Southern rust of corn observed in Illinois

Southern rust of corn has now been observed in different areas of Illinois.  Southern rust is one of two different rust diseases of corn that can be observed in the state (the other is known as common rust).  Because nearly every corn hybrid grown is susceptible to southern rust, yield reductions can occur if infection takes place early enough in the season.  Late-planted corn fields are at the highest risk for yield losses associated with southern rust and should be scouted for the presence of this disease.  Warm and humid conditions are most favorable for the southern rust pathogen infection and disease spread.

It is important to be able to differentiate southern rust from common rust, since the latter generally is not considered a threat to yellow dent corn hybrids because most are fairly resistant to common rust.  Southern rust pustules generally are smaller than common rust pustules and are orange in color compared to cinnamon-brown in color for common rust.  Southern rust pustules tend to be more densely scattered than common rust pustules and more chlorosis (yellowing) around the pustules generally will be observed with southern rust.

Because nearly every corn hybrid is susceptible to southern rust, foliar fungicides are the only management tool available.  If corn plants are at the R3 development stage (milk stage) or beyond, then it is less likely that southern rust will cause yield loss; however, on late-planted fields that are not yet at that stage, it is important to continue to scout those fields for southern rust.  A southern corn rust observation map for the United States is available through the the IPM PIPE system (http://www.ipmpipe.org/); however, southern rust may be present in counties that are not highlighted on the observation map.

 

Pustules of common rust (left) and southern rust (right) on corn leaves. Note that southern rust pustules are lighter in color.

Southern rust pustules beginning to form. Note the chlorotic (yellow) tissue around the pustules.


Will The Corn and Soybean Crops Get Finished?

Late planting and weather that continues to be cooler than normal into August has many wondering if the corn and soybean crops will reach maturity and harvest moisture within a reasonable time this fall. Crop conditions remain good for both crops, but crop development, including pod formation and filling in soybean and grain fill in corn, remains well behind normal. Corn is 10 days to 2 weeks behind normal, and soybeans are 2 to 3 weeks behind normal. The number of days behind will “stretch” as the weather cools, so late crops get even later. Ten days behind in mid-August will be become 15 or 20 days behind in mid-September, even if temperatures are normal.

We have often pointed out that late-planted corn tends to require fewer growing degree days to reach maturity than does early-planted corn. That’s often reflected in lower yields, as the crop experiences stress during the high temperatures and dry weather we often experience in mid-summer. This year, however, with temperatures generally below normal in recent weeks, we do not expect to see this accelerated development. Instead, we are seeing that corn development is following closely the normal number of GDDs required to reach each stage. This means less chance of premature death and so a better chance to fill grain completely. But for late-planted corn it also means late maturity.

How late can we expect the corn crop to mature? If we assume normal GDD accumulations in August and September, corn planted in the Champaign area on May 1, May 15, and May 31 will accumulate about 3,020, 2,850, and 2,530 GDD, respectively, by the end of September. If we further assume that a mid-season (111-day RM) hybrid needs 2,700 GDD from planting to maturity regardless of planting date, we can expect corn planted on these dates to reach maturity (black layer, about 32% grain moisture) about September 5-10, September 15-20, and mid-October, respectively. The crop seems to be on course to do this: corn planted in early May is at stage R3 (milk stage) now while corn planted in late May or early June is just finishing pollination. This reflects ongoing cool temperatures so far in August, and if these continue, maturity dates will be even later.

With a 50% frost date of about October 20 here, we would expect a mid-season corn hybrid planted in early June to mature before frost, if frost does not occur before its normal (50%) date. But drydown slows quickly as we move into October, and even early-planted corn will dry slowly after maturity unless September is unusually warm. Corn planted in mid-June (some fields in central Illinois are still not pollinated) is unlikely to mature before frost if temperatures are normal and frost comes at its normal time.

While the soybean crop has a dark green, healthy appearance in most fields now, podsetting is later than normal, and the crop planted the second half of May has not yet reached, or moved past, beginning podfill (stage R5). If temperatures continue to be cooler than normal, we can expect the crop to reach maturity only by late September or early October. We hope not to see a repeat of 2009, when more than half the Illinois soybean crop was harvested after November 1. That followed a very wet October.

