Foliar fungicide timing and interaction with soybean maturities: Effect on soybean disease suppression and yield

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1 Trial: 2005 Fungicide Growth Habit Study Foliar fungicide timing and interaction with soybean maturities: Effect on soybean disease suppression and yield Bruce Potter, University of Minnesota Department of Entomology and Southwest Research and Outreach Center Summary A strobilurin fungicide (Headline, BASF Corporation) was applied to a 0.8 and 2.1 maturity soybean at R2, R3 and R5 growth stages and a 2.6 maturity soybean at R1and R3 stages. Yield and quality differences by cultivar were observed. These were not influenced by fungicide at any timing under a low foliar disease, high yield environment. Fungicide applications did influence senescence and incidence of Septoria symptoms. A fungicide timing by cultivar maturity interaction for nodes/plant and pods/plant did not translate to yield. Background Even before identification in the southern United States during November, 2004, Asian soybean rust (ASR) caused by the fungus Phakospora pachyrhizi has been of concern to Minnesota soybean growers. Strobilurin herbicides have shown disease preventative properties as well as potential benefits in the perceived absence of disease. Questions on the yield benefits from fungicide applications to indeterminate soybean as well as optimal timing to this soybean type for preventative treatments for soybean rust exist. This trial was implemented to examine disease prevention and yield effects, based on timing, on a range of indeterminate soybean maturities. In the absence of ASR, the effects on endemic diseases and plant growth would be examined. Objective This trial was designed to examine: 1) The effect of strobilurin fungicide, Headline in this trial at various growth stage timings and maturities on soybean rust or other foliar disease under Southwest MN conditions 2) Effect of these fungicide applications, on plant health and 3) Effect of fungicides on yield. Site description and application description This trial was located at the University of Minnesota Southwest Research and Outreach Center, Lamberton, Redwood County, MN. Plots were sprayed with an CO2 backpack sprayer (R&D sprayers) XR flat fans 20 GPA and 35 PSI set 18 inches above the canopy. Plot design was a 3(cultivar) X 4(fungicide timing) factorial in a randomized block with four replications. Site and application details are shown in Table 1. Planting was delayed due to an excessively wet, cold spring. June October temperature was above normal. Due to adequate moisture and warm temperatures, corn and soybean yields were above average. Detailed temperature and rainfall information for this location can be found at Infection symptoms of several above ground soybean diseases were observed early in the growing season. Most prevalent were bacterial blight (Pseudomonas syringae) and Brown spot (Septoria glycines), the former particularly prevalent in the 0.8 maturity cultivar and not controllable with fungicides. Both these diseases were present in the lower canopy season long. Other late season diseases observed include pod and stem blight (Diaporthe phaseolorum var. sojae) Anthracnose (Colletotrichum spp.) and charcoal rot (Macrophomina phaseolina), the later disease often observed but

2 aggregated in distribution. Brown stem rot (Phialophora gregata) was present in this trial, but incidence was not rated.other diseases observed at below levels that could be rated include Cercospora blight (Cerocospora kikuchii), Frogeye leafspot (Cercospora sojina), downy mildew (Peronospora manshurica) and Stem canker (Diaporthe phaseolorum var. caulivora). Table 1. Site and treatment application conditions Location C2 U of M SWROC Lamberton Redwood County MN Soil type Normania Loam O.M. ph 5.7 P 15 ppm 159ppm Zn Primary tillage Fall 2004 Soil save 2004 Spring tillage 5/27/2005 Field cultivator 2x Cultivar AG0801 AG2107 DK2652 Planting date May 28,2006 Seeding rate JD maxemerge w'cones Planting depth 1.5 in Row spacing 30 in Seed bed cond. MEDIUM/TRASHY WET Weed control 4/27/2005 Prowl H pts PPI 6/22/2005 Roundup Weathermax 22 oz POST 6/22/2005 Assure II 5 oz POST 7/15/2005 Roundup Weathermax 22 oz POST 8/2/2005 Warrior 3.2 oz POST Application information date time Wind (mph) temp (F) %R.H. trt no 7/11/2005 7:30 AM S ,6,10 7/25/2005 8:00 AM S /28/2005 5:00 PM S /3/2005 6:40 PM W ,7,11 8/9/2005 7:15 AM S Equipment CO2 backback sprayer 20 GPA 40 PSI 8002 XR flat fan nozzles Methods Asgrow 0801, Asgrow 2107, and DeKalb 2652 soybeans were planted with a 4 row JD MaxEmerge planter on May 28. These cultivars were selected to represent shorter than adapted, adapted and longer than adapted maturities for the geographic area of the trial. Headline was arbitrarily selected to represent the strobilurin class of fungicides. Applications were geared to cultivar growth stage and all R3 and R5 treatments were not applied on the same day. The R5 growth was not applied to the 2.6 maturity soybean due to severe lodging. Insect interactions within this trial should have been minimal. Soybean aphid (Aphis glycines) populations were controlled by applying Warrior insecticide at 3.2 fl. oz. /acre to all plots on August 2nd. Aphids were assumed to be controlled well in advance of yield loss and populations remained very low for the remainder of the growing season. Due to prolong presence of green tissue, 2 nd generation bean leaf beetle populations were more abundant in the 2.6 maturity soybean than other cultivars during late September.

