Just keep on scouting.”

That’s the word from Kurt Maertens, a BASF technical services representative who covers eastern Iowa and western Illinois. A factor that makes scouting now for diseases so important is the amount of late-planted corn in Iowa, Maertens says.

“A lot of our corn got planted after Mother's Day this year, and a lot of that corn's just finished up pollinating,” he says. If diseases such as tar spot merit spraying with a fungicide, this corn is in an optimal window to benefit, he adds. Left unchecked, tar spot can shave yields 50 bushels per acre or more on susceptible hybrids.

Tar spot

Given this spring and summer’s prolific precipitation, disease so far has been minimal in many Iowa fields. Moisture is a determining factor in whether diseases such as tar spot progress in Iowa cornfields this month. There was plenty of precipitation in July. For example, parts of central Iowa received a deluge of rainfall in late July, with some areas receiving over 4 to 5 inches of rainfall, says Meaghan Anderson, Iowa State University Extension field agronomist.

However, moisture that fuels tar spot is more related to humidity than rainfall, according to findings by the Crop Protection Network, a consortium of Midwestern land-grant and Extension plant pathologists.

Plant pathologists saw tar spot development 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.

Also, watch for cool temperatures this month, for they can spur tar spot development. Plant pathologists with the Crop Protection Network 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.

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 Alison Robertson, Iowa State University Extension plant pathologist.

Other pests

Now is also a good time to do root digs in your cornfields to assess corn rootworm control, Maertens says.

“Over the last few several years, we’ve had very high corn rootworm populations,” he says.

Although it’s too late to control rootworm for 2024, rootworm digs and tools such as sticky traps that indicate rootworm beetle pressure can help corn farmers form a 2025 control plan.

In soybeans, the good news is, Japanese beetle populations are crashing. On the other hand, soybean aphid populations are increasing in northern Iowa, Maertens says. Cool temperatures can fuel rising aphid populations, he says.

In many areas of Iowa, tremendous crops are on the way, Maertens says.

“Keep on scouting and keep an eye on the crop,” he says. “The last thing we want to have is a poor finish.”