Beyond their effect on maturity, cool temperatures are continuing to have a somewhat negative effect on soybean crop. The crop has good photosynthetic capacity due to its healthy, complete canopy. And soil water use rates are less than normal, extending the water supply. But below-normal daytime temperatures (and clouds) mean less photosynthesis, and cool nights can physiologically limit growth rates and the photosynthetic rates the next day. The first effect we might see is below-normal pod numbers, or pods initiated in late August that might not fill.

Good soybean yields are still possible if the weather remains good into September, but seed filling rates will remain slow as long as temperatures remain low. We have sometimes seen cool temperatures trigger maturity before seeds are fully filled. The best scenario for soybeans would be a return to temperatures – both day and night – return to normal or a little above normal, with enough rainfall to enable the crop to photosynthesize fully as seeds fill. Even with that, we’re in for a wait to see how the crop finishes this fall.


Check Corn Pollination

We are starting to get reports that corn pollination might not have been as successful as expected in parts of Illinois. While the weather was generally favorable during the peak period of pollination, it was warm during the third week of July, and soil water may have been limiting during this period in some fields. Thus we would expect to see this in the areas with low rainfall in July.

This is not the lack of silking that we saw in many areas under the drought of 2012. Silks generally emerged well in most fields this year, but tassel emergence was slowed by dry soil conditions in some cases. The warm third week of July was followed by unusually cool weather, with some lows in the upper 40s and lower 50s the last weekend in July.

It’s possible that these unusual conditions limited pollen production or that silks became non-receptive (unable to accept and germinate pollen) before the end of pollen-shed. Indications are that scattered kernels from poor pollination are being found at the base of the ear more than at the tip, which might point toward lack of early pollen production and possible loss of silk receptivity.

Regardless of the cause, it’s time to get into fields to see how successful pollination has been. Once silks start to dry, remove husks and shake or pull on silks. Those that detach easily are from fertilized kernels, while those that stay attached are on kernels that have not been fertilized. Silks that emerge more than a week after silks first appeared and seem to be fresh probably emerged after rainfall, and there will typically be little or no pollen available to pollinate these. The target is to have around 500 kernels per ear at populations in the lower to mid-30,000 plant population.

There is little to be done about lack of kernel set at this point, but it will help us know what’s coming, and it may help us identify causes. When different hybrids show different degrees of this type of problem, it is often more a matter of timing than genetics; hybrids that silked on a certain date are often affected more than those that silked a day or two later. But if the same hybrid planted on different dates shows the problem, it may be genetic in nature, or possibly related to soils or management. For example, a better root system that can pull water a little more effectively can make a large difference in pollination success.

I’d be interested to receive reports and observations on this so we can try to put the puzzle together.


Assessing the risk of white mold (Sclerotinia stem rot) of soybean in 2013

White mold of soybean (a.k.a. Sclerotinia stem rot), caused by the fungus Sclerotinia sclerotiorum, is a disease that can occur in the northern half of the state in cool, wet years.  The most recent white mold epidemic in Illinois occurred during the 2009 season, where several fields in the northern half of the state were affected.

The white mold fungus overwinters in the soil as, small, black, and dense structures known as sclerotia.  These sclerotia germinate and form mushroom-like structures known as apothecia when soil remains moist for several consecutive days and soil temperatures are at 60 degrees F or below.  These apothecia generally will not form until the soil is shaded from sunlight due to soybean canopy closure.  Spores of the white mold fungus are shot out of the apothecia and land on senescing flower petals, where infections first occur on the soybean plants.  The white mold fungus becomes inactive when temperatures within the soybean canopy are above approximately 82 degrees, so infection and disease development may cease or slow down during periods of hot and dry weather.  The 2013 season started out similar to the 2009 season, with frequent rainfall and cool temperatures, but warmer and drier conditions have been observed more recently.

 

Fig. 1. Apothecia of the white mold fungus germinating from a sclerotium. Image courtesy J. Venette, North Dakota State University.

