Wallaces Farmer

Tar spot travails

Moisture from humidity — not rainfall — and temperatures are the main drivers for tar spot.

Gil Gullickson, editor of Wallaces Farmer

May 23, 2024

2 Min Read
Corn leaf showing signs of tar spot
HUMIDITY AND TEMPERATURE: Crop Protection Network plant pathologists found relative humidity under 90% over a two- to three-week span and extended 30-day temperatures ranging between 64 to 73 degrees F are key tar spot drivers. Courtesy of Darcy Telenko

Tar spot is a puzzler.

Fungal diseases thrive under high-moisture conditions, and tar spot is no exception.

“The disease triangle tells us we have to have three components — host, pathogen and environment — to get disease,” says Alison Robertson, Iowa State University Extension plant pathologist.

High-moisture conditions — a disease triangle environmental component — that favor many fungal diseases were also thought to favor tar spot. However, tar spot surfaced in central Iowa in late June during droughty conditions. Rain fell, but only in small amounts in early June.

Later in early September, Meaghan Anderson, an ISU Extension field agronomist, also confirmed tar spot running rampant though a Madison County cornfield in central Iowa.

“It was just odd,” Robertson says. “It hadn’t been particularly wet during that time.”

Humidity, not rainfall

Moisture does play a role in tar spot development, according to the Crop Protection Network, a consortium of Midwestern land-grant and Extension plant pathologists. However, moisture is more related to humidity than rainfall.

Tar spot developed when relative humidity was under 90% over a two- to three-week span. Although moisture early in the infection process might spur spore germination, plant pathologists found extended periods of excessive moisture — with relative humidity exceeding 90% and combined with higher temperatures — actually hindered disease progression.

They also found extended 30-day temperatures ranging between 64 to 73 degrees are the most important factor that aids tar spot development. Monthly temperatures exceeding this level reduce the chances of tar spot progression.

Predictive models

Scientists who developed tar spot predictive models found the most accurate one was correct 90.1% of the time it was used. The model is integrated into the publicly available app called Tarspotter that farmers can use to make fungicide decisions.

Several years of data have shown fungicide applications between VT to R3 can manage tar spot effectively, says Robertson.

Looking to 2025, the best way to guard against tar spot is to select resistant hybrids, she says. “The hybrid found in Madison County in September 2023 was really, really susceptible to tar spot,” she says.

“The scary thing about tar spot is a 50-bushel [per acre] yield loss can be common with tar spot,” reminds Ron Geis, a market development specialist for Corteva Agriscience. “It can be devastating.”

About the Author(s)

Gil Gullickson

editor of Wallaces Farmer, Farm Progress

Gil Gullickson grew up on a farm that he now owns near Langford, S.D., and graduated with an agronomy degree from South Dakota State University. Earlier in his career, he spent 13 years as a Farm Progress editor, covering Minnesota and the Dakotas.

Gullickson is a widely respected and decorated ag journalist, earning the Agricultural Communicators Network writing award for Writer of the Year three times, and winning Story of the Year four times. He is a past winner of the International Federation of Agricultural Journalists’ Food and Agriculture Organization Award for Food Security. He has served as president of both ACN and the North American Agricultural Journalists.

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