June 7, 2004
Several producers and dealers have encountered soybean leaves that are crinkled and puckered. While those may be the result of drift or sprayer tank contamination from a plant growth regulator herbicide (2,4-D or dicamba), it may also be a response to soybean herbicide applications or to environmental stresses. Remember that drift normally will have a discernable pattern, worse next to the "culprit field" and fading with distance. If volatilization has occurred, effects will often be greater in low areas. With either particle or vapor drift, broadleaf plants in fence lines or other areas between the fields in question should show leaf distortion, too. If there was a tank contamination, overlaps should have more severe distortion. Three good references are Plant growth regulator herbicide symptoms in soybeans Malformed soybean leaves , and Effect of dicamba on soybean yields
Viral diseases of soybean can also cause leaf distortion. However, even if soybean plants have been infected by viral diseases, it is too early in the season for the leaf symptoms to be exhibited.
There are still a number of soybean fields to be planted in the area. On average, soybeans planted in early June in central and southern Iowa have yielded about 10% less than those planted in early May. The yield potential does drop more rapidly after early June. Stick with an adapted full season variety through June. A good source of information on soybeans is "Soybean Replant Decisions" (Pm-1851) at http://www.extension.iastate.edu/Publications/PM1851.pdf.
As water recedes from flooded and ponded areas, many are asking the
question "how long can crops be under water and survive?" Corn and soybeans can
normally only survive complete submersion for 2 to 3 days (80 day air temperature) and
most forages can survive for 1 - 2 days. I have seen survival for considerably longer
periods of submersion, however. The cooler the air temperature, the longer the plants can
survive. Plants NOT totally submerged will survive considerably longer. By the time the
water has receded and the field dried out, it will be easy to see whether the crop has
survived or not. Some corn has been flattened by the rushing flood waters, but if it
survives, should be able to straighten up again. Any corn beyond V6 (6 leaves fully
emerged from the whorl) will likely be goose necked, because stalk elongation will have
begun. Flooding can lead to greater disease problems on all crops. A light rain in the
near future might be beneficial to wash off the mud on plants. If a replant is going to be
done and it will be to a crop different from the one originally planted, remember
rotational restrictions from any herbicides applied. Several good references on flooding
Watch Re-growth after Cutting Be sure to watch re-growth after the first cutting is made.
Feeding of alfalfa weevil should have ended in locations along and south of Highway 30, and feeding should end along Highway 20 by about June 14. Even if feeding should be over by the time first cutting re-growth begins, it is still best to watch to be sure.
High numbers of potato leafhoppers can be found in many hay fields. As re-growth
begins, be sure to also use a sweep net to monitor potato leafhopper numbers and treat if
numbers exceed the threshold. For more information on managing potato leafhopper, see
pages 107 - 110 of the June 21, 1999 Iowa State University Integrated Crop Management
Some may be needing to change their weed control strategies in corn. For a review of maximum corn sizes and weed sizes for various corn herbicides, see page 55 of the May 31, 2004 ICM Newsletter or http://www.ipm.iastate.edu/ipm/icm/2004/5-31-2004/weedsinlatecorn.html.
Purple and/or Yellow Corn
Portions of many corn fields have been exhibiting purple and/or yellow leaves. While most of the yellow leaves are symptomatic of lack of nitrogen uptake, occasionally the symptoms are of potassium deficiency (yellow running down the outside edges of the leaves, beginning with the lower leaves). The vast majority of the discoloration is a result of poor root function due to saturated soils. As root function improves, plant color should improve.
If potassium deficiency symptoms occur or persist, inspect the root system. I have been in several fields where the mesocotyl (area between the old seed and the main plant) is rotting. The primary (seed, seminal) roots, which develop from the old seed, may be a small portion of the total root system, but it is still highly functional at this time, and if the plant is cut off from the primary roots prematurely, it is forced to rely on the secondary (nodal) root system (the whorls of roots) before it is ready to do so, resulting in potassium deficiency symptoms. Generally, plants will recover and go on to be productive plants.
EVENTS IN THE NEAR FUTURE TO GET ON YOUR CALENDAR
June 17 SE Iowa Research & Demonstration Farm Spring Field Day
Featuring Elwynn Taylor
Topics: Crop Weather Outlook; Relay Intercropping of Soybeans into Wheat; Improve Profits with Low Linolenic Soybeans; Controlling Weeds in non-GMO Soybeans; Corn Nitrogen Needs in Crop Rotations. Tours start at 1:00 p.m. at the farm located SE of Crawfordsville. A special CCA session will be held in the morning. Contact me for details.
June 22 Pasture Walk, Stockton
The 6:00 pm walk will focus on a rotational grazing system using warm season grasses along a stabilized creek bank and crossing.
June 24 NE Iowa Spring Field Day, Nashua
Lunch and Static Displays 12:00 1:30PM ; and wagon tours from 1:30 4:30 PM.
June 29 Soybean Rust Update
This morning meeting will be a multi-state distance education program that will bring together the most knowledgeable people on the subject in the United States to provide insights on the status of the disease, most likely scenarios for it to move to North America, distinguishing between soybean rust and other leaf diseases, and the process that is in place to monitor for the disease and confirm its presence when it arrives. This will be offered at several locations, so contact your local ISU Extension office, watch the media, or contact me to find the location nearest you.
July 8 Pasture Walk, Andrew
The 6:30 pm walk will focus on rotational grazing, stockpiling, and grazing standing corn.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
Last Update: June 9, 2004
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