Iowa Field Reports: Heat Stress, Soybean Defoliators

Thistle caterpillar in soybeans. Photo by Erin Hodgson, Iowa State University

Check out what ISU Extension and Outreach field agronomists have been seeing and hearing about in fields across the state this past week. Some of the common issues observed in fields include insect defoliation in soybeans by pests like thistle caterpillars and Japanese beetles, weed management challenges, and crops starting to show signs of stress due to the heat and lack of moisture. Read on for more information about what’s happening in specific regions across the state.

Northwest Iowa

Joel DeJong (Region 1): “In the NW corner, corn is ranging from beginning to tassel for the earlier planted fields down to about V10 for the fields planted in early June. Soybeans have about as much range of growth, with the most mature at the R2 stage, from what I have observed. The recovery from post PPO herbicides seems to be going well. I can still find fields that need a post herbicide application.

“On Monday I received three calls on growth regulator herbicide injury on soybeans. It’s been about two weeks since application of many of those products, so this would be the time for those symptoms to be more noticeable. Second generation soybean gall midge adults are beginning to emerge, and you can still find field edges with plants wilting and dying. I still haven’t confirmed Soybean gall midges in Osceola and Monona counties in my area.

“Thistle caterpillars are starting to feed again. I have seen very little disease pressure so far, but this is the time to scout fields to see if yours has an issue.”

Paul Kassel (Region 2): “Some of the April and early May planted corn began to show tassels on Monday. However, the June planted corn is about at the V10 stage of development. The corn that was planted the week of June 2 is expected to begin pollinating around August 5. Soybean aphids have been found in the Sioux Rapids area along the Little Sioux River Valley.

“This is not unusual to find soybean aphids this time of year in this area, and soybean aphids are at very low levels at other locations. Damage from soybean gall midge larvae can be found in parts of my area. The site near Linn Grove in Buena Vista county, which was one of the areas where soybean gall midge was originally discovered, has some damage from gall midge. However,

North Central Iowa

Angie Rieck-Hinz (Region 3): “NC Iowa crops are looking good. We have corn ranging from V7 to R1 with reports of several R2 fields that were planted early with a shorter maturity. I spent a day last week looking at corn fields with several farmers and took some time to go through some hybrid test plots and found very little disease. Common rust and eyespot were the most frequently found diseases, but the heat this week should slow them down.

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“I also found some gray leaf spot. However, I only found a few lesions and never more than one per plant and even then, only on about 8 plants total. I also saw one lesion of northern corn leaf blight. I did receive some calls about bacterial leaf streak. Soybeans range from V5 to R2 with most soybeans at the R1 to R2 stage.

“I noticed last week the second generation of thistle caterpillars was starting. This generation is not normally of economic concern but continue to scout soybeans for defoliation from multiple insects. The threshold for defoliation when soybean fields are in the reproductive stages is 20%.

“Soybean aphid reports are also on the rise, but there have been no reports of reaching thresholds in NC Iowa. Waterhemp pressure continues to be a problem in soybean fields, especially later planted beans where it appears the canopy may not close. The majority of second crop hay has been cut and baled and some oats have been harvested.”

Northeast Iowa

Terry Basol (Region 4): “The last couple of weeks have been relatively warm and dry, giving good growth conditions for both corn and soybeans, as well as allowing producers to get their hay cut and harvested without too much moisture and insuring optimum quality. Corn ranges from V8 and V9 up to silking (R1) in the earliest planted fields. According to USDA-NASS as of July 14 4% of corn fields are silking in NE Iowa.

“We have seen some corn fields over the past couple of weeks exhibit rapid growth syndrome. For more information on rapid growth syndrome, see the ICM article titled: Twisted whorls, buggy whipping, yellow leaves.  Soybeans continue to progress and have started to bloom in NE Iowa. According to USDA-NASS, 18% of soybean fields are blooming in this part of the state.

“The week of July 1 provided the driest conditions with only 0.44 inches of precipitation falling at the NE IA Research and Demonstration Farm near Nashua, according to the Iowa Mesonet. Last week (July 8 to July 14) the farm received 1.26 inches of rain, giving a total of 1.7 inches for the past two weeks.”

East Central, Southeast, and South Central:

Rebecca Vittetoe (Region 8): “The earlier planted corn started tasseling and silking in my area last week. We’ve started to see the corn show signs of moisture stress, especially some of the later planted corn as it’s been rolling its leaves pretty good on these hot days. Not seeing much for foliar diseases in corn, just common rust and a little bit of gray leaf spot. Soybeans range from R2 down to V4.

“We are starting to see second generation thistle caterpillars, bean leaf beetles, and Japanese beetles cause defoliation in soybeans. The threshold to treat for insect defoliation is 20% for soybeans in the reproductive stages and 30% for vegetative soybeans. Most post herbicides applications have wrapped up. With the dry weather, there was quite a bit of second cutting hay put up. Do keep your eyes open for potato leafhoppers in alfalfa fields as I’ve been seeing and hearing about them.”

Virgil Schmitt (Region 9): “Rainfall last week was from zero to less than 0.5 inch. Last week just about every day was a field work day. April planted corn is mostly V15 to V16, mid-May planted corn is mostly V13 to V14, and June planted corn is mostly V9. Some tassel emergence began last week for the April planted corn and a few tassels are peaking out in the mid-May planted corn.

“There was some corn leaf rolling due to moisture stress over the weekend on the June planted corn. Even in fields where soil moisture was abundant; there wasn’t a large enough root system to be able to move water into the plant fast enough. Earlier planted corn was not showing stress like the later planted corn. Soybeans range from R2 down to V4. Post herbicides are wrapping up.

“There was quite a bit of hay harvesting activity last week and winter wheat was harvested. Oats are still totally green to turning. Katydids are starting to be heard. Inquiries about weed management on pastures, herbicide injury, and cover crops dominated calls last week.”

Josh Michel (Region 11): “Much of my area has remained dry for the past week. Some isolated areas may have received up to a tenth of an inch of rain. While the dry weather has allowed many second cuttings of hay to be put up and some much-needed post herbicide applications to be completed, the lack of soil moisture is quickly beginning to show in fields. Corn that is tasseling or beginning to tassel is especially of concern this week as high temperatures and depleting soil moisture will put additional stress on plants trying to pollinate.

“Some pastures are beginning to show signs of stress as well. April planted corn is VT to R1, while June planted corn is around V7 to V78. Early planted soybeans are around R2, while later planted soybeans are approaching V4. Japanese beetles continue to cause concerns with defoliation in soybean fields and now some corn fields with silks out.

“There are still some late post herbicide applications being completed. I’m also seeing some herbicide injury in soybean fields. Common questions this past week have been on fungicide usage in corn and soybeans, late season weed management, planting summer forages, and heat stress on crops.”


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