How do Agronomic Choices Affect Tar Spot Severity?

CPN 5013. Published January 5, 2024. DOI: doi.org/10.31274/cpn-20240119-0

Jill C. Check, Michigan State University; Adam M. Byrne, FMC Corporation; Maninder P. Singh, Michigan State University; Kurt Steinke, Michigan State University; William D. Widdicombe, Michigan State University; and Martin I. Chilvers, Michigan State University.

Summary

• There is no relationship between nitrogen (N) application rate and tar spot severity. In this experiment, there was no evidence that N application influences the severity of tar spot.

• Higher plant population densities resulted in lower tar spot severity. Plots with higher plant densities (40-46 thousand plants per acre) had lower tar spot severity than the lower plant densities (28-34 thousand plants per acre).

• Increasing plant population density is not a viable disease management strategy. Increasing plant density to lower disease severity did not improve the economics of production. Instead, planting density choices should be based on other factors, such as productivity environment and hybrid genetics.

• Genetic resistance is important for disease management. Hybrid susceptibility had a strong effect on tar spot severity and yield at nearly all trial locations and should be prioritized for disease management.

Introduction

Tar spot of corn, caused by the fungus Phyllachora maydis, has emerged as a significant pathogen of corn in the U.S. and Canada, responsible for losses totaling 229 million bushels from 2018 to 2019 alone (Mueller et al. 2020). While historically a major corn pathogen in Latin America (Maublanc 1904; Mottaleb et al. 2019), P. maydis was documented in the U.S. in 2015 in multiple counties across Illinois and Indiana (Ruhl et al. 2016) and has since spread across the northern and southeastern U.S. and Ontario, Canada (Corn ipmPIPE). Disease signs manifest as small black structures called stromata on the leaves (Figure 1).

Climate modeling indicates that a vast corn production area in the U.S. is at risk of tar spot outbreaks (Mottaleb et al. 2019), highlighting the potential for significant annual economic impacts and need for integrated management strategies. Cultural control strategies, such as avoiding excessive N fertilization and seeding rates, are recommended by Latin American researchers (Pereyda-Hernández et al. 2009; Mahuku et al. 2013; Rios Herrera et al. 2017), although the data used to form these recommendations are unavailable.

Nitrogen is crucial for plant growth and can influence disease severity in some pathogen-host systems (Huber and Watson 1974). Nitrogen promotes the growth of young, vegetative tissues, which are more susceptible to fungal infections (Dordas 2008). Nitrogen can also change plant physiology and biochemistry in ways to be conducive for disease development, including higher amino acid concentrations, reduced production of defense compounds and increased moisture content. The relationship between N and disease response varies among pathogens. For some fungi that need a living host to survive (which describes P. maydis), increased N can increase disease severity (Vereseglou et al. 2013). Despite increased disease severity, the benefits of N fertilization on grain fill and leaf area index may offset yield reductions in certain cases, highlighting the complex interplay between optimal N application rates and disease management (Devadas et al. 2014).

Higher planting densities can lead to increased yields but can also elevate disease risk (Adipala et al. 1995; Blandino et al. 2008; Fininsa and Yuen 2001). The close proximity of plants can increase disease development by increasing the rate of leaves intercepting inoculum (Burdon and Chilvers 1982). Additionally, high plant density affects individual plant health by intensifying competition for light, water, and nutrients. This competition can make plants weaker and more susceptible to infection. The plant canopy microclimate is influenced by planting density, which affects factors like light, temperature, humidity, and air movement which influence the rate of pathogen infection and reproduction.

Due to the spread and establishment of P. maydis in the U.S. and Canada, there is an urgent need to identify and describe risk factors and integrated disease management strategies for tar spot (Rocco da Silva et al. 2021). Therefore, this study aimed to fill holes in our current knowledge regarding the influence of agronomic factors on tar spot severity.

Research Goals

• To determine the effect of N application rate on tar spot disease development.

• To determine the effect of plant population density on tar spot disease development.

• To determine the effect of hybrid resistance on tar spot disease development in interaction with these agronomic factors.

• To assess the economic impact of using plant population density as a disease management strategy for tar spot.

The Research

Field trials were conducted across Michigan in 2019 and 2020 to separately assess the effects of N application rate and plant population density on tar spot severity. In 2019, trials were in Allegan and Montcalm counties. In 2020, trials were repeated in Ingham and Montcalm counties and two additional plant population density trials took place in Ottawa and Van Buren counties. Nitrogen application rate treatments consisted of 0.5, 1, and 1.5 time(s) the maximum economic return to N rate for Michigan (80, 160, and 240 lbs N/acre) (Rutan and Steinke 2018). N applications were made at the V4 growth stage in 2019 and the V6/7 growth stage in 2020. Applications consisted of broadcast granulated urea in 2019 and 2020, with a urease inhibitor included to reduce volatilization in 2020. Plant population density treatments used seeding rates of 28, 34, 40, and 46 thousand seeds per acre to represent low, moderate, high, and ultra-high seeding rates. All treatments were repeated across two corn hybrids, one previously rated as susceptible and one as moderately resistant, both with a relative maturity rating of 102.

