Integrating Nutrient Management in Soybean (Glycine max) Health.

The influence of plant nutrient status and efficiency of nutrient uptake on fungal diseases is a subject that has not received adequate scientific attention. Low plant levels of potassium (K) are correlated with thinning cell walls and low sugar foliar tissue accumulation, characteristics that promote fungal infection. Chloride (Cl) and Manganese (Mn) application is correlated with a reduction in the severity of several fungal diseases. Boron (B) deficiency has been associated with the presence of small fissures/cracks in the leaves that may facilitate entrance of diseases. Mn has a role in the synthesis of lignin and phenols increasing the thickness of external plant layers. Some soybean varieties such as glyphosate-resistant have presented low Mn uptake/translocation at early stage. This brings challenges for fungal disease control in the near future. Integrated approaches to manage both nutrient and fungicides are needed. PURPOSE: i) to determine the impact of K, Cl, Mn and B nutrition in soybeans on controlling frogeye spot (Cercospora sojina),  ii) to evaluate the nutrient interaction effects with and without the application of fungicides., iii) to compare the effects across treatments using Roundup-Ready^TM and conventional soybean varieties. METHODS: To evaluate these objectives, a 4 year field study was conducted in Illinois.  Measurements included soil analysis, tissue composition, and grain yield. Evaluation of frogeye spot was monitored using a visual index. Means for each year were analyzed as separate experiments due to differences in response between years. The experimental design was a split plot. The whole plot consisted of soybean varieties (Round-up ready and conventional) and the split plots were a combination of presence/absence of foliar fungicide application plus the addition of micronutrients and K sources. The data were analyzed using analysis of variance (ANOVA), with the general linear model PROC GLM procedure of the SAS statistical package. A probability of 0.05 or less was used to separate significant effects and means were compared using ‘contrasts’ and the ‘Least Significant Differences’ (LSD) procedures. RESULTS: Fungicide treatment alone reduced the incidence of frogeye leaf spot without affecting yield. Results indicated that none of the fertilizer treatments affected soybean grain yields in any year. Soybean variety had no interaction with either fungicide or fertilizer treatments. The foliar application of B and Mn increased leaf B and Mn and reduced the injury index, without affected yields, suggesting an increase on soybean response. Application of K did not increase the leaf level compared to the control plots presumably because of the high K supplying power of the subsoil.