Uncovering the environmental conditions required for Phyllachora maydis infection and tar spot development on corn in the United States for use as predictive models for future epidemics

Abstract

Phyllachora maydis is a fungal pathogen causing tar spot of corn (Zea mays L.), a new and emerging, yield-limiting disease in the United States. Since being first reported in Illinois and Indiana in 2015, P. maydis can now be found across much of the corn growing regions of the United States. Knowledge of the epidemiology of P. maydis is limited but could be useful in developing tar spot prediction tools. The research presented here aims to elucidate the environmental conditions necessary for the development of tar spot in the field and the creation of predictive models to anticipate future tar spot epidemics. Extended periods (30-day windowpanes) of moderate mean ambient temperature (18–23 °C) were most significant for explaining the development of tar spot. Shorter periods (14- to 21-day windowpanes) of moisture (relative humidity, dew point, number of hours with predicted leaf wetness) were negatively correlated with tar spot development. These weather variables were used to develop multiple logistic regression models, an ensembled model, and two machine learning models for the prediction of tar spot development. This work has improved the understanding of P. maydis epidemiology and provided the foundation for the development of a predictive tool for anticipating future tar spot epidemics.

Foliar inoculation ofPhyllachora maydisinto corn induces infection and local spread in field environments

“…For example, before and during the study, the air temperature varied from 18 to 28 °C (Fig. 3), similar to the reported ranges by Hock et al (1995) and by Solórzano et al (2023a) and as recently discussed by Webster et al (2023). The period following inoculations was generally dry with periodic rain over the first 19 days and then dry, but stroma and tar spot continued developing.…”

“…This study highlights the importance of incorporating surveillance of P. maydis in epidemiological studies of tar spot. It also emphasizes the importance of inoculum availability, viability, and/or amount, as it was clear in this study that the host and environment (Hock et al 1995; Solórzano et al 2023a; Webster et al 2023) were compatible with tar spot occurrence and that only the inoculum was needed to initiate the disease. During this study, tar spot did not occur naturally in any corn beyond the research area during the growing season, arguably due to the absence of inoculum or viable inoculum and indicating that environmental conditions alone do not predict tar spot occurrence.…”

“…Phyllachora maydis causes tar spot of corn ( Zea mays L.) throughout the Americas (Valle-Torres et al 2020) and has led to regional epidemics in the United States since 2015 that resulted in significant yield losses (Mueller et al 2020). Colonization and development of this fungus on corn are favored by a range of environmental conditions (Hock et al 1995; Solórzano et al 2023a; Webster et al 2023), but little is known regarding conditions favoring its off-season survival or conditions that favor initial infection in the field. However, the overwintered inoculum likely initiates tar spot during the growing season (Kleczewski et al 2019; Lipps et al 2022).…”

“…Colonization and development of this fungus on corn are favored by a range of environmental conditions (Hock et al 1995;Solórzano et al 2023a;Webster et al 2023), but little is known regarding conditions favoring its off-season survival or conditions that favor initial infection in the field. However, the overwintered inoculum likely initiates tar spot during the growing season (Kleczewski et al 2019;Lipps et al 2022).…”