 

Fig. 2. Soybean plant with symptoms and signs of white mold (a.k.a. Sclerotinia stem rot). Image by C. Bradley.

 

So, what does the risk of white mold look like for 2013?  This is not an easy question to answer since we have had fairly favorable weather for white mold initially with some stretches of non-favorable weather in-between.  In fields that were not planted late and have been in full-canopy for the last couple of weeks, it is likely that apothecia have developed and that some infections may have occurred already.  However, the progression of infection and disease development likely came to a stop with the hot and dry conditions observed last week.  For fields planted later that just recently closed their canopy (or have not yet closed their canopy), apothecia likely have not emerged because of the hot dry weather experienced last week.  The weather from this point forward will likely dictate how bad white mold will be in 2013, although, I believe that it is safe to say that it will be less severe and less widespread than it was in 2009.

In University of Illinois research trials, some fungicide products have shown efficacy against white mold.  Foliar fungicides will not provide complete control of the disease, but may reduce disease.  The results of University of Illinois trials conducted in 2009 and 2010 are shown in Tables 1 and 2.  Note that some of the more popular, frequently marketed fungicides are not listed in the tables since many do not have white mold on their label because of no to poor efficacy.  In these trials, the primary targeted growth stage to apply foliar fungicides was at R1 (beginning flower).  Because of the late-planted soybean fields this year, R1 may occur before canopy closure.  If this is the case, then an application at canopy closure (rather than R1) might be more effective in protecting against white mold.  Also note that some treatments in these research trials were applied twice during the season (R1 and again 7-9 days later).

 

Table 1. Results of soybean foliar fungicide research trials focused on white mold conducted in 2009 at the University of Illinois Northern Agronomy Research Center (DeKalb County).

Treatment Rate/A Incidence (%) 

8-11-09

Incidence (%) 

9-14-09

Yield (bu/A)
Untreated check 75 95 24
Topsin 4.5 L 20 fl oz 43 96 24
Proline 3 fl oz 38 95 24
Domark 5 fl oz 68 98 23
Cobra herbicide 12.5 fl oz 15 51 42
Endura (2x)* 8 oz 38 86 39
Aproach (2x)* 8 fl oz 35 80 40
LSD 0.05** 33 15 8

*All treatments were applied at the R1 growth stage (July 20, 2009).  Treatments followed by “(2x)” were applied again 9 days later.

**Least significant difference (alpha level = 0.05).  Treatment values that differ by this number can be considered significantly different from one another.

 

Table 2. Results of soybean foliar fungicide research trials focused on white mold conducted in 2010 at the University of Illinois Northern Agronomy Research Center (DeKalb County).  Funded in part by the Illinois Soybean Association.

Treatment Rate/A Incidence (%) 

8-11-09

Incidence (%) 

9-14-09

Yield (bu/A)
Untreated check 18 95 62
Topsin 4.5 L 20 fl oz 9 83 61
Proline 3 fl oz 11 93 64
Domark 5 fl oz 7 76 63
Cobra herbicide 6 fl oz 6 86 56
Endura 8 oz 4 79 69
Aproach (2x)* 8 fl oz 11 79 66
LSD 0.05** 11 NS 8

*All treatments were applied at the R1 growth stage (July 10, 2010).  Treatments followed by “(2x)” were applied again 7 days later.

**Least significant difference (alpha level = 0.05).  Treatment values that differ by this number can be considered significantly different from one another.  “NS” indicates that no treatments were significantly different from each other.

 

Overall, the highest level of white mold control will be achieved when several management practices are integrated (i.e. choosing the most-resistant varieties, utilizing recommended seeding rates, applying a foliar fungicide, and applying a biocontrol product).  For more information about white mold and management of this disease, go to http://www.planthealth.info/whitemold_basics.htm, where a recently-developed 7-page publication on white mold can be downloaded and several podcasts on white mold can be accessed (all funded by the North Central Soybean Research Program).


