Sudden death syndrome (SDS) of soybean (Glycine max [L.] Merr.) is a soilborne disease of increasing importance in high‐yield environments. This study was conducted to determine which combinations of soil fertility parameters and soybean cyst nematode (Heterodera glycines Ichinoe) (SCN) second stage juvenile (J2) population levels were associated with SDS disease severity. Also, the effect of SDS disease severity on leaf nutrient content and on soybean yield and seed germination were determined. These studies were conducted at the Pine Tree Station, in Colt, on a Crowley silt loam (fine, montmorillonitic, thermic Typic Albaqualfs) over 3 yr with the soybean cultivar Lee 74. Soil factors associated with SDS were increased levels of soil available P, electrical conductivity (soluble salts), organic matter, and exchangeable Na, Ca, and Mg. Decreased levels of leaf N, Mg, Cu, and Mn and increased levels of leaf Ca were related to increased levels of SDS severity. Sudden death syndrome severity was negatively correlated with yield in 2 of 3 yr and with seed quality in 1 of 2 yr. Soybean cyst nematode second stage juvenile population density decreased as SDS severity increased in the study relating soil and leaf nutrient levels fo SDS. This may reflect a decrease in the ability of a diseased root system to support nematode reproduction. In the yield studies, however, SCN was negatively correlated to yield and positively correlated to SDS in 1 of 2 yr. Soybeans grown in high production environments of increased soil fertility and irrigation appear to be more susceptible to SDS. Sudden death syndrome affects yields by a reduction in seed size and number and may, under the proper environmental conditions, affect seed germination. The role of SCN in SDS is still unclear. Research Question Sudden death syndrome (SDS) is an important soilborne disease of soybean in parts of the South and Midwest. The disease is associated with high‐yield production'environments. This study compared the effects of soil and plant chemical factors on SDS severity and the relationship between SDS severity on yield and seed germination. Literature Summary Sudden death syndrome usually appears after flowering in vigorously growing, well‐watered soybeans, and is often associated with the soybean cyst nematode (SCN). Losses due to this disease vary greatly, but can be severe. Like other soilborne diseases, SDS distribution usually is aggregated in the field. Disease aggregation as well as inoculum distribution is strongly influenced by the soil environment. Since SDS is associated with high‐yield production environments, soil chemical factors may have a strong effect on SDS severity as they do with other soil‐borne diseases. While SDS appears to affect yield, the relationship of disease severity and yield has not been demonstrated nor has the effect of SDS severity on seed germination been determined. Study Description The study was conducted at the Pine Tree Station, Colt, AR, in a field with a history of SDS. Soil and plant samples were collected,...

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Yields of irrigated soybean in Arkansas are threatened by two problems: chloride toxicity and sudden death syndrome (SDS). Soybeans are sensitive to chloride, which accumulates in the upper soil profile when water with high salt content is used for irrigation. Sudden death syndrome is a soilborne disease often associated with well-irrigated fields. Even though these problems both affect irrigated soybeans, there are no reports on the effect of chloride toxicity on SDS. To determine if there is an effect of chloride toxicity on SDS, a test was established at the Cotton Branch Station, Marianna, AR, in 1995 and 1996. Four cultivars were selected that were either susceptible to SDS (S) or resistant to SDS (R) and either translocated chloride to the leaves (includer, I) or confined chloride in the roots (excluder, E). The cultivars were Hartz 6686 (SE), Terra Vig 6653 (SI), Hartz 6200 (RE), and Asgrow 6297 (RI). Soil chloride concentrations were adjusted by the addition of KCl. Before planting, KCl was applied to achieve the recommended concentration of K over the entire field. At V4, chloride treatments were applied by either adding no additional KCl (low Cl) or adding 1,120 kg of KCl per ha (moderate Cl) or 2,240 kg of KCl per ha (high Cl). Soil samples were taken within the center two rows of each plot at planting, flowering (R2), and harvest and assayed for populations of Fusarium solani and Heterodera glycines. Soil chloride concentrations were determined at R2, and leaf chloride levels were determined at R3. Weekly disease ratings were made on SDS and converted to area under the disease progress curve (AUDPC). Plant lodging and the incidence of southern blight (Sclerotium rolfsii) were determined during mid-reproductive growth. Leaf chloride concentrations were influenced by both chloride treatment and cultivar: elevated concentrations occurred with the includer cultivars in the moderate and the high Cl treatments. Soil concentrations of chloride reflected the chloride treatments in 1995, but not 1996. Soil populations of F. solani did not respond consistently to either chloride treatment or cultivar; however, H. glycines egg densities increased with increased soil chloride treatments in Hartz 6686 (SE) and Terra Vig 6653 (SI) at R2, but not at harvest. Increased soil chloride treatments increased SDS in both years with Hartz 6686 (SE), but did not affect this disease in the other cultivars. Higher soil chloride treatments decreased yield in all cultivars except Hartz 6200 (RE) in 1996. Although Terra Vig 6653 (SI) did not develop severe levels of SDS foliar symptoms, it did have increased lodging and significant increases in southern blight with moderate and high soil chloride treatments. These results indicate that growers with fields that have both elevated concentrations of soil chloride and SDS should select SDS-resistant excluder cultivars to minimize yield losses due to both problems.

Plant nutrient deficiencies may be diagnosed with DRIS (diagnosis and recommendation integrated system) or critical nutrient level (CNL) procedures. While extensive efforts have been made to improve diagnoses by DRIS and M‐DRIS, little has been done to refine CNL diagnoses for soybean [Glycine max (L.) Merr.]. In this study, we compare diagnoses made by M‐DRIS with those from CNL and modify these procedures to improve their diagnostic accuracy. Two databases of R2 (full bloom) soybean leaf nutrient concentrations from 828 field observations of known K, Mn, Zn, or P deficiencies or sufficiencies were used for comparing the procedures. We optimized critical nutrient levels (modified CNL) using the same cases from a database that was used earlier to improveM‐DRIS. Using this optimization database, the M‐DRIS and modified‐CNL procedures had ≤ 13 misdiagnoses each of K nutrient status compared to 81 by unmodified CNL. M‐DRIS was more accurate than the two CNL procedures in diagnosing P deficiencies from cases in the optimized and independent data sets. Unmodified CNL produced 18 misdiagnoses for Zn status from cases in the independent data set, versus >27 for M‐DRIS or modified CNL. Generally, M‐DRIS was the same or more accurate than CNL methods in diagnosing nutrient status. We recommend CNL concentrations for R2 stage soybean of 3.1 g P kg−1, 15.1 g K kg−1, 17 mg Mn kg−1, and 21 mg Zn kg−1. For M‐DRIS, we recommend critical index values of zero for K and Mn, 18 for P, and −19 for Zn.

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