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On Thursday, 14 August, Curvularia leaf spot was observed in a field trial (Fig. 1A) and at a demonstration site in Central Iowa.  Small circular-to-oval-shaped (up to 1/8” diameter and ¼” long) lesions were observed in the mid- and upper canopy. The spots have a whitish center and are surrounded by a dark-green water-soaked margin that eventually becomes orange-brown with a yellow halo (Fig. 1B).

Many reports of caterpillars feeding at the tips of corn ears have prompted discussions about how to distinguish between common ear-feeding caterpillars. In particular, western bean cutworm infestations across the state have been misidentified as corn earworm, probably because of the location of feeding and western bean cutworm being an infrequent pest in Iowa. Here are some tips to help you distinguish the two caterpillars.

Iowa and other Midwestern states are reporting very high levels of West Nile virus (WNV) this month. Data is generated from ongoing mosquito surveillance efforts coordinated by Dr. Ryan Smith at Iowa State University. This week had the highest observed WNV activity at this point in the summer observed in over 20 years. This trend is of serious concern for the next 8 weeks when WNV transmission risks are the highest. 

Southern rust has continued to spread across Iowa and increase in severity since the disease was first reported in mid-July. This is not surprising considering the weather we have been having. July was warm and extremely wet across the state, and the precipitation has continued through this first 10 days of August.

Favorable conditions for southern rust development

The Corn and Soybean Field Guide is back in stock at https://go.iastate.edu/9EJVNV.

Since June 1^st, the wettest parts of Iowa have received around 20 inches of rain. The rainiest areas in the state include central Iowa, primarily Story and Boone Counties, and north-central Iowa, Winnebago and Worth Counties.

Corn rootworm beetles have emerged and are thick in some Iowa cornfields. This is the time of year we get questions about managing adult rootworms for the next growing season. The short answer: targeting adults is challenging and can be expensive. We recommend focusing on larval management, but there are scouting guidelines for adults and some products that work better than others.

Understand rootworm biology

There is still time to register for the 2025 Nitrogen Use Efficiency Workshop to be held in Ames at the Iowa State University Alumni Center August 4-6.

I recently attended the Iowa State University Weed Science Field Day at the Curtiss Farm outside of Ames. During this field day, I noticed an area of shorter, yellow corn as compared to the surrounding corn (Figures 1 and 2). My initial thoughts were 1) this area was a wet area and probably had experienced significant denitrification, or 2) perhaps they missed applying nitrogen in this research block. 

Tassels shedding pollen while still wrapped in the flag leaf is an uncommon occurrence in Iowa and across the Corn Belt. I think I've only seen it once before in my 20+ year career as an agronomist. However, based on conversations with colleagues, it appears to be widespread throughout the region. It does not seem to be brand-specific, but it does appear to be hybrid-specific.

Over the past few days, I have received several reports of southern rust from southwest and southeast Iowa.  In addition, I and others have observed southern rust in central Iowa (Fig. 1). Incidence (number of plants affected) and severity (area of leaf diseased) is very low (Fig.2). 

Fig. 1.  Map showing counties where southern rust has been observed in Iowa (July 16, 2025). This map available at CPN is updated daily as reports are received. 

With older tools like Tarspotter and Sporecaster no longer supported, the Crop Protection Network (CPN) has introduced the Crop Disease Forecasting Tool, a new free web-based tool to assess risk for diseases in corn and soybeans. This tool builds on years of university-led field data collection and modeling to provide field-specific risk forecasts for foliar diseases.

July typically brings a lull in soybean diseases. By this time, plants have grown out of early-season issues, and late-season diseases usually haven’t arrived yet. But every season brings its own challenges. With the frequent rains this year, some of the typical “August diseases” are starting to show up earlier than expected.

Are you seeing Japanese beetles in field crops? Watch this 4-minute video to learn more about how to estimate Japanese beetle defoliation and use economic thresholds in soybean: 

 CPN: Soybean insect defoliation training tool link