Does the Application of a Fungicide after a Herbicide Reduce Soybean Injury and Increase Yield?

A total of four field experiments were conducted during 2017, 2019 and 2020 in Ontario, Canada to determine if applying a fungicide 2 - 3 days after a herbicide, applied POST, reduces visible injury, increases crop vigour and increases yield of soybean. At 3 DAB (days after fungicide application), the POST application of glyphosate, fomesafen, bentazon, thifensulfuron-methyl, cloransulam-methyl and imazethapyr caused 0, 11%, 5%, 18%, 9% and 12% visible injury in soybean, respectively. The injury decreased over time with less than 5% injury at 8 WAB (weeks after fungicide application) in all treatments evaluated. The application of pyraclostrobin/fluxapyroxad after the application of herbicides evaluated did not reduce soybean injury. Soybean vigour with glyphosate, fomesafen, bentazon, thifensulfuron-methyl, cloran-sulam-methyl and imazethapyr applied POST without the fungicide application was 100%, 91%, 95%, 84%, 91% and 88%, respectively at 3 DAB. The soybean vigour increased over time to 95% - 100% at 8 WAB. The application of pyraclostrobin/fluxapyroxad after the herbicide application did not improve soybean vigour, except with thifensulfuron-methyl where soybean vigour was improved 6% when followed by pyraclostrobin/fluxapyroxad. There was no effect of herbicide and fungicide treatments on soybean yield except for thifensulfuron-methyl and imazethapyr without the fungicide treatments which reduced soybean relative yield 7% and 10%, respectively. The application of pyraclostrobin/fluxapyroxad after the application of imazethapyr increased soybean yield 3%. Based on these results, applying pyraclostro-bin/fluxapyroxad fungicide 2 - 3 days after glyphosate, fomesafen, bentazon and cloransulam-methyl does not affect soybean injury, vigour or yield, but it can slightly enhance the vigour and yield of soybean when applied after thi-fensulfuron-methyl and imazethapyr.


Introduction
Soybean [Glycine max (L.) Merr.] is an economically important crop produced in southwestern Ontario. In 2020, growers in Ontario seeded over 1.2 million hectares of soybean and produced nearly 4 million tonnes valued at over $1.6 billion making it the second most important grain cash crop grown after corn in the province in terms of total farm gate value [1]. Weeds and diseases if not managed can reduce the yield, quality, and profitability of soybean.
The common problematic weeds in Ontario include common lambsquarters (Chenopodium album), Canada fleabane (Conyza canadensis), common ragweed (Ambrosia artemisiifolia), eastern black nightshade (Solanum ptycanthum) and pigweeds (Amaranthus spp.) [2]. Soybean growers in Ontario often utilize glyphosate, fomesafen, bentazon, thifensulfuron-methyl, cloransulam-methyl and imazethapyr applied POST to manage these weeds [2]. However, some of these herbicides can cause transient crop injury which may affect soybean vigour and yield [2] [3]. Diseases of concern in soybean production under Ontario environmental conditions include septoria brown spot (Septoria glycines), phytophthora root and stem rot (Phytophthora sojae), powdery mildew (Microsphaera diffusa), downy mildew (Peronospora manshurica), brown stem rot (Phialophora gregata), sudden death syndrome (Fusarium verguliforme), white mould (Sclerotinia sclerotiorum) and Asian soybean rust (Phakospora pachyrhizi) [4] [5]. Soybean growers often use a relatively new class of strobilurin fungicides which are known as quinone-outside inhibitor (Qol) fungicides to control these diseases [6] [7]. Foliar applications of strobilurin fungicides after major abiotic stress events have been promoted to reduce soybean injury, increase vigour, improve plant health, and increase yield [7]- [14].  [24]. Some soybean producers use foliar fungicides in the absence of diseases to reduce crop stress and increase seed yield [24]. Fungicide application after a herbicide application may have the potential to reduce herbicide induced plant stress. This assumption has not been investigated for soybean production under Ontario environmental conditions. To our knowledge, there is no published information on the benefits of applying a foliar fungicide three days after the application of a herbicide on soybean injury, vigour and yield under Ontario environmental conditions. Lack of information, or incorrect information, leads to unnecessary pesticide applications which results in increased pesticide loading in the environment, increased pressure for the evolution of pest resistance, elevated crop production costs, and reduced net returns for soybean producers.
The objective of this research was to determine if applying a fungicide 2 -3 days after a herbicide, applied POST, reduces visible soybean injury, increases crop vigour and elevates yield. The experimental design was factorial, with herbicide treatment and fungicide treatment as the two factors. Field trials were established as RCBDs and each treatment (factor combination) was replicated four times. Treatments included a non-treated control, glyphosate, fomesafen, bentazon, thifensulfuron-methyl, cloransulam-methyl and imazethapyr applied POST alone and followed by pyraclostrobin/fluxapyroxad fungicide (2 to 3 days later) at rates listed in Table 1.