Corn and Soybean Crops at Mid-Season

The third week of July brought above-normal temperatures to much of Illinois. This is expected to be temporary, with temperatures the fourth week expected to return to normal. Because soils in most areas had enough water to carry the crop through the week, we don’t think the high daytime temperatures were much cause for concern.

By July 14, 21 percent of the Illinois corn crop was pollinating. This included most fields planted before May 10 to 15. Because planting was so concentrated in the third week of May, we expect as much as half the crop to pollinate by July 21. That’s a week or so later than normal. Higher than normal night temperatures this past week might have hurt pollination success some in areas where soils are starting to dry out. By the heavy silking that preceded the full emergence of tassels that we have noted in recent years is very much in evidence again this year. This indicates that silks should be present when pollen is being shed.

In some fields that were planted in mid-May or later, especially those planted at high populations, stalk diameter is noticeably smaller than we often see in earlier-planted corn. Later-planted corn that has plenty of soil moisture often grows taller than early-planted corn because of higher temperatures during internode elongation. But photosynthetic rates (dry weight increases) are not higher in late-planted corn, and so taller plants usually mean less weight per foot of stalk length. Plants with smaller stalks often have less leaf area, and so less ability to set and fill a large ear. It’s too early to know if this will decrease yield potential, but it is one of the ways in which late planting leads to lower yields.

If we have a return to better soil moisture conditions along with lower night temperatures over the next two weeks, we can expect kernel set to be good. But after a wet June, soil moisture is becoming a concern in some places. July rainfall has been less than normal over much of Illinois, with less than an inch so far in parts of western and northern Illinois.

The soil water supply was good coming into July, but how well the crop is tapping into this supply will make a difference as we move into the second half of the season. Fields that were planted by mid-May have had a chance to produce a good crop canopy, which in turn has enabled deeper root growth. It’s doubtful that roots are as extensive as they were in July 2012, but as we saw last year, good root systems don’t help when there’s no available water in the soil.

The major issue with water supply now is in the crop that was planted in late May or early June, and that won’t pollinate until late July or early August. These plants are in mid-vegetative stages, and are often showing mid-afternoon drought stress symptoms (leaf curling) where recent rainfall has been limited. This crop has not grown enough to have used up the available soil water to 3 or 4 feet deep. Rather, the root system is simply not as deep or as well-connected as it would be had planting been earlier or if soils had not been so wet earlier.

Another concern with the wet weather earlier in the spring is nitrogen supply for the crop. Leaf color remains good in areas with adequate soil water where the crop has continued to grow well. But in areas where soils have dried and some drought effects are starting to appear, leaf color had lightened some. In most cases this is because nitrogen reaches the roots with water, and when water uptake slows, so does N uptake. Rainfall in such places should return leaf color to normal green. If leaves have been pale during the pollination period, however, success of kernel set can be lowered.

It is possible that enough nitrogen has been lost in some lower-lying parts of fields to mean shortages for the crop, even after root systems recover. Supplemental applications of N have been made in some such areas, usually as urea, sometimes with urease inhibitor. Such applications might produce enough added yield to provide a positive return if they are made before or even during pollination, but are likely to be effective only when rain falls to move them into the soil.

As we move through pollination and into the grainfilling stages, the major concern will be how well the leaf canopy holds up to provide maximum amounts of photosynthate (sugars) to fill kernels. Soil water supply will be the major factor, followed by leaf health and nutrient supply. We expect the crop to take about 8 weeks to reach maturity after pollination is complete, and maintaining green, healthy leaves through this period is the only way to maximize kernel fill and yield. If the crop takes less time from pollination to maturity, it will have stopped filling prematurely and will produce lower yields.

The soybean crop has had a slow start, but warmer temperatures have finally produced a surge of growth in fields planted by early June. According to NASS, 32 percent of the soybean crop was blooming by July 14. That’s a little behind average, and less than half of what it was in 2012, but for a crop planted as late as this year’s, it’s reasonable. As is typical with late-planted soybeans, plant height at the beginning of flowering is fairly short this year.

