Impact assessment of residual soil-applied pre-emergence herbicides on the incidence of soybean seedling diseases under field conditions

Highlights

• PRE herbicides did not consistently affect soybean seedling root rot in optimal and late-planted soybeans.

• Community composition was functionally associated with the development of seedling disease epidemics.

• Increased soybean seedling root rot in environments with higher incidence of Phytophthora and Pythium spp.

Abstract

A multi-environmental field study was conducted in 2017 and 2018 in Nebraska to investigate potential interactions between soybean seedling diseases and soil-applied residual pre-emergence (PRE) herbicides. Experiments were established from mid-May to early June in fine-textured, poorly drained soils with a history of seedling establishment problems. PRE herbicides consisted of chlorimuron-ethyl, flumioxazin, metribuzin, saflufenacil, and sulfentrazone applied at labeled rates, in addition to non-treated control. Assessments included soybean injury, seedling root lesion severity (DSI), plant height, population, biomass, and yield. Additionally, symptomatic seedling roots were sampled for fungal and oomycete organisms to expand comprehension of potential biotic associations. Greater soybean injury and reduced root biomass were observed in two distinctive environments following PPO-inhibiting PRE herbicide applications. Exceptionally in one environment, where DSI seemed (P = 0.07) lower for metribuzin in comparison to saflufenacil, PRE herbicides did not affect seedling root rot severity and no yield differences occurred among treatments. Community composition depicting Fusarium, Phytophthora, Pythium, and Rhizoctonia genera varied considerably across environments (P < 0.001) and DSI classes (P = 0.002), representing distinctive ecological environments under investigation. Phytophthora structured a large portion (>40%) of the total primary pathogenic isolates recovered in the highest DSI environment, whereas Pythium frequency ranged from 4.6% to 22% across all surveyed environments, and Rhizoctonia recovery was low (<10.3%) and sporadic. Across environments with varying DSI and soilborne pathogen composition, results indicated a lack of consistent interaction between soil-applied residual PRE herbicides and the incidence of soybean seedling diseases in optimal to delayed planting situations.

Introduction

Seedling diseases pose a major threat to soybean production. Annual losses are estimated at 1.3 million metric tons in North America (Allen et al., 2017; Bandara et al., 2020). A composite of soilborne pathogens is associated with seedling diseases in the United States, including Fusarium, Pythium, Phytophthora, and Rhizoctonia (Ajayi-Oyetunde and Bradley, 2017; Radmer et al., 2017; Rizvi and Yang, 1996). Symptoms include seedling damping-off, root rot, stunting, and uneven crop emergence that can additionally provide weeds a competitive advantage over the crop for the remaining of the growing season. Soybeans are most vulnerable to infection during the first days of emergence. Furthermore, edaphic and climatic conditions play a significant role in disease epidemics (Rojas-Flechas et al., 2017b; Workneh et al., 1999). Unraveling vulnerability factors associated with seedling diseases can help outline management strategies to alleviate the impact of this malady.

