Evaluation of Foliar Fungicides on Soybean in West Central Missouri: 2016

Published: March 31, 2017

A question often asked by Missouri soybean growers, and likely, by soybean growers across the Midwest, is whether or not it is worthy to apply a fungicide to control foliar diseases in soybean. The answer to this question calls for an important consideration: Several foliar diseases occur each year in soybean in Missouri, yet their level of intensity and resulting impact on yield in a growing season depend on the susceptibility of a given soybean cultivar and the suitability of the environmental conditions for disease development. Early-season diseases like Septoria brown spot, for example, may have a significant negative impact on the yield of a susceptible or moderately resistant cultivar if weather conditions are favorable for disease early in the season and towards the reproductive stages R4 to R6. Late season foliar diseases like frogeye leaf spot, Cercospora leaf blight and soybean rust can significantly reduce yield if weather conditions are favorable for epidemics towards mid- to late season.

Generally speaking, fungicide treatments are justifiable when they reduce disease impact on yield such that the profit from the gained yield advantage is much greater than the cost associated to the fungicide application. Many commercial fungicides are available for control of foliar diseases of soybean, and treatments are typically recommended for the beginning of blooming and later reproductive stages. Timing and rate of application are important when a decision to do a fungicide treatment has been made. This report summarizes the results of two fungicide trials carried out in West Central Missouri in the 2016 growing season. The purpose of the trials was to evaluate the effect of several fungicides (applied at different timing and application rates) on the development of foliar fungal diseases and yield in soybean.

Description of the site and experimental design
The trials were conducted in a non-irrigated, conventional till field (14.5% residue cover) in Johnson County, MO, near Warrensburg. Plots were planted to the soybean cultivar AsGrow 4034 at a seeding rate of 140,000 seeds per acre. Agronomic management of the plot followed the recommended practices for soybean in Central Missouri. The experimental plots consisted of four 30-ft long rows, spaced at 30 inches. Plots were delimited by mowing two-foot alleys when plants were at the developmental stage V2. The treatments were assigned to the experimental units in a randomized complete block design with four replications per treatment.

Treatment Applications and Disease Assessment
The fungicide treatments in both trials were applied at reproductive stages R1 and R3 and at different application rates (Tables 1 and 2). Treatments were applied with a CO2 pressurized sprayer calibrated at 38 psi and a flow rate of 22 gal of water/acre. Intensity of foliar diseases was assessed in early August 2016 in both trials. The amount of disease per plot was determined in two 20-inch wide x 40-inch long x 40-inch tall cubic quadrats that were placed systematically on each of the center rows of each plot. Septoria brown spot (SBS; Septoria glycines) incidence was measured in the lower, inner side plant canopy of the center row on which the quadrat was placed. Amount of frog eye leaf spot (FLS; Cercospora sojiina) and bacterial blight (BB; Pseudomnas syringae pv. glycines) was assessed as the number of trifoliolate leaves with symptoms of these diseases on the top canopy of the plants contained in the quadrat. Assessment of bacterial blight was done as an extra to the trial. However, fungicide treatments were not aimed at controlling this disease.

Figure 1. Symptoms of Septoria brown spot on first trifoliolate leaf of soybean (Photo taken on July 8, 2016)

Figure 2. Symptoms of Septoria brown spot in lower leaves by midseason (Photo taken on August 5, 2016)

Figure 3. Symptoms of frogeye leaf spot on soybean leaf by midseason (Photo taken on August 8, 2016

Figure 4. Symptoms of bacterial blight on soybean leaf by midseason (Photo taken on July 21, 2016)

RESULTS

Effects of the fungicides on the foliar diseases
In both trials, the incidence of SBS in the lower canopy was significantly reduced with the application of fungicides as compared with the untreated control (Tables 1 and 2). Amount of FLS was not significantly reduced with the applications of any of the fungicides. This was presumably due to the very low FLS pressure in the season. The incidence of SBS in both trials varied from 0.17 to 58% whereas the number of trioliolate leaves with symptoms of FLS varied from 0 to 2 leaves per quadrat. Consistently, the fungicide applications at developmental stage R1 resulted in plots with lower SBS and higher yield (Tables 3 and 4).