The soybean crop canopy is starting to develop and close, but some of the very late-planted crop will flower for some weeks before the canopy is effectively complete. As we saw last year and in some previous work, as long as flowering and early podsetting take place under good conditions, having the canopy close early might not be that critical. The long-held idea that failure to close the canopy by first flower means lowered yield potential does not seem to hold true, at least with indeterminate soybeans that continue vegetative growth for weeks after first flower.

While we want to have good canopy color and cover in mid-July, yield potential for the 2013 soybean crop is far from being set at this point. We might consider it as we would the corn crop a month before pollination. There’s a long ways to go, and we won’t have a good idea about yield potential for at least a month.


Brownstown Agronomy Research Center Field Day – July 25

The 2013 Brownstown Agronomy Research Center Field Day, presented by the University Of Illinois Department Of Crop Sciences, will be held on Thursday, July 25. Extension researchers and specialists will address issues pertinent to the current growing season. Tours will start at 8 a.m., with the second and third groups leaving the headquarters around 8:20 a.m. and 8:40 a.m. The tours will last about two and a half hours and will be followed by lunch provided by U of I Extension.

Shaded tour wagons will take participants to each stop. These topics will be addressed:

  • Nitrogen Sensors & Variable-rate N Applications – Dennis Bowman
  • Wheat Disease I.D. & Management – Dr. Carl Bradley
  • Emerging Developments in Weed Management – Doug Maxwell
  • Crop Rotation:  Another Risk Management Tool – Dr. Emerson Nafziger
  • Agronomic and Environmental Assessment of Cover Crops – Dr. Angie Peltier

 The 208-acre Brownstown Agronomy Research Center has been conducting crop research on the claypan soils of southern Illinois since 1937. More than 30 research and demonstration projects are conducted at the Center every year. Visitors are always welcome.

The research center is located south of Brownstown on IL Route 185, approximately 4 miles east of the IL Route 40 / 185 junction.

For more information, contact Robert Bellm (618-427-3349); rcbellm@illinois.edu
Visit us on the web at http://web.extension.illinois.edu/barc/


Stormy weather and Goss’s wilt go hand in hand

Goss’s wilt of corn often is most severe after fields are exposed to high winds and/or hail damage, because the causal bacterium, Clavibacter michiganensis subsp. nebraskensis, readily infects corn leaves through wounds.  With the recent storm activity across the state, growers should be on the lookout for the appearance of Goss’s wilt symptoms.  Goss’s wilt lesions on the leaves generally have wavy margins with a water-soaked appearance on the edges of the lesions.  Dark spots, known as “freckles”, almost always can be found within the lesions.  The affected areas of the leaves will have a shiny appearance when observed in the sunlight, and bacterial exudates may be on the leaves that resemble drops of maple syrup.

 

Goss's wilt symptoms on a corn leaf. Note the wavy margins of the lesion, the dark spots ("freckles") inside the lesion, and the water-soaked appearance of the lesion margin.

 

Bacterial exudates (circled in red) of the Goss's wilt pathogen, which appear as drops of maple syrup.

 

Goss’s wilt incidence was at an all-time high in the 2011 season, with over 30 Illinois counties having positive detections via samples sent to the University of Illinois Plant Clinic.  This high incidence observed in 2011 means that the causal bacteria is present in the state and can cause infections again in the 2013 season if the conditions are favorable.  Fields that are at the most risk to Goss’s wilt are those that have been grown to continuous corn, have been planted to a susceptible hybrid, and have received wind or hail damage.  Because other diseases and disorders can resemble Goss’s wilt, it is important that suspicious samples be sent to the University of Illinois Plant Clinic for the most accurate diagnosis.  Be cautious of results received from test kits designed for the related bacterial canker of tomato pathogen, Clavibacter michiganensis subsp. michiganensis, because these kits have been shown to give false positives.

Unfortunately, no consistently-performing in-season control options are available for Goss’s wilt management.  Because Goss’s wilt is caused by a bacterium, foliar fungicides are not effective.  Other products may be marketed as providing control of Goss’s wilt, but field testing of these products by universities have shown these to be inconsistent and/or ineffective in providing reliable control of Goss’s wilt.  The best way to manage Goss’s wilt is to rotate to a non-host crop, such as soybean, and plant the most resistant hybrid available the next time corn is in the field.  Because the Goss’s wilt bacterium survives in corn residue, fields with severe Goss’s wilt can be tilled after harvest to help decompose the residue, which may help reduce the inoculum level in the field.


