S. A. Ebelhar, and J. D. Hernandez, C. D. Hart, and J. Bond 1
Graham and Webb (1991) describe resistance in the host-pathogen relationship as the ability of plants to limit the penetration, development and/or reproduction of invading pathogens. Although both factors “tolerance and resistance” are genetically controlled, the environment and particularly plant nutrition status of the host can certainly have an impact. We have challenging years to come with soybean production and disease management. A primary challenge will be to remain competitive when Asian rust appears in our region. Therefore, an integrated management and control program will be needed.
Research for tools 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 to consider. Any potassium (K) plant deficiency will be associated with thin cell walls, smaller, thinner and shorter roots, lower sugar accumulation in the foliar tissue, and accumulation of unused nitrogen (N). All of which encourages disease infection, particularly soybean Asian rust (PPI, 1998). Potassium is involved in numerous functions in the plant such as in enzyme activation, cation/anion balance, stomatal movement, phloem loading; assimilate translocation and turgor regulation (Peoples and Koch, 1979). Perrenoud (1990) reviewed several thousand references and concluded that the use of K decreases the incidence of fungal diseases in 70% of cases, in the same review he reported that yield was always increased in plants infested with fungal diseases. This suggests a great potential by including appropriate K nutrition management in foliar fungus affected soybean plants, and particularly for soybean Asian rust integrated management (Piccio and Franje, 1980).
Chloride (Cl), usually in the form of potassium chloride (KCl), has been shown to reduce the severity of several fungal diseases (Fixen, 1993). Manganese (Mn) is essential in reducing the incidence of foliar disease in crops (Graham and Webb, 1991; Huber and Wilhelm, 1988). However, newer glyphosphate resistant soybean varieties have shown lower capacity for soil up-take of Mn or to translocation Mn within the plant (Huber et al., 2004). Manganese plant nutrition reduces the incidence of foliar disease in most crops. The main reason is its role in the synthesis of lignin and phenols and thus, in controlling pathogens (Graham and Webb, 1991). Research has shown that when Mn concentrations are low in soil-plant systems there is greater susceptibility for the attack of fungal pathogens (Huber and Wilhelm, 1988). Boron (B) is related to the production of small fissures and cracks that may be the initial entrance or access to fungal diseases. The new genetic information about genetic modified soybean and its relationship with uptake of nutrients combined with new 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.
A field study continued in 2007 at two locations in southern 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 1) determine the effects of potassium, chloride, boron and manganese nutrition on the response of soybeans to diseases (possibly including Asian soybean rust), 2) evaluate the nutrient interaction effects with and without the application of fungicides, and 3) compare these effects across Roundup Ready and conventional herbicide programs.
In 2005, three soybean varieties (Pioneer brands 94B54 [Roundup Ready®, RR] and 94B53 [non-RR], and Asgrow brand 4502 [RR]) were planted at a planting rate of 175,000 seed/acre in 30" rows. In 2006, only the two Pioneer varieties were planted. In 2007, the soybean varieties were changed to Pioneer 93M96 (RR) and 93B86 (non-RR) due to unavailability of previous varieties. Prior to planting and fertilizer application, soil samples were collected from the 0-6", 6-12", 12-18" and 18-24" depths. These samples were dried, crushed, and analyzed for nutrient composition (Tables 1-3). In general, soil P, K, and Mn levels were higher in 2005 than 2006 or 2007, but S levels were lower. Cl levels were much lower in 2006 and 2007 than 2005.
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. 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 2005 and 2006, roughly corresponding to the V4 and V10 soybean growth stages, but only once in 2007 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 listed above. The fungicide treatment consisted of 14 oz/acre Quilt® applied at the V10 growth stage in 2005, but in 2006 and 2007 Headline® replaced Quilt and was 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. Dates are summarized in Tables 1 and 2 below. In 2007 at DSAC, the non-RR soybeans had to be replanted due to the misapplication of Roundup to these plots.
Trifoliate leaves were collected 3 weeks after fungicide applications and evaluated for diseases and phytotoxicity. Twenty of the upper-most fully developed leaves were also collected for nutrient composition analysis. Soybean grain yields were taken at physiological maturity.
Weather. The weather in 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 probably accounts for good yields occurring at both locations despite the early season low rainfall.
In 2006, rainfall was normal to above normal at both locations (Figures 1 and 2). There appeared to be less problems with phytotoxicity, especially at DSAC. In 2007, below average rainfall early in the growing season and late significantly reduced yields over prior years at both locations, especially at DSAC (Figure 3). The single application of Mn and B resulted in very little problem with phytotoxicity compared to previous years.
2005 Results. The application of foliar treatments of B and/or Mn led to phytotoxicity problems at both locations (Table 3). These effects were not severe but very consistent. There was a very slight (insignificant) yield reduction when both B and Mn were applied together, which indicates that the leaf injury was not severe enough to affect soybean development. None of the fertilizer treatments significantly affected soybean yields.
At BARC, the fungicide application caused a significant increase in phytotoxicity and probably led to a significant reduction in yield (Table 3). At DSAC, there was no effect of fungicide on phytotoxicity or grain yields, even though there was a significant effect on frogeye leaf spot, the only measurable disease at this location. The application of the fungicide treatment significantly reduced the number of lesions (eyes), but had no effect on yield. Apparently the presence of frogeye leaf spot was not severe enough to cause yield loss. There were no other discernable disease presence at either location.
