C.O.R.N. Newsletter: 2017-20
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Japanese Beetles in Corn and Soybeans
We have been hearing reports of Japanese beetles in corn and soybean. These beetles are large with a shiny copper and green color. Foliage feeding in corn is almost never economic, though economic damage from silk clipping is possible (though rare). Consider a rescue treatment when silks are clipped to less than ½ inch and, fewer than 50% of the plants have been pollinated, and the beetles are still numerous and feeding in the field. Japanese beetles will also feed on soybean foliage.
While the damage might look startling, it is very rare that this reaches economic levels from Japanese beetle. A rescue treatment is advised when defoliation levels reach 30% in pre-bloom stages, and 20% in bloom to pod fill. These defoliation levels apply to the plant as a whole, not just certain leaves. A visual guide to defoliation is useful because it is very easy to over-estimate defoliation in soybean. If there are other foliage-feeding insects present in soybean the same percent defoliation guidelines can be used for all of them collectively.Defoliation guide for soybean (University of Nebraska)
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How to separate flooding injury from Phytophthora seedling and stem blight
Background
I received many calls last week from areas south of and along Interstate 70 that received more than 3 inches of rain and I expect next week they will come from northern Ohio where 4 or more inches of rain landed in some or our plots.
Flooding Injury
Flooding injury can range from lower roots killed to the whole plants if submerged for a long enough period of time. 
The damage and the yield loss associated with flooding injury are directly related to the amount of time the plants are in a low soil oxygen or high carbon dioxide state. The longer the time, the more damage. Most of the plants that I observed and collected last week will recover and in fact many had new roots coming at the top of the root, near the soil line. Most of the nodules were killed, and had turned gray and soft, but on some plants, the nodules at the top were still pink to red when you split them open. The common and tell-tale symptom of flooding injury are the white rat-tail appearance of the roots. The outer root tissue, the cortical layer, can easily be pulled off the roots and leaves the white, woody-feeling, center of the root. The plants will be yellow for a short time while they are recovering, but they will catch up. It is still early enough of a growth stage in soybean that the neighbors that survive will compensate for any plants that were killed either from other root pathogens or just killed out right from the flooding.
Phytophthora
Root rots and Phytophthora turn roots brown to black. These flooded soil conditions are just what the water molds need to infect soybean. When the fields are saturated these water molds form fruiting structures that have zoospores that swim through the soil water to the roots. These root pathogens produce all kinds of enzymes that break down the cell walls, the plants can fight back but the end result are soft brown roots. If the plants have resistance to these pathogens, they will also develop new roots and continue to grow – but there is a yield hit for severe damage.
If plants have low levels of partial resistance to Phytophthora, then the stem rot phase will develop following infection. The plants will turn yellow and a brown canker will develop on the stems. The Rps genes (1c, 1k, 3a, and 6) will give some protection to some of the strains of Phytophthora in these fields, but not to all. We can identify more than 50 different strains of Phytophthora from a single field. For Ohio, Rps1c has not been effective to more than half of the strains since the mid-1980s and Rps1k and Rps3a since the early 2000s.In Ohio, we are very dependent on the partial resistance, (also called tolerance in the seed industry) portion of the package. Both the Rps genes and partial resistance will hold up under flooding stress. In fact, we recently identified one locus that has
both partial resistance and flooding tolerance with our collaborators at University of Missouri. If you see a lot of plants develop Phytophthora stem rot in your field – you know two things. The first is that the Phytophthora package for that field is not right. For Ohio farmers, check the partial resistance score, a range of 1 to 9. Don’t forget to read the fine print at the bottom of the table to see if it is a 1 to 9 where 9 is good and 1 is poor or vice versa? Then check the variety rating. If the plants have Rps1c or Rps1k and they develop stem rot – then you will also know that that these Rps genes are not doing the job. Look at the proportion of plants – is it a few plants here and there or every plant has stem rot. For fields with a lot of stem rot – focus on the best partial resistance score and add a gene stack with more than one Rps gene.Soybean Field Problem
Characteristics
Phytophthora stem rot,
Phytophthora sojae
- Soybeans dying during early vegetative growth stages. The tissue is dried, brown, and collapsed.