Soil-applied residual PRE herbicides represent a pivotal component of weed management and herbicide resistance mitigation programs in conventional and genetically modified soybeans (Norsworthy et al., 2012). Adoption of PRE herbicides into a diversified weed management strategy allows rotation of herbicide sites of action while providing early-season residual weed suppression and increasing post-emergence herbicide efficacy (Arneson et al., 2019; Jhala et al., 2017; Knezevic et al., 2019). Protoporphyrinogen oxidase (PPO)-inhibiting herbicides, including flumioxazin, saflufenacil, and sulfentrazone, have been increasingly adopted as part of weed management programs in soybeans in the United States (Sarangi and Jhala, 2018). For instance, between 2012 and 2020, soybean acreage treated with sulfentrazone increased from 8% to 21%. Similarly, during the same period, saflufenacil and metribuzin consumption increased by 138% and 500%, respectively (USDA, 2012, 2020). Increased reliance on PPO-inhibiting PRE herbicides is linked to their superior efficacy on troublesome weeds, including glyphosate-resistant biotypes (Krausz et al., 1998; Oliveira et al., 2017). Sarangi et al. (2017) observed a reduction in glyphosate-resistant common waterhemp (Amaranthus rudis L.) density from 107 to 13 plants m⁻² upon the use of flumioxazin + chlorimuron-ethyl applied PRE followed by fomesafen + glyphosate POST compared to glyphosate applied alone. Meanwhile, sulfentrazone and flumioxazin applied PRE provided 81%–92% and 88%–98% control of kochia (Kochia scoparia (L.) Schrad), respectively, within 10 weeks after treatment (Hulse, 2012). Occasionally, however, PPO-inhibiting PRE herbicides cause injury to soybean, including leaf burn, desiccation, and chlorosis leading to stand reduction and yield losses (Miller et al., 2012; Zhaohu et al., 1999). A variety of factors have been suggested to enhance PPO-inhibiting herbicide injury, including applications made with an interval superior to three days from planting or during soybean emergence, soybeans cultivated in conventional tillage systems, moisture for herbicide activation, and edaphic factors (Hager, 2014; Mahoney et al., 2014; Reiling et al., 2006). Not all soybean varieties respond equally to PPO-inhibiting chemistries and tolerant lines should be considered under high-injury risk scenarios to mitigate yield losses (Taylor-Lovell et al., 2001). The risk of PRE herbicide injury is greatest when seedling emergence coincides with cool and saturated soil conditions, which is also conducive to the occurrence of certain early-season soilborne diseases in soybeans (Kirkpatrick et al., 2006; Serrano and Robertson, 2018).

Herbicides cause profound physiological changes in plants, which may alter their susceptibility to soilborne pathogens (Duke et al., 2007; Hale et al., 1981). Herbicide-stressed plants liberate more root exudates and the chemical nature of these leaked root components can stimulate or inhibit pathogen propagule germination (Brown and Curl, 1987; Lee and Lockwood, 1977). Alternatively, some herbicides can reduce disease severity, as reported for lactofen and Sclerotinia sclerotiorum in soybeans (Dann et al., 1999; Nelson et al., 2002). The mild oxidative stress resulting from PPO-inhibiting herbicide uptake can induce the synthesis of antimicrobial phytoalexins (Landini et al., 2003), but the magnitude of responses is likely dependent on a combination of factors including the pathosystem and crop stage under evaluation. There have been studies demonstrating PRE herbicides do not predispose seedlings to infection, particularly in cotton (Gossypium hirsutum L.) and soybean (Agamalian, 1964; Bauske and Kirby, 1992; Heydari and Misaghi, 1998), but controversial evidence has also been documented under field and controlled conditions (Bowman and Sinclair, 1989; Bradley et al., 2002; Carson et al., 1991; Espinoza et al., 1968; Harikrishnan and Yang, 2002; Neubauer and Avizohar-Hershenson, 1973). Specifically for residual PPO-inhibiting herbicides applied to soils, Daugrois et al. (2005) reported that sulfentrazone and flumioxazin resulted in increased colonization of sugarcane (Saccharum officinarum L.) roots by Pythium tolurosum. Conversely, Wilcut et al. (2001) reported a lack of interaction between flumioxazin and foliar and soilborne diseases in peanuts (Arachis hypogaea L). More recently, Priess et al. (2020) observed a greater incidence of Pythium stem rot resulting from flumioxazin application in one growing season. However, flumioxazin did not affect colonization rates of Fusarium, Macrophomina, and Rhizoctonia, nor affected plant density or yield across two environments (Priess et al., 2020).

Given the importance of PRE herbicides to weed management in soybeans and recurrent inquiries regarding synergism between PPO-inhibiting herbicides and early-season soilborne diseases (Giesler, 2017; Jhala, 2017; Wise et al., 2015), more research is needed to address the agronomical relevance of this potential interaction under field conditions. This study reports on the effect of a single mode of action, soil-applied residual PRE herbicides on the severity of soybean seedling diseases, injury, plant height, population, biomass, and yield across multiple environments prone to disease development in optimal and late-planted conditions.