Please note: While I have not observed Tar spot in Paulding County, we have had many reports of Tar spot in Hardin and Hancock Counties, and in previous years in Fulton County. Please contact Sarah Noggle if you believe you have Tar spot.
CPN 2018. Published August 19, 2021. DOI: doi.org/10.31274/cpn-20210820-1
By: Darcy Telenko, Purdue University; Martin Chilvers, Michigan State University; Alison Robertson, Iowa State University; Albert Tenuta, Ontario Ministry of Agriculture, Food, and Rural Affairs; and Damon Smith, University of Wisconsin-Madison.
Tar spot has quickly become a widespread concern on corn this season (2021) across much of the upper Midwest U.S. and portions of Ontario, Canada. This is especially concerning after reasonably localized epidemics resulted in low or no yield reductions over the past two seasons. This season the tar spot fungus has infected corn plants early and is rapidly increasing in many areas of the upper Midwest corn belt. The speed at which the epidemic is now moving and the crop growth stage across much of these acres (ranging from tassel to early dough) has resulted in questions about what in-season management approaches might provide an economic benefit.
Characteristic tar spot signs on a corn leaf. Image: Darcy Telenko
When is the best time to apply a fungicide for tar spot management?
Like most of the other diseases of corn, the timing of fungicide application to hedge your bets against tar spot generally is at tasseling (VT) to the silking (R1) growth stage. Recent regional research has demonstrated that while there might be little yield benefit with an application at the V6 growth stage, a single application of fungicide at VT-R1 on average can result in as much as 7 bushels or more yield compared to not treating. This is compared to just 2-3 bushels at the V6 timing and suggests that farmers are more likely to recover their fungicide costs if applying just one application at VT-R1. In the absence of tar spot and southern rust, spraying at V6 AND VT-R1 also has not resulted in economically positive returns. This practice, on average only results in an additional 1 bushel of yield compared to one application at VT-R1. There is no considerable return on investment (ROI) with two-pass fungicide programs for many corn diseases. But what about the tar spot situation this season? What do the data say about a second fungicide application to manage tar spot if I have already sprayed at VT-R1 and the disease is continuing to increase?