Soil and Plant Factors Associated with Sudden Death Syndrome of Soybean

Abstract

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, chemical factors analyzed, and SCN populations determined once in 1985 and twice (from different locations in the field) in 1986. Yield, disease ratings and SCN populations were determined in 1985, 1986, and 1987. Applied Questions What combinations of soil and plant chemical factors are associated with SDS? Sudden death syndrome severity was greater where levels of available soil P, Mg, and organic matter were higher, indicating increased soil fertility. In addition, higher levels of exchangeable Na, Ca, and electrical conductivity were associated with higher levels of SDS severity, possibly indicating higher levels of irrigation. These results agree with previous observations that SDS occurs in soybeans growing under high‐yield environments. Analysis of the plant nutrient factors showed lower levels of N, Mg, Cu, and Mn and higher levels of Ca with higher levels of SDS, indicating that either root function was impaired or there was translocation of these elements to other parts of the plant. What effect does SDS have on soybean yield and seed germination? Increased SDS severity was correlated to lower yields in 2 of 3 yr. Lower yields were due to smaller seed and fewer seed in 1 yr and smaller seed in the other year. Differences between years may reflect when disease started, how fast disease progressed, and how severe disease became. Besides yield, seed germination was reduced by SDS in 1 yr through an increase in the number of dead and hard seed. Sudden death syndrome may make the plants susceptible to seed infection earlier than in nondiseased plants, increasing infection by seed pathogens in suitable environments. As a result, if SDS is widespread in soybeans grown for seed, it may be advisable to apply a foliar fungicide to insure acceptable seed quality. Does SCN affect SDS development? The role of SCN in SDS is not clear. In the study investigating the role of soil and plant chemical factors in SDS, SCN population levels were low where SDS was severe, indicating that the diseased soybeans may be poor hosts for the nematode. On the other hand, in one year of the yield study SCN was positively correlated with SDS severity, indicating that the nematode may increase disease. Soil cyst nematode was not related to SDS in the other year of the study. The role of SCN will require more study to determine its importance with SDS.