm2 (area) calculation. F. virguliforme 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, R1, 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.

(3) Determine the relationship between soil compaction and SDS disease severity. (i) Background: 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. (ii) 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 #2). The second half of the plot will be planted (with inoculum added, as above) and a tractor will be repeatedly driven between the rows to mimic soil compression. The experiment will include four replications of susceptible and resistant soybean varieties.

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1. Traditionally, soybean planting dates in the Kansas River Valley have been delayed until after mid-May to help avoid the development of SDS. Planting date studies over the past two years have shown that SDS symptoms have been more severe in earlier plantings (first of May). As the planting dates progressed later the SDS severity decreased. However, the highest yields in these studies occurred with the earlier planting dates, in spite of increased SDS disease severity. Yields decreased by an average of almost 0.5 bu per day for more tolerant varieties when planting was delayed after the end of April or the first of May, and 0.3 bu per day for the more susceptible varieties. In both years, the SDS severity was not as high as previously observed at KRV.

2. As predicted, ILeVO reduces SDS disease severity. Moderately resistant soybeans show improved yield compared to susceptible regardless of ILeVO use.

3. Plants with longer root systems appear to have decreased SDS severity compared to those with shorter root systems. The SDS pathogen itself decreases root length for both moderately resistant and susceptible genotypes.

4. For both years of the compaction study (2015 and 2016), there was no impact of compaction on disease severity or yield. Thus, compacted soil may not influence sudden death syndrome in the Kansas River Valley. This may be due to the soil types that are common in the area. However, sub- surface compaction appears to be related to increased disease severity, although this finding is preliminary and non-significant.