Iowa Field Reports: Rains Bring Relief from Heat

This past week brought some much needed rainfall to areas across the state, which helped the crops through the hot temperatures. The pest of the week across the state seems to be thistle caterpillars causing defoliation in soybeans, which are particularly bad in central and west-central Iowa. Read on to see what else ISU Extension and Outreach field agronomists are seeing and hearing about in fields across the state.

Northwest Iowa

Joel DeJong (Region 1): “In the NW corner of Iowa we received a nice 1 to 2 inches rain this past week, with less to the south. Corn is tasseling in most of my counties with the exception of those along the Minnesota border where the corn was planted in June and is around the V12 stage. The rainfall events came with some high winds, and some fields not quite tasseled yet experienced some significant greensnap.

“Soybeans are mostly in full bloom (R2), with a few beginning to set pods (R3). There were lots of questions about managing thistle caterpillars, and a few continuing questions on soybean gall midges along field edges this past week. Although it was a hot week, the unexpected rainfall really helped crops in this corner of the state tolerate it well.”

Paul Kassel (Region 2): “A recent review of projected corn crop development in Clay County shows that the early May planted corn is expected to reach the black layer stage of development by mid-September.  A 100-day maturity hybrid planted in early June is expected to pollinate by the first week of August and reach the black layer stage by the first week of October.

“This is somewhat good news for the early June planted corn and means that the recent hot weather has helped pushed the late planted corn along. There have been a few problems with off-target drift issues with dicamba in soybeans. Thistle caterpillars are quite abundant, but they have not reached the 20% defoliation threshold in this part of the state so far.

“Continue to scout fields and monitor insect feeding to determine if a treatment is warranted. Soybean gall midge larvae can be found in some areas around Linn Grove in Buena Vista, but the damage has been light so far.”

North Central Iowa

Angie Rieck-Hinz (Region 3): “The topic of biggest concern this past week has been second generation thistle caterpillars.  Localized pressure in a few fields has warranted treatment, but so far the majority of fields I have looked at do not warrant treatment. Japanese beetles continue to increase across NC Iowa. Continue to scout for soybean defoliators. Another concern in soybeans are cases of off-target movement of dicamba to non-dicamba tolerant beans. This is not just a NC Iowa concern, but one that is statewide.

“Disease pressure continues to be low in both soybeans and corn. There have been foliar fungicide applications going on in corn for the past week. I am grateful for the cooler weather, especially with the corn pollinating. The forecast for the next 10 days looks to have fairly moderate temperatures.”

Northeast Iowa

Terry Basol (Region 4): “Corn and soybean crop development continues to move along, especially given the warm and humid weather conditions that we sustained over the past week. Corn ranges from V14 up to VT, and soybeans range from R1 to R3. Oat harvest has also started in NE Iowa over the last week. According to the Iowa Mesonet, the NE IA Research and Demonstration Farm near Nashua received 0.94 inches of rain from July 14 through July 21.”

Central Iowa

Meaghan Anderson (Region 7): “Despite the heat last week, we got some really nice rainfall across central Iowa that brought a lot of welcome relief to crop fields (and my lawn).  This last week has been a true thistle caterpillar apocalypse in my world. Soybean field calls dominated my schedule with thistle caterpillars defoliating numerous fields and actually reaching threshold levels of defoliation.

“Aside from the fields I looked at, I’ve seen many fields that appear to have significant defoliation from the road and have been encouraging everyone in central Iowa to check their June-planted soybean fields. Dicamba drift issues have been popping up across the area as well. Most soybeans are at R2, with some fields at R3 and some at R1.

“I’ve seen a few corn fields that are still in vegetative stages, but most have tasseled and are now in reproductive stages. I’ve looked in a couple fields of April-planted corn to find short and fat ears. Planes and helicopters have been flying in the last week over corn fields, but we are still seeing relatively low levels of disease pressure. Continue to scout fields for any diseases.”

Southwest and West Central

Aaron Saeugling (Region 6): “SW Iowa continues to see thistle caterpillar feeding in soybeans. Additionally, Japanese beetle damage has increased. Corn is in the late vegetative stages to R2. Foliar fungicide applications are in full swing.  Soybean fields are mostly R1 to R2. The major issues I’ve seen this past week as been insect feeding and defoliation as well as volatility damage calls. Small grains harvest is being completed with lower than expected yields being reported so far.”

East Central, Southeast, and South Central:

Rebecca Vittetoe (Region 8): “Some much needed and unexpected rain fell across EC Iowa last week with rainfall totals ranging from 0.25 to 2 inches. The rain also brought some strong winds, causing some damage in areas. Corn ranges from late vegetative to R2, and soybeans are mostly in the R2 to R3 stage. There are still some late-planted soybean fields in the vegetative stages.

“Thistle caterpillars, Japanese beetles, adult corn rootworm beetles, and potato leafhoppers are the common insect pests I’ve seen this past week. Watch for silk clipping in corn by Japanese beetles and rootworm beetles. Additionally, for soybeans, remember the threshold to treat for insect defoliation is 20% for soybeans in the reproductive stages and 30% for vegetative soybeans. Still seeing low disease pressure in both corn and soybeans.

“I’ve mainly been seeing a little common rust in corn, but I did see some gray leaf spot showing up in the lower part of the corn canopy late last week. Also, this past week I got more calls regarding dicamba drift issues in soybeans.”

Virgil Schmitt (Region 9): “Rainfall last week was from less than 0.5 inch generally north of Highway 92 to between 1.0 and 2.0 inches generally south of Highway 92. Corn ranges from V10 to R1. There is probably some April planted corn at R2, but I have not seen any. There was considerable corn leaf rolling on the June planted corn due to moisture stress on the hot days last week. The earlier planted corn showed less stress.

“Starting to see a little gray leaf spot, but overall there is little disease pressure in the corn at this point. Soybeans are mostly at the R1 to R3 stage. Like the corn, there is little disease pressure in the soybeans as well. Oats are turning, and pasture are turning brown. Additionally, adult grasshoppers are flying. Inquiries about uneven corn, alfalfa seedings, potato leafhoppers, weed management, and herbicide injury dominated calls last week.”

Josh Michel (Region 11): “Much of my area received some very welcomed precipitation over the past week. Rainfall totals ranged from 0.5 to 3 inches. Some isolated areas around Lake Rathbun had closer to 4 plus inches of rain. There were reports of wind damage to corn fields from the recent storms, but I haven’t heard of any significant green snap or hail damaged corn.

“Most of the second cutting hay was put up before the rain. There have been reports of potato leafhoppers, so scout alfalfa fields for them. Pastures were also starting to look heat and moisture stressed, but the recent rain and cooler temperatures should help provide some relief. Many oat fields were harvested last week.

“April planted corn ranges between R1 to R2 and the June planted corn ranges from V8 to V9. The later planted corn showed some heat and moisture stress last week. Reports of common rust and some gray leaf spot are beginning to pop up. Early planted soybeans are around R2 to R3 with later planted soybeans around V5. Foliar fungicide applications, Japanese beetles, heat/moisture stress, thistle caterpillars, late season weed escapes, and herbicide injuries were common questions received last week.”


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