Before using a foliar fungicide, it is important to scout and determine the type of disease(s) present, as only fungal pathogens can be controlled with these products. Fungal diseases that can be managed with foliar fungicides include anthracnose, Septoria brown spot, Cercospora leaf blight, frogeye leaf spot, pod and stem blight, and soybean rust. Bacterial diseases such as bacterial blight or bacterial pustule, and viral diseases such as soybean vein necrosis virus are not controlled by fungicides.
Several fungal diseases are also not well-controlled by foliar fungicides, due to time of infection or where it occurs in the plant. For example, stem canker can cause severe yield losses in soybeans, but results of fungicide applications have often been inconsistent. This is likely because infection occurs during early vegetative growth and fungicides are often not applied in time to prevent it. Charcoal rot and sudden death syndrome also can cause severe yield losses, but because infection occurs in the roots, they are not controlled by foliar fungicides.
Still other diseases may be controlled by fungicides, but because yield is rarely reduced by these pathogens, treatment is generally not recommended. Alternaria and Phyllosticta leafspot and downy mildew are examples of such diseases.
Figure 1. Septoria brown spot (left) and frogeye leaf spot (right) are 2 diseases capable of reducing yield in soybeans that can be managed with foliar fungicides.
White mold is another disease that requires precise timing for fungicide control. Several foliar fungicides may be useful against white mold when applications are targeted at early flowering (R1) and there is sufficient penetration of spray to the lower soybean canopy. Products labeled for white mold control include synthetic fungicides (DuPont™ Aproach® fungicide, Topsin® fungicide, Domark® fungicide and Endura® fungicide), a biological fungicide (Contans® fungicide) and the herbicide lactofen (Cobra® herbicide and Phoenix® herbicide).
Research results with DuPont Aproach fungicide have been very positive when compared to older fungicide choices such as Domark and Topsin. In research trials conducted by Ohio State University, Michigan State University, and the University of Illinois in 2009 to 2011, DuPont Aproach fungicide reduced white mold severity and increased yield by an average of 7.2 bu/acre.
Other Physiological Effects
Although leaf diseases are frequently present in soybean fields at some level, they are often thought to have minimal, non-economic impacts on yield. This leads to the possibility that physiological benefits apart from disease protection often claimed for these fungicides may indeed play a role in increasing soybean yields. These benefits may include reduced ethylene production, improved CO2 assimilation, increased water use efficiency, increased stress tolerance during flowering and pod fill and delayed plant senescence.
Foliar Fungicide and Insecticide Applications
Insecticide applications to soybeans have also increased in recent years due to soybean aphid proliferation throughout most of North America. However, unlike claims for fungicide products, insecticide benefit in the absence of pests has not been heavily promoted. Nevertheless, more growers are exploring the benefits of insecticide use, particularly by including an insecticide in the tank when making a fungicide application. Although this practice is efficient from an application standpoint, growers should be aware that precise timing is usually required for optimum effectiveness of 1 or both spray components.
Pioneer Small-Plot Research Trials
Pioneer small-plot research trials were conducted over 5 years to evaluate the effect of the strobilurin fungicides Headline® and Quadris® and fungicide + insecticide combinations applied at soybean growth stage R3 to R4 (pod stages) on soybean yield. Trials were conducted at 4 to 10 locations per year at sites in Illinois, Indiana, Iowa, Minnesota, and Nebraska. Trials were planted in 30-inch rows at 160,000 seeds/acre. Fungicides and insecticides were applied at labeled rates in a total volume of 15 to 20 gallons/acre.
Table 1.Treatments and rates used in small-plot study.
|Headline||R3 to R4||6 oz|
|Quadris||R3 to R4||6.2 oz|
|Headline + Asana®||R3 to R4||6 oz and 6.4 oz|
|Quadris + Warrior®||R3 to R4||2 oz and 2.56 oz|
Research results showed that applying a strobilurin fungicide with or without an insecticide at growth stage R3 to R4 frequently results in a significant yield response.
Figure 3. Soybean yield response to Headline in Pioneer small-plot research trials.
In the 2008 research trials, all 4 strobilurin fungicide and strobilurin fungicide + insecticide applications significantly improved yield over the non-treated check across locations in Illinois, Indiana and Nebraska. At 1 location in Minnesota, treatments including an insecticide greatly improved yield due to high soybean aphid pressure at the site. Asana® applied alone increased yield by 20.5 bu/acre, Headline® + Asana® by 24 bu/acre, and Quadris + Warrior by 18.2 bu/acre.
Figure 7. Aerial image of a soybean field in Minnesota sprayed for soybean aphids. A non-sprayed strip at edge of field shows devastation caused by aphids.
Factors that Influence Disease Risk
The response to a foliar fungicide treatment will vary depending upon conditions that favor disease development. The higher the disease pressure, the more likely there will be an economic response to treatment.
Fungicide resistance prevention should also be considered in treatment decisions. The strobilurin class of fungicides, although very effective at controlling many corn diseases, is considered high risk for resistance development in fungal species. Strobilurin resistance has already been documented in disease species in other crops, and has recently been discovered in frogeye leaf spot in soybeans. Widespread indiscriminant use of fungicides increases the selection pressure on fungal pathogens, which can accelerate resistance development.
1Mark Jeschke, Agronomy Research Manager, Pioneer
2Rebecca Ahlers, Former Production Agronomist, Pioneer