Corn Roots, Wet Soils, and Nitrogen

The June weather pattern in Illinois was variable, and the month is ending with rainfall totals ranging from a little less than normal in parts of western Illinois to nearly double the normal amounts, with some totals as high as 7 to 8 inches, in parts of southeastern and northern Illinois.

While getting rainfall in June is certainly preferable to getting little or none as happened in Illinois in 2012, standing water and wet soils can badly damage a rapidly-growing corn crop. In June of 2010, 2 to 4 inches of rain fell during the fourth week of June over most of central and northern Illinois. With the crop planted early and developing rapidly under high temperatures, standing water resulted in serious and irreparable damage to root systems. This lowered yields in low-lying fields and parts of fields, even where rains fell later in the season.

When soils remain saturated for more than a day or two, the lack of oxygen causes nutrient uptake to slow quickly, and root tips start to die off. It helps that temperatures have not been above normal; cooler water carries more dissolved oxygen, and also slows growth and nutrient uptake. Also, plants during vegetative growth have much better ability to grow back damaged root systems once soils drain than do plants during or after pollination.

These factors, along with the very good crop color (which indicates good root activity and adequate supplies of soil N) before the rains in late June, point to good chances for recovery of crop yield potential  in fields and parts of fields where the water is no longer standing. In the short run, plants may lose some of their green color before roots are fully functional again, but this will likely be a temporary condition. While many worry that any stress during mid-vegetative growth will lower yield potential, there’s not much evidence that a few days with reduced photosynthetic rates has much effect on yields, at least if this occurs more than a week before tasseling.

Regardless of how quickly the crop returns to normal after an event like temporary flooding, questions will remain about how standing water might affect the amount of nitrogen left in the soil to meet the needs of the crop. Warm, saturated soils lose nitrogen (as gas back into the air) through the process of denitrification. We do not think that such losses have been very large in most fields, given the temperatures and the fact that most flooding was temporary. In better-drained fields, denitrification would be less, but percolating water has probably moved some of the nitrate-nitrogen deeper, perhaps below the root system or into tiles lines.

In central Illinois we have accumulated about 1,100 growing degree days (GDDs) since May 1, and about 930 GDDs since May 15. By the time corn accumulates 1,000 GDD, reaching about stage V13, it has accumulated about 20 percent of its dry weight and about 40 percent its season-long nitrogen accumulation (Abendroth et al., 2011) During this period the crop takes up 3 to 3.5 lb of N per acre per day, and by the time of pollination, it will have taken up about 60 percent of its nitrogen and produced about 40 percent of its dry weight.

At the time the crop reaches stage V13 (about head-high), it still has to take up 110 to 120 lb of N, and in years when June is wet, a common question is whether or not the crop might run out of nitrogen, leaving the crop short. While the need for 20 or more lb of N per week would seem to raise the possibility of a shortage, the production of plant-available N from soil organic matter through the process of mineralization is also at its maximum rate in mid-season.

For a crop with a good root system growing in a soil with 3 percent organic matter, mineralization at mid-season likely provides at least half the N needed by the crop on a daily basis. This means that normal amounts of fertilizer N, even if there has been some loss, should be adequate to supply the crop.

Though we could measure soil N present or apply urea by air on the wetter field or parts of fields where the crop shows deficiency, it would seem prudent to wait to see if the crop recovers its green color before going to this expense. The loss of crop color in wet soils is due mostly to loss of root function, and roots will need to recover before the canopy does. Even without adding more N, odds are good that the crop will recover and thrive in the coming weeks, providing the weather remains favorable.

Reference:

Abendroth, L.J., R.W. Elmore, M.J. Boyer, and S.K. Marley. 2011. Corn growth and development. PMR 1009, Iowa State University Extension, Ames, Iowa.