3 As cultivars matured, Septoria was estimated by a visual rating of lower canopy in five locations in each. The following rating scale was used: 1) No identifiable disease observed, 2) Brown spot detected, 3) Brown spot pronounced in lower canopy 4) Brown present in mid to upper canopy possibly with lower canopy leaf loss, 5) Brown spot pronounced in mid to upper canopy. Simultaneously, senescence (plant health or delayed maturity) was visual rated as % of canopy lost. The brief window to detect differences was missed for the 2.6 maturity soybean due to a frost. At maturity 5 plants/plot were rated for pod and stem blight, anthracnose and charcoal rot symptoms. 5 plants/plot were measured for height and dissected to determine the yield components nodes/plant and pods/plant. Results and discussion Results are presented in Tables 2-4. The differences in senescence and brown spot symptoms were not obvious but could be detected in the lower canopy after lower leaf loss had started. In this trial, senescence differences were short-lived and did not affect harvestability. Table 2. Factorial analysis of Headline timing and maturity Treatment Replication Maturity Fungicide Maturity x timing Timing Yield YES YES NO NO Oil YES YES NO NO Protein YES YES NO NO 200 seed weight YES YES NO NO Senescence NO YES NO NO Septoria NO YES YES NO nodes/plant NO YES YES YES pods/plant YES YES YES YES Pod & stem blight symptoms NO NO NO NO Anthracnose YES YES NO NO Charcoal rot NO NO NO NO As should be expected, differences in yield and growth between cultivars were observed. The short maturity soybean (0.8) had the lowest yields and less vegetative growth than other varieties. The 2.6 maturity had the greatest vegetative growth and pod sites (nodes) while the 2.1 maturity had the highest yield and number of pods. Differences in the yield components nodes and pods per plant as well as the diseases Septoria and anthracnose were associated with maturity (cultivar). Septoria (in the two varieties rated) and yield components were related to fungicide timing while senescence was not. Maturity (cultivar) and fungicide timing interactions were significant only for yield components and anthracnose. However, the differences in yield components measured did not translate to yield and these differences may be due to sample error. Data are presented in detail in Table 3 (senescence and disease) and table 4 (yield and soybean growth). The incidence of anthracnose symptoms was lower in the 0.8 cultivar than 2.1 and 2.6 maturities. All fungicide treatments reduced anthracnose incidence Septoria was reduced with all fungicide timings, particularly R1-2 and R3, compared to untreated. The 0.8 cultivar had less anthracnose at the R2 and R3 timings than

4 untreated. Anthracnose incidence by fungicide timing differences were not observed in other cultivars and may be related to senescence. Interestingly differences in pod and stem blight, another stem disease by cultivar or fungicide timing were not observed. These data do provide evidence that Headline does influence disease. They also provide evidence that prophylactic fungicide applications do not necessarily provide economic yield benefits under low disease pressure. These data point out the need to develop scouting guidelines and disease predictive models to maximize economic benefit from foliar fungicide applications in soybean. Additionally, it is apparent that better diagnostic tools than relaying on symptoms alone for rating disease differences are needed. Acknowledgments: This study was funded in part by the MN Rapid Response Fund and MSRPC. Technical support was provided by Jeff Irlbeck, Jodie Getting and Steve Quiring Trade Name Common Name Class Company Headline pyraclostrobin strobilurin BASF Corporation Warrior l-cyhalothrin pryrethroid insecticide Syngenta Crop Protection, Inc. The information given in this publication is for educational purposes only. Reference to commercial products is made with the understanding that no discrimination is intended and no endorsement by the University of Minnesota any specific product(s) used in this implied.

5 Table 3. The effect of triazole and strobilurin foliar applied fungicides on senescence and disease. University of Minnesota Southwest Research and Outreach Center, Lamberton, MN. (2005). B. Potter

6 Table 4. The effect of triazole and strobilurin foliar applied fungicides on soybean growth and yield. University of Minnesota Southwest Research and Outreach Center, Lamberton, MN. (2005). B. Potter

Soybean plant health: Foliar fungicide and insecticide effects on soybean disease suppression, senescence and yield I.

Soybean plant health: Foliar fungicide and insecticide effects on soybean disease suppression, senescence and yield I. Bruce Potter, J. Irlbeck and J. Getting, University of Minnesota, Southwest Research