Disease severity ratings were taken approximately weekly from the start of symptom development through crop maturity. Ratings were used to calculate relative area under the disease progress curve (rAUDPC) to summarize disease development through the growing season and compare treatment effects on disease development. To investigate the profitability of using plant population density for tar spot management, the economically optimal planting density (EOPD) was estimated for these trials at three different grain sale prices: USD $3.81, 5.72, and 7.62 per bushel. EOPD was found by using yield data to estimate total return and subtracting seed cost based on planting density and fixed costs equivalent across all treatments (fertilizer, pesticides, fuel, etc.) based on 2022 estimates for rotational corn on average productivity soils (Purdue Crop Cost and Return Guide 2022).

Treatments were compared using statistical models. Each trial was analyzed separately by location and year since there were external factors that influenced analysis.

Results

The spectrum of N application rates had no significant effect on disease development at any trial location across both years (Table 1). However, hybrid susceptibility was demonstrated to have a strong significant effect at all locations. At experimental locations there may have been interacting environmental and nutritional factors that influenced plant N uptake, such as soil moisture conditions, and consequently the results observed here. However, the consistency in results across four site years with variable conditions offers strong evidence that N application rate did not influence tar spot disease severity that would be relevant to disease management.

Table 1. Mean relative area under the disease progress curve (rAUDPC) across nitrogen (N) application rates and hybrids at all site years.

^y Numbers represent N application rates in lbs per acre

^z Values within columns of a given site year followed by the same letter are not significantly different (P < 0.05)

Plant population density had a significant effect on disease development, with higher planting densities resulting in less disease development (Figure 2). For every 1,000 additional plants per acre, disease development decreased by 2% (measured as rAUDPC). Additionally, hybrid susceptibility had a significant and strong effect, with an average 41% disease (rAUDPC) reduction when using the resistant hybrid compared to the susceptible hybrid.

When light, water, nutrients, and other resources are not limited, yield increases with plant population density (a positive linear relationship) (Assefa et al. 2016). In real-world scenarios, yield and plant density follow a quadratic relationship where the agronomically optimal plant density is the peak of the curve. When input costs are factored in (seed costs, chemical inputs, etc.) and total return is used in place of yield, the peak of this curve represents the EOPD. In these trials, EOPD was estimated at 30 to 31 thousand seeds per acre across the grain prices used to represent grain price fluctuations over the last ten years. These seeding rates were closest to the lowest two used in this study (28 and 34 thousand seed/acre) and demonstrate that plant population density should not be increased to manage tar spot, and that any positive effects of reduced disease severity were offset by overcrowding of plants. Instead, other management strategies are more important for disease management, such as hybrid resistance and timely applications of efficacious fungicides (check out the CPN Tar Spot Web Book).

Conclusions and Future Work

Our research found that N application rate did not affect tar spot severity and surprisingly, lower plant population densities were associated with greater disease, challenging previous beliefs. High plant densities did not increase tar spot risk, offering reassurance to farmers. Despite lower disease levels, high plant densities did not yield increased profits. Hybrid susceptibility significantly impacted disease severity and yield, with moderately resistant hybrids consistently outperforming susceptible ones. This study suggests genetic resistance will be more effective in tar spot management than adjusting plant density or N application. Although these trials were conducted only in Michigan, the agronomic practices used here are generally representative of practices used in neighboring states. Therefore, these findings can likely be used to inform tar spot management for this broader area. Future work should focus on assessing the risk of additional agronomic practices, such as tillage and fertilizer regimes involving other macro- and micro-nutrients.

This Research Update Summarizes the Work Described in the Following Peer-Reviewed Research Article

Check, J. C., Byrne, A. M., Singh, M. P., Steinke, K., Widdicombe, W. D. and Chilvers, M. I. 2023. Effects of nitrogen application rate and plant density on severity of tar spot of corn. Plant Health Progress. 24:416-423. Article / Google Scholar

Additional Resources

• Check out the CPN tar spot web book for more information on tar spot disease management: CPN tar spot web book.

• Bookmark the corn ipmPIPE map to track positive confirmations of tar spot throughout the growing season: Corn ipmPIPE.

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Acknowledgements

Authors

Jill C. Check, Michigan State University; Adam M. Byrne, FMC Corporation; Maninder P. Singh, Michigan State University; Kurt Steinke, Michigan State University; William D. Widdicombe, Michigan State University; and Martin I. Chilvers, Michigan State University.