High rainfall in some areas the past couple of weeks has produced another wet spring in Iowa. This leads to questions about nitrogen (N) loss and need for supplemental N application to corn. Unfortunately, this question has become almost the norm - I have written approximately 20 articles on the subject since 2007.
Integrated Crop Management News
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Japanese beetle is an invasive insect that feeds on corn and soybean plus many other plants. This pest has been in Iowa since 1994 but its distribution in field crops is considered sporadic around the state. Statewide populations in field crops have been variable since 2014 and it is unclear if pressure will be significant this year. Several reports around Iowa indicated high numbers of grubs within fields, but it is not clear if they were Japanese beetle or another closely-related species. Adult emergence well before corn silking is noteworthy.
The soybean cyst nematode (SCN) is a major pathogen of soybean in Iowa and throughout the Midwest. Damage often goes unrecognized until lower-than-expected yields are harvested because above-ground symptoms may not be obvious, especially when adequate or excess rainfall occurs during the season. A key way to check fields for this pest is to dig soybean roots and look for the small, round, white adult SCN females. It typically takes 5 to 6 weeks or more after planting for the first SCN females of the growing season to develop and appear on roots.
Building soil health is important to sustain soil resiliency and productivity. Many conservation practices can maintain and enhance physical, chemical and biological soil properties that contribute to overall soil biological functions as fundamental drivers that support plant growth and productivity. However, these properties are complex and interrelated as each function is influenced by a central building block, soil organic matter (SOM). Soil health is one of the co-benefits of improving SOM through soil carbon sequestration or storage.
Tracking degree days is a useful tool to estimate when common stalk borer larvae begin moving into cornfields from their overwintering hosts. Foliar insecticide applications, if needed, are only effective when larvae are migrating and exposed. Start scouting corn for larvae when 1,300-1,400 degree days (base 41°F) have accumulated. Counties south of I-80 in Iowa reached this important benchmark this week (Figure 1), and therefore scouting for migrating larvae should begin now to make timely treatment decisions. Stalk borer larvae in northern counties will migrate later in June.
Corn rootworm egg hatch in Iowa typically occurs from late May to the middle of June, with an average peak hatching date of June 6 in central Iowa. In 2018, the average hatching date will be ahead of the average, despite having cool April temperatures. Development is driven by soil temperature and measured by growing degree days. Research suggests about 50 percent of egg hatch occurs between 684-767 accumulated degree days (base 52°F, soil). Most areas in Iowa have reached peak corn rootworm egg hatch (Fig. 1). Larvae will start feeding on corn roots if available.
Isolates of Cercospora sojina showing reduced sensitivity to quinone outside inhibitor (QoI, strobilurin) fungicides were recently recovered from soybean in Iowa. This pathogen causes frogeye leaf spot, an important foliar soybean disease that can be managed with fungicides. With the confirmation of fungicide-resistant isolates, it becomes especially important to understand when to spray and what products to use for long-term control of this disease.
ISU Extension and Outreach Field Agronomists continue to receive calls regarding fomesafen carryover injury to rotational corn. There are several factors resulting in this injury: 1) continued problems with waterhemp result in late-season applications, 2) fomesafen is relatively persistent, and 3) many areas of Iowa received less than average late-season rainfall in 2017. In most cases, this carryover injury has been limited to relatively small sprayer overlap areas, though some fields are showing injury on a more widespread area.
The black cutworm (BCW) is a migratory pest that cuts and feeds on early vegetative-stage corn. Black cutworm moths arrive in Iowa and other northern states with spring storms each year. These moths lay eggs in and around crop fields and emerging BCW larvae cut seedling corn. This pest is sporadic, making it essential to scout fields to determine if management is needed. Scouting for BCW larvae helps to determine if an insecticide application will be cost effective.
Iowa’s most significant soybean insect pest, soybean aphid, has host-alternating biology. This species has multiple, overlapping generations on soybean in the summer and moves to buckthorn in the winter. Fall migration to buckthorn is based on senescing soybean, and decreasing temperatures and photoperiod. For the majority of the year, soybean aphids are cold-hardy eggs near buckthorn buds (Photo 1). As spring temperatures warm up, soybean aphid eggs hatch and produce a few generations on buckthorn before moving to soybean (Photo 2). Tilmon et al.