May 24, 2012
Pythium Damping Off
Many fields are also showing some disease problems, although it is likely that in many of the fields, it is a combination of several factors in addition to disease that is leading to the stand reductions. If the mesocotyl (structure that goes from seed up to nodal roots) rots before the nodal roots have developed, the corn essentially then has no root system so it dies. Many fields are being replanted in SE Iowa where damping off is at least part of the problem. There are many strains of the fungus and the fungicide seed treatments are not effective on all of the strains. In addition, the fungicide seed treatments each have efficacy on a somewhat different array of the pythium strains. Alison Robertson, ISU Extension Plant Pathologist, has collected plants from many fields in the area to determine what strain(s) is/are causing the problem to help farmers and dealers better target fungicide seed treatments in the effected fields in future years. Stay tuned. For more information on damping off, including pictures, see http://www.extension.iastate.edu/CropNews/2012/0516robertson.htm.
Corkscrewed Mesocotyls and Leafing Out Underground
There are fairly widespread problems with the emergence and early growth of corn in the area. One problem is associated with the corn planted around April 23-27, which went through several days of soil temperatures below 50F right when the corn was germinating. We have also had some crusting in areas that received the heavy rains. This has lead to a lot of leafing out underground, corkscrewed mesocotyls, and other abnormal growth which has reduced corn stands. Soil temperatures, crusting, and chemicals, either individually or in combination, may contribute to the problem. Bob Nielsen at Purdue has an excellent article at http://www.agry.purdue.edu/ext/corn/news/timeless/Corkscrews.html.
One problem that is now showing up across much of the area is rootless corn. Corn is flopping over and dying because the nodal roots, which emerge about 3/4 to 1 inch below the soil, are not developing. In some cases this is due to shallow planting, settling of loose soil, and/or soil erosion, resulting in the roots emerging at or above the soil surface. In some cases it is where the corn was planted when the soil was too wet so the planter caused sidewall compaction, resulting in the seed slot not closing and the soil becoming hard and dry so the roots cannot penetrate. However, many of the fields that are starting to show this problem were planted under near ideal soil moisture conditions. With the dry weather we have had in the last 2-3 weeks, our silty clay loam soils have the tendency to shrink and crack and the easiest place for them to crack is anywhere the soil has already been fractured.
A rain would do a lot of good in overcoming this problem The only other option to consider would be to cultivate to mound some soil up against the corn to provide some physical support for the plants and encourage root growth. Unfortunately this may also tend to dry out the soil even more. My grandfather used to talk about plowing corn. When cultivating, the goal is to operate shallowly, not to be plowing. In this case, can we operate shallowly (thereby, bringing little or no moisture to the surface) and still move enough soil to the base of the plant? This will depend on the implement, the adjustment, and the operator. Hopefully well get a rain in a few days to solve the problem.
Some corn herbicides can also lead to poor nodal root development. This occurs most often when 2,4-D is applied with an amide herbicide (such as acetochlor, metolachlor, or dimethenamid) around planting time and it is followed by a heavy rain. This is part of the reason for the 10 day window (7 days prior to 3 days after planting) when 2,4-D should not be applied to corn. The easiest way to tell this from other factors associated with rootless corn is to look for a sprayer pattern.
Roger Elmore has a good article on rootless corn at http://www.agronext.iastate.edu/corn/production/management/early/rootless.html.
Making Replant Decisions
Stands of 30,000+ will result in maximum yields. Half a corn stand does not mean half the yield, but a yield reduction of 20-25% would be typical. The cost of re-planting and yield loss from late planting needs to be compared to any yield loss from stand losses to make a good decision. The following table can help with re-plant decisions:
Influence of planting date and plant population on corn grain yields in Iowa
------------------ Corn Yields (% of maximum) -----------------
Stand April 20 - May 5 - May 15 - May 25 - June 5 -
X 1,000 May 5 May 15 May 25 June 5 June 15
35 100 96 87 70 54
30 99 95 86 69 53
25 95 91 83 67 51
20 89 85 77 63 48
15 81 78 71 57 44
10 71 68 62 50 38
Note that, on average, a perfect stand planted at this time will produce about 70% of the fields yield potential. This is nearly identical to a uniform stand of 10,000 plants established on or before May 5.