Varietal effects on phytotoxicity, yields, and diseases were small and appear to be due to difference in genetics as there were no significant variety x fertilizer treatments effects and few
significant variety x fungicide treatment effects. The only highly significant variety x fungicide interaction was for frogeye control at DSAC. In this situation, some varieties responded to fungicide application and others did not. In the case where there was no response to fungicide, it appears that the variety has a built-in resistance to frogeye, as the no fungicide treatment had a very low incidence of frogeye lesions.
In general, the effects of fertilizer treatments and fungicide on trifoliate leaf nutrient composition were small and/or insignificant (Tables 5 and 6). The foliar application of Mn, alone or in combination with B, increased leaf N at BARC and leaf Mn at both locations. The foliar application of B, alone or in combination with Mn, increased leaf B at both locations. The application of B and Mn together had no effect on the uptake of B compared to application of B only, but there was a significant decrease in leaf Mn compared to application of Mn only. Variety effects again appear to be small and probably relate more to the genetic make-up of the plant than interactions with treatments.
There were few significant interactions between fertilizer treatments, fungicide and/or variety. At BARC, there was a significant interaction between fungicide and fertilizer treatments for leaf B and Mn levels. It appears that when the Mn or B or both were applied with fungicide, there was significantly more of the nutrient taken up by the leaves than when the nutrients were applied with a fungicide. This may relate to the surfactant package used with the fungicide increasing the uptake of the nutrients. A significant variety x fertilizer treatment interaction existed for leaf Mn levels at DSAC. Here the RR varieties had significantly lower levels of leaf Mn with application of fertilizer Mn than the non-RR variety, a relationship similarly identified by researchers at Purdue (Huber, et al., 2004).
2006 Results. Frogeye leaf spot was the only significant disease of magnitude at the two locations in 2006. In 2006, this disease was rated on a 1-10 scale with 1 being no disease and 10 being severe. Disease was again fairly light (Table 4). At BARC, frogeye was also very spotty, which probably explains why there was a lower yield response to fungicide at this location. At DSAC the frogeye was fairly consistent, and apparently severe enough that there was a 6.6 bu/acre yield response to fungicide application. As in 2005, foliar nutrients appeared to have little effect on disease or soybean yield in 2006.
At BARC, a Crop Circle® sensor was used to evaluate phytotoxicity. It appears that the foliar application of Mn and especially B is causing some phytotoxicity problems (Figure 4), and could explain the slight yield depression with these treatments. Future research may be needed to determine if one application date or perhaps lower rates of products at the two dates might alleviate this problem.
As in 2005, 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 increase Cl levels in the plant, but a similar trend at DSAC was not statistically significant. BARC also had a gain in leaf S with the application of potassium sulfate. But as in 2005, none of these increases in plant tissue levels led to increased yields, perhaps due to the low disease presence during grain fill or perhaps due to phytotoxicity problems masking any beneficial effects of the micronutrients.
2007 Results. Frogeye leaf spot was again the only significant disease in 2007 and pressure was low from this disease (Table 6). Levels were higher at DSAC than BARC, but was probably not high enough to dramatically affect soybean grain yields. There was significantly more disease noted in the RR variety than with the non-RR at both locations (similar to 2006), and fungicide application at DSAC significantly reduced disease and increased yield. The application of Cl, Mn, and B significantly reduced disease levels at DSAC, but again with such low disease pressure, there was no effect on soybean yields.
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 none significantly increased yields (Table 9). 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.
Foliar application of B and/or Mn caused some phytotoxicity problems in 2005 at each of the two locations studied, but less of a problem in 2006 and 2007. This could be related to limited rainfall at these locations around the time of foliar application in 2005 compared to 2006, and the reduced rates used in 2007 had less effect. The fungicide application at BARC in 2005 significantly increased the level of phytotoxicity and reduced grain yield, but no problems were observed in 2006 or 2007 with the different fungicide used. At DSAC, the fungicide treatment reduced the incidence of frogeye leaf spot, but there was no effect on yield in 2005, but in 2006 there was a positive yield response to fungicide application at both locations. In 2007, only at DSAC was there a reduction of disease with an associated increase in yields.
None of the fertilizer treatments studied significantly affected soybean grain yields in any year. 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 yields. Application of other nutrients such as K or S usually did not significantly increase leaf levels of these nutrients compared to the check plots in 2005 and 2006, presumably because of the nutrient supplying power of the soil, but did in 2007.
Table 1. Study soil tests and important dates, 2005.
Table 2. Study soil tests and important dates, 2006.
Table 3. Study soil tests and important dates, 2007.
Figure 1. Rainfall at Brownstown, 2006.
Figure 2. Rainfall at Dixon Springs, 2006.
Figure 3. Rainfall and 30-yr normal rainfall for DSAC and BARC, 2007.
Figure 4. NDVI measurements at BARC, 2006.
1S. A. Ebelhar is Agronomist and C. D. Hart is Research Specialist, Crop Sciences Department, University of Illinois J. D. Hernandez is Asst. Professor and J. Bond is Assoc. Prof., Southern Illinois University.
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