- Phytophthora stem rot on plants with very low levels of partial resistance. Note the characteristic chocolate brown canker going up the stem. The Internal tissue is also brown and decayed.
Flooding Injury - Large numbers of plants in low lying areas of field
- Wilted appearance, some yellowing of leaves
- Almost a shepherds hook appearance on the tops
- New leaves on mildly affected plants were growing
- Note grayish color on some of the older leaves.
Figure 5. These are the roots from plants collected in the flooded areas of fields
Note the “rat-tail” appearance of the roots the outer cortical tissue of the roots was killed during the flooding and can be easily pulled off the roots. What is left is woody and tough. Also note on these roots that there are new roots forming at the tops of each of these roots.
The nodules are missing or dead from the flooding, but new ones will form.
If this was Pythium or Phytophthora- the roots would be soft and brown
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Manure, PSNT and N recommendations
The question has come in several times this year, probably due to the excessive rain, from folks wanting to know how much more nitrogen (N) to add. Recently the question came from a livestock operation.
For several years the use of the PSNT – Pre-Sidedress Nitrogen Test – has been recommended in manured or terminated legume situations. Ohio has done field test with PSNT and where soil test result exceed 25 ppm NO3-N additional N is unlikely to increase yield. Both Pennsylvania State University and Purdue University have taken the PSNT a step further providing guidelines to adjust N rates where the PSNT result does not exceed the no application threshold. We often point producers in those directions to learn more.
After a manure application or termination of a legume, nitrogen will become available for corn during the growing season. We can predict somewhat how much is available for the crop and recommend a reduction in added commercial fertilizer nitrogen.
- Sample 12 inches deep (or as deep as you can) in the field when corn is about 12 inches tall. Collect 15 to 25 cores per sample. More if there is a history of banding nutrients. Air dry the soil and send to a lab requesting the PSNT soil nitrate test.
To determine how much N to apply:
- From PennState University – if the soil nitrate test is above 21 ppm, then you would expect no economic benefit from added nitrogen.
- If the test shows less than 21 ppm NO3-N, then use this calculator to determine the need.
1
Soil NO3-N level in ppm from PSNT
2
Yield expectations – in bu/A
3
Field history = The total of scores from three factors: Manure applied since last harvest + Previous crop + Manure history
- Manure applied (None 0.75, Any 3.5)
- Previous crop (Corn 0, Soybean 1, Forage legume 3.5, other 0.
- Manure history, past 3 years (None = 0, Any = 1.75)
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Recommendation = Yield – (PSNT x History score)
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- For example: A recent PSNT test I was shown was 20ppm NO3-N, the yield last year was 165 bu/A, the field has a history of applying manure and is corn after corn – so the history score is 3.5 + 0 + 1.75 = 5.25. Our recommendation would be: N rec = 165 – (20 x 5.25) = 60 N/A
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- Purdue University suggests no additional N is required if the nitrate test is above 25ppm.
- Use a table to determine the level of N to add if the NO3-N level is below 25ppm. The table published with the AY-314-W factsheet was created prior to our current MRTN – Maximum Return To N – multistate developed corn N recommendation calculator (http://cnrc.agron.iastate.edu). The Corn Nitrogen Rate Calculator calculates the MRTN based on state, crop rotation, price of N and price of corn.
- A personal communication this past winter from Jim Camberato at Purdue suggested a modification to the factsheet table relying on the MRTN calculator.
Soil NO3-N
ppm
Adjustment to MRTN recommended N rate
Lbs. N/A
0-10
Full rate
11-15
-30
16-20
- 45
21-25
- 90
>25
No sidedress N
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- For example: From the above grower with a 20 ppm soil nitrate PSNT test we use the 16-20 row and reduce our MRTN rate by 90 lbs. N/A. A quick run of the CNR Calculator suggests 174 lbs. N/A for Ohio at present – I subtract 45 and get a suggestion of 129 lbs. N/A. Yes this differs from the Pennsylvania recommendation but keep in mind that both are suggestions based on past history. One way to see how this year and your conditions may differ to provide a better corn nitrogen recommendation is to use a crop sensor.
