Graham and Webb (1991) consider that pathogens can be reduced by limiting their penetration, development, and reproduction on host plants. Most of the research in controlling soybean foliar diseases has focused on either fungicide application or genetic resistance. However, the influence of plant nutrition status on susceptibility and tolerance of crops to diseases is an important aspect that has not been given enough importance. The lack of sufficient levels of potassium (K) can be correlated with thin plant cell walls, smaller, thinner and shorter roots, lower sugar accumulation in the foliar tissue, and accumulation of unused nitrogen (N). Those are characteristics that encourage disease infection, particularly soybean Asian rust (PPI, 1998). Perrenoud (1990) reviewed several thousand research publications and concluded that the use of K decreases the incidence of fungal diseases by more than 70%. In addition, yield was always increased in plants infested with fungal diseases. The usage of K has the potential to increase resistance/tolerance in foliar fungus affected soybean plants (Piccio and Franje, 1980). Chloride (Cl) and Manganese (Mn) have been shown to reduce the severity of several fungal diseases as well (Fixen, 1993; Graham and Webb, 1991; Huber and Wilhelm, 1988).
The development of new soybean varieties, such as glyphosate-resistant, has shown lower capacity for either soil up-take or translocation Mn within the plant (Huber et al., 2004). It is unclear whether there will be a positive or negative effect over foliar fungal disease management. Manganese plant nutrition reduces the incidence of foliar disease in most crops due to its role in the synthesis of lignin and phenols. Therefore, this can be a factor in controlling pathogens (Graham and Webb, 1991).
Research has shown that plants with low Mn concentration have greater susceptibility for the attack of fungal pathogens (Huber and Wilhelm, 1988). Boron (B) is another element that is correlated to the production of small fissures and cracks that might be the entrance or access of fungal diseases. The new genetic information about “genetic modified” soybean and its relationship with nutrient uptake, combined with arriving diseases, may bring new challenges for disease control in the near future. To face this upcoming challenge in soybean production, alternative ways to handle both nutrient and fungicides management is needed; particularly in Illinois where soybean production is prevalent. To accomplish these unresolved questions, a field study was conducted from 2006-2008 at two locations in Illinois - the University of Illinois (UI) Dixon Springs Agricultural Center (DSAC) and the UI Brownstown Agronomy Research Center (BARC).
The objectives of the study were to
Treatments identified in Table 1 were applied to soybeans grown under two soil types in Southern Illinois. The soybean treatments were established in an area where the selected disease was present, to ensure distribution of the pathogenic form all over the treatments. A split-plot field study with four replications was conducted at two locations; The University of Illinois Dixon Springs Agronomy Center (DSAC), and the University of Illinois Brownstown Agronomy Research Center (BARC). Whole plots consisted of two soybean varieties (1 Roundup-ready and 1 conventional). Split plots consisted of a combination of +/- fungicides conventional treatment, manganese (Mn) and boron (B) plus a comparison of potassium (K) sources in the two available formulations (potassium chloride [KCl] vs. potassium sulfate [K2SO4]).
Measurements included soil samples for pH, P, K, Mn and Cl with Mn and Cl determined to depths from 0 to 18 inches in increments of 6 inches (Table 2). Tissue samples for K, Cl, B and Mn were taken at early pod set. Evaluation of soybean eye frogeye incidence was monitored throughout the growing season. An index value of 1 was assigned to a “no incidence” and 9 to the highest level of incidence. Plot size was 10' by 30' with the center 5' x 30' harvested for grain yield at the end of the growing season. Initial soil test values of pH, O.M., P, K, S and micro-nutrients were determined using standard methodologies (Table 2). Plot areas were tilled in the fall. A corn soybean rotation was used with soybeans planted in 15 inch rows at optimum seeding rates. Planting date, weed, and pest control was managed as a commercial crop.
Prior to planting and fertilizer application, soil samples were collected from the 0-6, 6-12 and 12-18" depths. These samples were air dried, crushed, and analyzed for nutrient composition (Table 2). Pre-plant fertilizers included a comparison of KCl (0-0-60-45[Cl]) and K2SO4 (0-0-50-18[S]) at a rate of 75 lb K2O/acre plus a check with no K (Table 1). At this rate the KCl would deliver 56 lb Cl/acre and the K2SO4 would deliver 27 lb S/acre. Foliar treatments included an application of either Mn or B or both applied in addition to the KCl pre-plant treatment. The foliar treatments were applied twice in 2006, corresponding to the V4 and V10 soybean growth stages, but only once in 2007 and 2008 at the V10 growth stage. The Mn treatment was 0.5 lb Mn/acre/application supplied as a 5% mannitol chelate (Brandt Consolidated) and the B treatment was 0.25 lb B/acre/application as Solubor DF. Each of the six fertilizer treatments above were applied in combination with a plus or minus fungicide application to each of the soybean varieties (Table 1). The fungicide treatment consisted of Headline® applied at a rate of 6 oz/acre. A split-split plot design was used at each location with soybean variety serving as whole plots and a factorial arrangement of fungicide X fertilizer treatments serving as sub-plots. There were four replications per location.
