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Lack of rain also means lack of insects and diseases —so far

By Staff | Aug 19, 2020



The crops may not be favoring the dry weather, but neither are insects or diseases.

Crop dusting planes have been seen spraying fungicides, but due to these drier conditions, is there a reason to be spraying?

“We’re not seeing a lot of disease pressure,” said Angie Rieck-Hinz, Iowa State University Extension and Outreach field agronomist in North Central Iowa.

Rieck-Hinz said there has been some areas of gray leaf spot, but those findings have been very few and on the lowest leaf of the plant.

“It doesn’t seem to be expanding very fast or moving up the plant,” she said. “This is obviously field by field and most of the disease pressure has been very low. Think about how that might influence fungicide decisions.”

In order to decide if a fungicide application is actually needed; Rieck-Hinz says to go out into your fields and scout. Especially consider those corn on corn fields.

“Did you have disease present last year and have conditions been favorable this year? Go back and evaluate your hybrid selection for disease resistance or tolerance. All of those decisions should come in to play before you decide you need a fungicide application,” she said. “Fungicides pay for themselves when we have disease present and when that disease threatens yield and in most places, we are nowhere near that level of disease pressure.”

There has been some talk of Tar Spot being found in corn fields -specifically in Polk and Cerro Gordo counties, but Rieck-Hinz said the pressure is also very low compared to last year.

“Last year it came on late enough we didn’t have to worry about it,” she said. “And this year, I would say the way we are finding Tar Spot, it may not be a big threat. But, we always want people to scout so they know what is out there.”

As far as diseases and fungicide applications on soybeans, Rieck-Hinz said now would be the time to be scouting those soybean fields as fungicide applications typically happen in the R3 growth stage.

However, disease pressure is low in soybeans as well.

“Really, the only thing I have seen out there for disease pressure in soybeans is phytophthora brown spot, but it is low on the canopy and we get that every year,” she said.


Rieck-Hinz said there is a lot of chewing on leaves from insects in soybean fields, but nothing she has seen that has called for an insecticide application.

“We don’t have an overwhelming amount of insect pressure,” she said. “It’s good to scout and then make those decisions, but in reality, we have a lot of leaf feeding out there in soybeans right now Japanese beetles, thistle caterpillars, bean leaf beetles, grasshoppers, green clover worm you need to combine the damage from all of those pests, if you have them, and if you exceed 20 percent defoliation across the entire field, then you would warrant some kind of insecticide treatment.”

Soybean aphids, so far this year, Rieck-Hinz said the reports have been very low.

As far as thistle caterpillar, a pest that needed some attention last year, are now in the second generation.

“In my experience, looking in these soybean fields, those thistle caterpillars are over an inch long, so they’re going to stop eating and pupate soon,” she said. “So, if you are worried about thistle caterpillar, you have to continue to scout and you would need 20% defoliation and we have not seen levels of defoliation like that from thistle caterpillar in north central Iowa.”

Another insect to be on the lookout for is spider mites – especially if the dry weather continues.

These insects, Rieck-Hinz suck on leaves, and are extremely hard to see on the plant.

Rieck-Hinz suggests putting a piece of white paper between the rows and shake the plant and then look for small, moving insects on the paper. They are a cream to pale green color almost translucent, she said.

Oftentimes, producers will make that decision to spray a fungicide and add a tank mix of insecticide. But is that necessary?

“An insecticide is not going to provide any control if there’s nothing there to be killed,” said Rieck-Hinz. “The insect or pest as to be present.”

Regardless if the weather stays dry or if rains come to bring some relief, scouting your fields, being aware of what is going on out there, Rieck-Hinz said is crucial for the rest of the growing season.

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