Materials and Methods
Herbicide (including adjuvants used) and fungicides rates used were based on manufacturers' recommended rates in soybean in Ontario, Canada.    Table 3. When no fungicide was applied, thifensulfuron and imazethapyr reduced soybean vigour 6% (Table 3).

Results and Discussion
Interestingly, when a fungicide was applied 2 to 3 days after the herbicide appli-  cation there was no decrease in soybean vigour. The application of pyraclostrobin/fluxapyroxad 2 to 3 days after the application of a thifensulfuron-methyl applied POST increased soybean vigour 6% at 8 WAB. These results are similar to Swoboda and Pedersen [6] who found that soybean biomass increased 10% with foliar application of pyraclostrobin. Joshi et al. [25] reported that pyraclostrobin increased nitrogen fixation in soybean which can result in enhanced plant growth and increased biomass.
Soybean maturity as indicated by the seed moisture content at harvest was not affected with all treatments evaluated ( Table 2). There was an interaction between herbicide and fungicide so the simple effects on soybean yield are presented in Table 3. Thifensulfuron-methyl and imazethapyr without a subsequent fungicide application reduced soybean yield 7% and 10%, respectively; the application of glyphosate, fomesafen, bentazon and cloransulam-methyl did not  reduce soybean relative to the non-treated control ( Table 3). The application of pyraclostrobin/fluxapyroxad after the application of imazethapyr increased soybean yield 3% (Table 3). Pyraclostrobin/fluxapyroxad fungicide applied to the non-herbicide treated control plots reduced soybean yield 5%, the authors attribute this observation to experimental variability (Table 3). In other studies, Swoboda and Pedersen [6] found no seed yield advantage in absence of diseases with foliar application of strobilurin fungicides in soybean. Other researchers have also found no significant yield increases in soybean with foliar applied fungicides in absence of diseases [26] [27]. Henry et al. [23] reported as much as 100 kg·ha −1 seed yield increase with the application of pyraclostrobin in soybean when disease and insect pressure was minimal. Seed mass was also increased 3% with pyraclostrobin applied POST in soybean [23]. However, the authors concluded that a yield increase may not result in an increased economic benefit as input costs may be greater than the yield benefits [23]. In another study, pyraclostrobin applied POST caused no increase in soybean health or yield in the absence of diseases. Additionally, Kandel and Mueller [28] who studied soybean

Conclusion
This study concludes that glyphosate applied POST without and with a follow up application of pyraclostrobin/fluxapyroxad fungicide caused no visible soybean injury and there was no decrease in soybean vigour and yield. Fomesafen, bentazon, and cloransulam-methyl applied POST caused up to 11% visible injury and reduced vigour up to 9% but caused no reduction in soybean yield. The follow-up application of pyraclostrobin/fluxapyroxad fungicide after fomesafen, bentazon, and cloransulam-methyl POST application provided no reduction in soybean injury and no increase in soybean vigour and yield. Thifensulfuron-methyl and imazethapyr applied POST caused up to 18% visible injury in soybean and reduced soybean vigour up to 16%. The follow up application of pyraclostrobin/fluxapyroxad after thifensulfuron-methyl increased soybean vigour 6% and the follow up application of pyraclostrobin/fluxapyroxad after imazethapyr increased soybean yield 3%. Results indicate that pyraclostrobin/fluxapyroxad fungicide can be safely applied 2 -3 days after POST application of glyphosate, fomesafen, bentazon, thifensulfuron-methyl, cloransulam-methyl and imazethapyr. The use of pyraclostrobin/fluxapyroxad after the application of the thifensulfuron-methyl was associated with increased vigour in soybean at 8 WAB. The use of pyraclostrobin/fluxapyroxad after the application of the imazethapyr was associated with increased soybean yield. Further studies are needed to determine the mechanisms involved.