Trifoliate leaves were collected 3 weeks after fungicide applications and evaluated for levels of frogeye spot. Observation was also made for possible phytotoxicity. Twenty of the upper-most fully developed leaves were randomly collected, dried, digested and analyzed for nutrient composition. Soybean yield was measured. The data was analyzed using PROC MIX of SAS institute version 9.2 (SAS, 2007) and treatment means differences were separated using the least significant difference (LSD alpha=0.05).
The year before the experiment was established, growing season 2005, consisted of periods of very dry conditions, especially at BARC (data not shown). This may have led to little or no disease presence at BARC and light disease presence at DSAC. Low rainfall did affect the growth and development of the soybean plants and could account for the increase in phytotoxicity of some of the treatments (discussed later). August and September rainfall was near normal and most likely accounted for relative high yields occurring at both locations, despite the early season low rainfall. In 2006, rainfall was normal to above normal at both locations. In 2007, below average rainfall both early and late in the growing season significantly reduced yields over prior years at both locations, especially at DSAC. The single application of Mn and B resulted in non-observable phytotoxicity compared to previous years. In 2008 rainfall was excessive at the beginning of the planting season causing a delay in the planting schedule.
Frogeye leaf spot index was increased with B and Mn application. However, disease presence was relatively low and had mixed effects over yields at both BARC and DSAC locations (Table 3). At DSAC the presence of frogeye was consistent, and apparently, severe enough that there was a 6.6 bu/acre yield response to fungicide application. The foliar nutrients application seems to have had little effect on disease level or soybean yield (Table 3). At BARC, a Crop Circle® sensor was used to evaluate phytotoxicity. The foliar application of Mn and especially B caused some level of phytotoxicity, and could explain the slight yield depression with these treatments.
There was a significant increase in leaf concentrations of applied foliar nutrients in most cases in 2006, except for Mn levels at DSAC. Soil application of KCl at BARC significantly increased Cl levels in the plant, but a similar trend at DSAC was not statistically significant (Table 6). BARC also had a gain in leaf S with the application of potassium sulfate. However, none of these increases in plant tissue levels led to increased soybean yield (Table 6).
Frogeye leaf spot presence was not prevalent this year. The low presence can be explained by environmental conditions that minimized the presence of this disease (low humidity and moisture and high temperature). Disease index was higher at DSAC than BARC, but did not affect soybean grain yields (Table 4). There was a significantly higher presence of the disease in the RR variety than with the non-RR at both locations (similar to 2006). Fungicide application at DSAC significantly reduced disease index levels and increased yield. The application of Cl, Mn, and B reduced disease index levels at DSAC, but again with such low disease pressure, there was no effect on soybean yields (Table 4).
As in previous years, the application of K, S, Cl, Mn and B increased these nutrient levels in soybean leaf samples (with the exception if Cl at BARC), but without increasing yields (Table 7). The RR variety had higher levels of K, S, and Cl, but lower levels of B and Mn than the non-RR variety at both locations. The Mn and Cl trends were similar to 2006, although the Mn differences were not significant in 2007.
Frogeye spot presence was relatively low in this year (Table 5). In BARC the use of different varieties produced differences in the index level of the disease and yield. However, there was no response in the increase/reduction of the disease index. Additionally, there was no yield gain due to the addition of fertilizers. In DSAC only, the application of Cl+Mn+B significantly reduced the disease index but was not reflected in an increase of yield.
The application of K, S, Cl, Mn and B increased these nutrient levels in soybean leaf samples without increasing yields (Table 8). The RR variety had higher levels of Cl in BARC that the non-RR.
The fungicide treatment reduced the incidence of frogeye leaf spot in 2006. There was a positive yield response to this application at both locations. In 2007, only at DSAC, was there a reduction of disease with an associated increase in yields. No clear response was observed in 2008. None of the fertilizer treatments studied significantly affected soybean grain yields at any year/location. Variety differences were varied and there were few interactions between variety and fungicide or, between variety and fertilizer treatment. The foliar application of B usually increased soybean leaf B and the application of Mn usually increased leaf Mn, but neither affected soybean yields. Application of other nutrients such as K or S did not significantly change disease index levels compared to the check plots in any year.
The addition of potassium (either sulfate or chloride forms) had no consistent results in reducing the presence of frogeye spot (Cercospora sojina) by either year or location, indicating possible spatial variability. The addition of fungicides reduced the disease index level in both locations. In general, the addition of Mn resulted in a reduction of the disease index in most locations and years. However, this response was not always significant.
The results indicated that there were differences in the presence of the disease and nutrient uptake based on variety type (RR vs. Non-RR). However, no differences were observed when the additions of fertilizers were included in the final observations. These results suggest that there is a potential to use nutrient management combined with fungicide application in reducing the index of frogeye spot in Illinois.
A spatial variability statistical analysis will be performed to separate possible effects due to this variable (work in progress).
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