Crop sensors
An alternative to using soil sampling is to use an in-crop sensor to determine corn nitrogen needs. Penn State suggests the use of a chlorophyll meter test; this would give you an idea of the nitrogen status of the crop based on the color. This requires a high-N reference strip although some suggest that with the variables of manure application perhaps (or perhaps not – Scharf in Missouri) you could use a virtual reference strip – meaning a place in the field where greater amounts of nitrogen are available. Using the chlorophyll meter you could determine the difference in N status between the low N and high N areas of the field.
Work done in Ohio, Oklahoma, Missouri and other areas suggest using an NDVI sensor to determine crop color. Work here since 2009 suggests that, yes, we can determine crop N needs with sensors such as the GreenSeeker and others. These would work in situations with manure or commercial fertilizer. Again as with the chlorophyll meter above we must have a high-N reference strip in the field. Wait until V8-V12 corn growth stage, compare the low and high N areas of the field and then use a calculator to determine any additional crop need for sidedress N. From work I am doing this year waiting until the V12 stage is a better indication of N status than the V8 growth stage, unless you had saturated soils and lost N due to denitrification. The calculator for Ohio resides on the Oklahoma State University Soil, Water and Forage Analytical Laboratory website, noted below.
Resources
From Pennsylvania State University: http://extension.psu.edu/plants/crops/grains/corn/nutrition/pre-sidedress-soil-nitrate-test-for-corn.
From Purdue University: https://www.agry.purdue.edu/ext/pubs/AY-314-W.pdf.
From the University of Missouri: https://plantsciences.missouri.edu/nutrientmanagement/nitrogen/pdf/sensor_manual.pdf.
From Oklahoma State University: http://soiltesting.okstate.edu/sensor-based-n-rate-calculator.
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What is the Nutrient Value of Wheat Straw?
Wheat harvest is now underway. What is the nutrient value of the straw? The nutrient value of wheat straw is influenced by several factors including weather, variety, and cultural practices. Thus, the most accurate values require sending a sample of the straw to an analytical laboratory. However, “book values” can be used to estimate the nutrient values of wheat straw.
In previous newsletters, we reported that typically a ton of wheat straw would provide approximately 11 pounds of N, 3 pounds of P2O5, and 20 pounds of K2O. A 2013 analysis of wheat straw collected at the OARDC in Wooster contained 14-18 pounds of N, 3-4 pounds of P2O5, and 20-23 pounds of K2O. These values were across four wheat varieties and three spring nitrogen application rates (60, 90, and 120 lb. N/acre). The 2013 values corresponded fairly well with the previously reported “book values.” Nitrogen values in 2013 were slightly greater than “book values” which may have been a result of wheat height/size. If plants are shorter/smaller, percentage nitrogen tends to be greater than taller/larger plants due to a dilution factor as the plant grows. Regardless, the 2013 analysis provides validity to the nutrient value of straw given in previous newsletters.
The nitrogen in wheat straw will not immediately be available for plant uptake. The nitrogen will need to be converted by microorganisms to ammonium and nitrate (a process called “mineralization”). Once the nitrogen is in the ammonium and/or nitrate form, it is available for plant uptake. The rate of which mineralization occurs depends on the amount of carbon and nitrogen in the straw (C:N ratio). The USDA reports a C:N ratio of 80:1 for wheat straw which means there are 80 units of carbon for every unit of nitrogen. Mineralization rapidly occurs when the C:N ratio is ≤ 20:1. At a C:N ratio of 80:1, mineralization will be much slower. (For comparison, corn stover is reported to have a C:N ratio of 57:1.) Rate of mineralization is also influenced by soil moisture and temperature. Since mineralization is a microbial-driven process, mineralization will be slowed (halted) in the winter when temperatures are cold. Thus, no N credit is given for wheat straw since it is not known when the N will mineralize and become available to the following crop.
Besides providing nutrients, straw has value as organic matter, but it is difficult to determine the dollar value for it. Removal of straw does lower soil potash levels. If straw was removed after heavy rainfall, some of the potash may have leached out of the straw, lowering the nutrient value of the straw. However, a soil test should be done to accurately estimate nutrient availability for future crops.
Crop Observation and Recommendation Network
C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.
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