TAR SPOT OF CORN: POPULATION DYNAMICS, ECONOMIC IMPACT AND MANAGEMENT IN MIDWESTERN UNITED STATES

Tar spot is a new foliar disease of corn in the United States. Tar spot was first detected in 2015 and is now among the most important corn diseases in the Midwest. Tar spot is caused by the obligate biotrophic fungus, Phyllachora maydis Maubl, from the genus, Phyllachora which consist of over 1,200 species of host-specific fungi. Due to the recent emergence, studies relating to P. maydis population dynamics in the U.S. are limited. How much genetic diversity, variation, and level of gene flow are occurring within and among these populations? Knowledge of the population dynamics is imperative for understanding the pathogen’s biology, ecology, epidemiology, and management. Currently, no corn hybrid is fully resistant to tar spot. Foliar fungicides are currently the most effective option for disease management, but best practices for fungicide management remain unknown. Better information is needed on fungicide efficacy and fungicide application timing to reduce tar spot severity, protect yield, and increase profitability for Indiana corn growers. 

This research dissertation presents four chapters to answers those questions and bridge the gaps between the knowns and unknows of this novel corn-Phyllachora maydis pathosystem. Chapter 1 presents a literature review on tar spot of corn, its economic impact, the causal pathogen, its host, lifecycle, distribution, and known management strategies as a resource for understanding the pathosystem in the U.S. Chapter 2 examines the genetic population structure, diversity, geneflow and mode of reproduction in Midwest U.S. by employing microsatellite (SSR) markers. Chapter 3 presents results from multi-year, multi-location, small-plot field trials on the net return of foliar fungicides and fungicide timing on tar spot management in Indiana. Lastly, Chapter 4 concludes by evaluating of an integrated management strategy for tar spot by examining the integration of tillage, corn hybrids and fungicide application in reducing tar spot severity while protecting yields.

Results provided in this research dissertation will be used to guide future studies and provide stakeholders such as researchers, corn growers, extension personnel in academia and industry with valuable information needed to guide effective disease management decisions.