Marestail is one of the most widespread and troublesome weeds in Iowa croplands. It can grow to a height of 1.5 to 6 feet, produce up to 200,000 seeds, and can reduce soybean yields up to 80% if not controlled (Figure 1). Marestail seeds are light and disperse across landscapes with winds. Seeds have little dormancy and can germinate soon after seed shed. In general, 75% of seedlings germinate in fall, remain in rosette-stage until spring, begin stem elongation in April, and start flowering in July. About 25% of seeds germinate in the spring.
Integrated Crop Management News
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Iowa State University has evaluated the nematode control and yield performance of soybean varieties that are resistant to the soybean cyst nematode (SCN) in hundreds of experiments conducted over nearly three decades. The results of the 2020 experiments are now available and are highlighted in this article.
Each year, the ISU Soybean Research Laboratory conducts insecticide efficacy evaluations for soybean aphid, and two emerging pests, Japanese beetle and soybean gall midge, were added in 2019. Details about pest biology, research plot design, sampling methods, and additional results can be found here.
Waterhemp is a dioecious (male and female flowers on separate plants) pigweed with a high genetic diversity. Consequently, this pigweed has a high propensity to adapt to control tactics and has evolved resistance to herbicides from as many as 6 or 7 different herbicide groups (HGs). Out of 75 waterhemp populations collected from corn/soybean fields in Iowa in fall 2019, almost 25% had a four-way multiple resistance to HGs 2 (ALS inhibitors), 5 (atrazine), 9 (glyphosate), and 14 (PPO inhibitors).
There are hundreds of soybean varieties with resistance against the soybean cyst nematode (SCN). Most of the varieties available for Iowa in the last several decades have had resistance genes from a breeding line called PI 88788. SCN resistance from PI 88788 is losing or has lost its effectiveness. Farmers should grow soybean varieties with other sources of resistance, if possible. This article lists soybean varieties available for 2021 in Iowa with resistance sources that are different from PI 88788.
With support from the soybean checkoff through the United Soybean Board, Iowa State University researchers evaluated foliar fungicides on soybean in 2020. Because dry weather affected all seven field locations, this year was really an evaluation of fungicides largely in the absence of disease. Seventeen fungicides were tested at the R3 (beginning pod) growth stage, at the recommended label rate.
Resistant soybean varieties are critical for managing the soybean cyst nematode (SCN). There are nearly 850 SCN-resistant varieties in a newly updated publication from extension. Almost all of the varieties have the breeding line PI 88788 as the genetic source of resistance. Many SCN populations in Iowa now have high levels of reproduction on varieties with this resistance. Only 39 varieties in the new publication have resistance genetics from a source other than PI 88788. This article discusses the range of SCN-resistant varieties available and includes recommendations for using resistant varieties to manage SCN most effectively.
The dry, warmer-than-normal growing season this year presents significant challenges for managing soil and crop residue this fall.
Sampling soil this fall following the dry conditions this past summer, and in some places continuing up to this time, may result in lower than expected soil-test results for phosphorus (P), potassium (K), and pH. Especially if soil samples are collected before any significant rainfall. Therefore, farmers and crop consultants should interpret those soil-test results with caution.
Fall is an ideal time to collect soil cores from fields to determine if the soybean cyst nematode (SCN) is present and to assess the population densities (numbers). There are reasons to sample fields in which soybeans and corn were grown in 2020. Multiple private soil testing laboratories can process samples for SCN as can the ISU Plant and Insect Diagnostic Clinic. Guidelines for fall SCN soil samples are provided in this article.