This table comes from the latest Iowa research and modeling which is found at http://www.agronext.iastate.edu/corn/production/management/planting/replanting.html and on page 12 of the new Corn Field Guide (CSI001).
Numerous gaps of up to 4-6 feet can reduce yields by an additional 5-6%.
The usual method to check corn populations is to measure off 1/1000 of an acre in a row. That is 262 in 20 rows, 175 in 30 rows, 146 in 36 rows, and 139 in 38 rows.
I have observed and continue to receive reports of Black Cutworm injury, some of which is great enough to warrant spraying. When soils are very dry, cutworms tend to restrict their tunneling and feeding to underground areas where moisture is greater, which is what is currently being observed. In order to get an insecticide to where the cutworms are, there will either need to be rain or a shallow cultivation. It is possible that incorporation with an aggressive rotary hoe will work. Fields should continue to be scouted until they reach the V5 growth stage so they can be treated if needed. Fields that are low-lying and poorly drained, those next to areas of natural vegetation, and those where there was reduced tillage or had considerable weed growth prior to planting are at greatest risk. A general rule of thumb is to treat if you find 2-3% of the plants cut and the worms are less than 3/4 inch long. However, higher corn prices may alter that rule. An Excel decision-aid and more information are at http://www.extension.iastate.edu/CropNews/2012/0425sissonjessehodgson.htm.
Late Spring Soil Nitrate Test
With the heavy rains in some areas we have likely lost some nitrogen again this year, but it least so far we havent had the long stretch of wet weather which should help to reduce the losses compared to the last few years. A good way to check the N status of the soil is to take one-foot depth soil samples when the corn is 6-12 tall. At least 16 soil cores (24 is better) should go into each sample and about a cup of this (soil bag full) sent to a lab for analysis. Cores should be pulled in a systematic way going across corn rows (i.e. first core pulled in the row, next one-1/8 the distance between rows, next 1/4 the distance between rows, etc.). For more information on the process see http://www.agron.iastate.edu/soiltesting/pm1714.pdf. An information sheet for sending samples to ISU is at http://www.agron.iastate.edu/soiltesting/LSN.pdf. The cost for analysis is $5/sample.
Bean Leaf Beetles
We havent seen many bean leaf beetles for several years now, but with the mild temperatures this winter, we are noticing them again. There have been several reports of beetles flocking to the earliest emerging soybeans. Fortunately young soybeans can tolerate a lot of leaf loss without much of an effect on the final yield, but if stands are being reduced and/or about 2 beetles per plant are found on seedling (VE-V1) soybeans, an insecticide treatment may pay off. Insecticide seed treatments are effective on bean leaf beetles. The beetles do have to feed on the soybeans before the insecticide seed treatment can kill them, so also expect a little damage even where a seed treatment has been used. The beetles can do a lot more damage during pod fill in August, regardless of whether or not a seed treatment was used.
FOR YOUR CALENDAR
SOUTHEAST IOWA RESEARCH and DEMONSTRATION FARM, Crawfordsville
SPRING FIELD DAY and 25th Anniversary Celebration (afternoon) &
SPECIAL SESSION FOR CCAs (morning)
JUNE 21, 2012
Details are posted at http://www.extension.iastate.edu/Pages/eccrops/meetserc.html.
NORTHEAST IOWA RESEARCH and DEMONSTRATION FARM, Nashua
SPRING FIELD DAY
JUNE 28, 2012, 1 4:30 p.m.
Details are posted at http://www.extension.iastate.edu/Pages/eccrops/meetnerf.html.
TECHNIQUES and TECHNOLOGIES TO AVOID SPRAY DRIFT
July 17, 2012, Field Extension Education Laboratory (FEEL) near Boone, IA
Two half-day sessions (morning session repeated in the afternoon) (no cost to participants) will be conducted, focusing on:
- Nozzle selection/use with demonstration on spray table
- Balancing efficacy and drift
- Environmental factors, adjuvants and limitations, field demonstration, etc.
More information and registration is at http://www.aep.iastate.edu/spray/.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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