Effect of fungicides on yield
Average soybean yield ranged from 69 to 75.1 bu/acre in trial 1 and 69.2 to 74.4 bu/acre in trial 2 (Tables 3 and 4). Mean yield response as a result of a fungicide application varied by timing of application (Tables 3 and 4), with relatively increased yields observed in plots that received fungicide applications at R1. Overall yield difference among the treatments in the two trials was not statistically significant (P > 0.05; Tables 3 and 4). However, the application of Domark at R1 (trial 1) and Preemtpor at R1 (trial 2), showed a yield advantage of 6.0 and 5.0 bu/acre, respectively, over the untreated control.

Table 1. Description of the fungicide treatments and their efficacy in control of foliar diseases in soybean; Trial 1. Johnson County, MO. 2016 growing season.

(x) Measured as incidence in the lower, inner side of the plant canopy on two 20-inch wide x 40-inch long x 40-inch tall quadrats that placed systematically in the lower plant canopy in inner side of center rows.

(y,z) Measured as the number of trifoliolate leaves with disease symptoms on the top canopy of the plants contained in two 20-inch wide x 40-inch long x 40-inch tall quadrats.

Means followed by the same letter within the disease intensity columns are not significantly different according to the least significant difference test (P = 0.05). Fungicide treatments were not aimed at controlling bacterial blight.

Table 2. Description of the fungicide treatments and their efficacy in control of foliar diseases in soybean; Trial 2. Johnson County, MO, 2016 growing season.

(x) Measured as incidence in the lower, inner side of the plant canopy on two 20-inch wide x 40-inch long x 40-inch tall quadrats that placed systematically in the lower plant canopy in inner side of center rows.

(y,z) Measured as the number of trifoliolate leaves with disease symptoms on the top canopy of the plants contained in two 20-inch wide x 40-inch long x 40-inch tall quadrats.

Means followed by the same letter within the disease intensity columns are not significantly different according to the least significant difference test (P = 0.05). Fungicide treatments were not aimed at controlling bacterial blight.

Table 3. Soybean yield (bu/acre) in plots treated with fungicides at different timing and application rates. Johnson County, MO, 2016 growing season.

Means followed by the same letter in the Yield column are not significantly different according to the least significant difference test (P = 0.05). Least significant difference = 5.4

Table 4. Soybean yield (bu/acre) in plots treated with fungicides at different timing and application rates. Johnson County, MO, 2016 growing season.

Means followed by the same letter in the Yield column are not significantly different according to the least significant difference test (P = 0.05). Least significant difference = 5.0

What can be learned from the 2016 growing season and these fungicide trials?
In West Central Missouri, weather conditions towards the end of the 2016 growing season were not conducive to high levels of late-season foliar diseases in soybean. However, conditions at the beginning of the season were favorable for early appearance and relatively high incidence of Septoria brown spot, with disease levels reaching about 60% in untreated plots by middle season. Warm, wet weather characterized by frequent and well distributed rain events occurred throughout June and July; about one rain event per week in June and two rain events in July in the area where these trials were carried out. The results of the present trials showed that all the fungicides consistently reduced incidence of Septoria brown spot in R1 and R3 applications. Yet, only the application of tetraconazole (Domark 230 ME) and fluoxastrobin + flutriafol (Preemptor) at R1 resulted in a significant yield advantage (about 6.0 and 5.0 bu/acre, respectively) over the untreated control. Considering that the cost of a ground application of Domark 230 ME at the above application rate was $13.60/acre and that the average market price of soybean at the end of 2016 was $9.54/bu, the fungicide treatment with Domark 230 ME in this area and year provided a return of $43.7/acre. Treatment with Preemptor provided a return of $27.7/acre. The observed yield advantage with the application of Domark is in agreement with a 2008 report, where a yield advantage of 3.7 to 7.7 bu/acre was observed in treatments with Domark across several locations in the Midwest, though in those studies the applications were carried out at R3.

In summary, the results of these trials indicate that in West Central Missouri, early occurrence of Septoriabrown spot followed by favorable weather for disease development towards mid-season may justify a fungicide application at R1. In southern and northern MO regions, Septoria brown spot or other foliar diseases may occur earlier or later in the season than what was observed in the locations where these trials were carried out. Therefore, early scouting for disease and attention to weather conditions and disease development can help determine whether or not the disease may reach high levels, thereby call for a fungicide treatment.

Acknowledgements: The authors thank FMC and GOWAN for their contribution and trust on us to conduct testing of theirfungicides. The authors also thank the UCM SkyHaven Airport Land Manager for allowing use of the field for the trials, and the Variety Trial Crew of University of Missouri for assisting with the planting and harvest of the plots.