The Harrison County Pest Resistance Project initially began in 2017 as a result of the introduction of Palmer amaranth in the county four years prior. As most farmers in the area are grappling with herbicide resistant weeds but do not have Palmer in their fields yet, the project was expanded to include waterhemp, marestail, giant ragweed and disease trials as well. Read below to learn more about local disease and weeds and why they were included in this project.
Diseases
According to Daren Mueller with ISU Extension, disease resistance in corn not yet observed, but disease resistance in soybean crops is an issue in Iowa. In 2018, brown spot and frogeye leaf spot were seen at all locations tested. There were seven statewide trials at ISU research farms; all had resistant strains of the frogeye pathogen, which were especially resistant to the QoI fungicides. The pathogen that causes Septoria brown spot was also tested for resistance. Not all, but a significant percent of samples came up resistant to QoI fungicides. In the past, as long as you spray fungicide and get good coverage, it didn’t matter what fungicide was used. Now there is a large efficacy difference in products; you can’t rely on QoI fungicide, either as solo active ingredients are part of pre-mix products. The good news is that newer products that do not rely on QoI that were much more effective. We must vary our product selections from year to year to avoid further resistance development..
Weeds
Waterhemp
Waterhemp, like Palmer amaranth, redroot pigweed, smooth pigweed, and Powell amaranth, is a member of the amaranth family (genus Amaranthus). While all pigweeds look fairly similar, waterhemp can be distinguished by its lack of hairs on leaves or stem, and petioles (leaf stems) that are shorter than the leaves. Stems of waterhemp can range from light green to red.
Waterhemp is a broadleaf species with seeds that can germinate from early spring into summer. Waterhemp is dioecious, meaning male and female flowers occur on separate plants. The cross-pollination required contributes to the high genetic diversity in waterhemp populations, which makes it more likely for resistance to develop and easy for it to spread. This extensive list of documented herbicide resistance makes waterhemp management challenging moving forward.
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Waterhemp Resistance |
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| Herbicide group | Mechanisms of action | Product examples |
| 2 | ALS-inhibitors | Pursuit, Classic |
| 4* | Growth regulators | 2,4-D |
| 5 | Photosystem II inhibitors | Atrazine |
| 9 | EPSP synthase inhibitors | Roundup |
| 14 | PPO-inhibitors | Cobra |
| 15† | Long chain fatty acid inhibitors | Dual, Outlook |
| 27 | HPPD-inhibitors | Callisto, Laudis |
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* Resistance found in Missouri, not yet in Iowa. |
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† Resistance found in Illinois, not yet in Iowa. |
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Palmer amaranth
Palmer amaranth is a member of the amaranth family and a close relative of waterhemp and other pigweeds. Several characteristics can help distinguish Palmer amaranth from its close relatives. Palmer amaranth, similar to waterhemp, lacks hair on its stems.
Palmer has a symmetric growth pattern, often likened to a poinsettia. In addition, if you grab the flower head of a female Palmer plant, it will be painful due to presence of sharp bracts. Perhaps the best way to distinguish Palmer from other pigweeds is the presence of very long petioles, often longer than the length of the associated leaf.
A new introduction to Iowa, Palmer amaranth is a concern due to its fast growth rate, high seed production, and competitiveness with field crops. Palmer amaranth has a number of documented resistant biotypes. A free Palmer amaranth identification guide can be downloaded at https://store.extension.iastate.edu/product/14794.
| Palmer Amaranth Resistance | ||
| Herbicide group | Mechanism of action | Product examples |
| 2 | ALS-inhibitors | Pursuit, Classic |
| 3 | Microtubule inhibitors | Treflan, Prowl |
| 4 | Growth regulators | 2,4-D |
| 5 | Photosystem II inhibitors | Atrazine |
| 9 | EPSP synthase inhibitors | Roundup |
| 14 | PPO-inhibitors | Cobra |
| 15 | Long chain fatty acid inhibitors | Dual, Outlook |
| 27 | HPPD-inhibitors | Callisto, Laudis |
Marestail
Marestail, or horseweed, is a winter or sometimes summer annual species in the sunflower family. Marestail usually emerges in the fall or early spring. Fall germinating plants over-winter as a small rosette and bolt the following spring. As a winter annual, marestail is primarily a weed in no- or reduced-tillage systems.
Marestail can produce 200,000 wind-dispersed seeds per plant, which are able to travel significant distances.
Marestail is most effectively controlled while still in the rosette stage. Fall herbicide applications can be very effective.
| Marestail Resistance | ||
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Herbicide group |
Mechanism of action | Product examples |
| 2* | ALS-inhibitors | Pursuit, Classic |
| 9 | EPSP synthase inhibitors | Roundup |
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* Resistance not yet in Iowa. |
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| Giant Ragweed Resistance | ||
| Herbicide group | Mechanism of action | Product examples |
| 2 | ALS-inhibitors | Pursuit, Classic |
| 9 | EPSP synthase inhibitors | Roundup |
| 27 | HPPD-inhibitors | Callisto, Laudis |
Giant ragweed
Giant ragweed, like its close relative common ragweed, is a member of the sunflower family. Giant ragweed is very competitive, growing up to 12 feet in height. Giant ragweed has large, opposite leaves with 3 to 5 lobes, often serrated at the edges. Giant ragweed has separate male and female flowers on the same plant. Male flowers are inconspicuous, yellow to green, and arranged in cylindrical spikes at the upper terminals; female flowers occur at leaf axils. Giant ragweed pollen is a significant contributor to late season hay fever. Giant ragweed germination occurs in a relatively short window, usually in April in Iowa