Purple and Yellow Corn Plants

The corn crop that was planted in May is up and growing in most fields, but there have been numerous reports of fields with uneven plant sizes and colors, including purple and yellow plants. Many are wondering if this will decrease potential yields.

Based on past experience many people expect to see purple corn when soils are cool and dry during early plant growth stages, or in that rare field with low soil test phosphorus levels. The purple color is from a pigment that forms when there is more sugar in the leaves than the plant can utilize. Low phosphorus inhibits sugar movement out of leaves, and cool, dry soils reduce root growth and sugar movement to the roots. Both of these increase sugars in the plant and can make the tissue turn purple.

Soils are not cool and dry in Illinois now, but surface soils are drying out in many fields, and it’s likely that roots growth up to now has been restricted some by having soils too wet and in some cases also compacted by tillage and planting operations. In fields where the plants have taken up enough nitrogen and water to grow well but roots remain constricted, purpling might be common now, especially in those hybrids that have a tendency to form the purple pigment.

We expect the problem of purple corn to correct itself as root growth continues. It will help if wet soils continue to dry out, but in fields with dry surface soils, root growth might benefit from some rainfall. The high sugar content that leads to purpling means that the plant is producing sugars through photosynthesis, which is a good sign. There is no evidence that temporary purpling affects yield of the crop, though factors such as soil compaction that can lead to purpling might also reduce yields if the weather is dry later in the season.

Other fields are showing the yellow color that characterized nitrogen deficiency. In some cases this diagnosis is strengthened by our being able to see patterns such as N applicator knife tracks where the plants are greener. As is the case with purpling, we tend to see more yellow plants in the lower areas of the field. This is both where soils were wetter at planting time, so are more compacted, and where we would expect more N loss and poor root growth due to wet soils.

Some have already responded to yellow corn by applying a higher rate of sidedress N than had been planned, or by, or applying sidedress N on top of a full rate of N already there. Some may even have applied foliar N or broadcast urea by air to try to get N into plants quickly. If soils are still wet in such fields, plants are continuing to struggle with poor root growth and poor root function, so adding N might not have much immediate effect.

It is likely that the problem of yellow corn is, like that of purple corn, more related to poor root growth than to low level of soil nutrients. Student interns working at the Orr Center in Perry sampled soils the first week of June in a study where different N rates were applied as UAN in the first week of April. That site received some 17 inches of rain in April and May, and we expected that some of the N would have been moved to below the top two feet.

In fact, we found as much or more total N (nitrate plus ammonium) in the top two feet the first week of June than we applied as UAN the first week of April. We recovered about 90 lb. of N where we had applied 60 lb., and just over 240 lb. of N where we had applied 240 lb. So it’s likely that some of the N there now was produced by mineralization, and that some fertilizer N was moved down below two feet deep, but the net amount available to the crop after high-loss conditions certainly has not been drastically reduced.

As soils dry out in most areas of Illinois and temperatures stay warm, it’s likely that many fields with yellow corn plants will improve, in some cases rapidly. Late planting and warm temperatures do tend to favor top growth over root growth, but we expect that as leaves grow and start improve in color and as soil oxygen levels increase as soils dry, sugars will become more available to the roots as well as the tops, and this will further improve root uptake of nutrients.

If crop color remains poor even after a week of drying soils and good growing conditions, then it is possible that N movement to below the rooting depth is affecting the ability of the crop to grow out of this problem. Our soil measurements suggest that in most cases N is likely still present, but a small “booster” shot of N might help the crop revive and reach the N more quickly. The crop has roots extending to the middle of the rows by the time it have 4 or 5 leaves, so injecting N between the rows should work to get it into the plant.  Broadcast urea will need some rainfall to reach the roots.


How Late Can We Plant Corn and Soybeans?

The latest report from NASS indicates that 96 percent of the corn and 62 percent of the soybean crop in Illinois had been planted as of June 9. Some of the corn is struggling, however, with 13 percent of the crop rated as poor or very poor. Much of this is due to heavy rainfall, which has caused problems with stands, including areas in fields where the crop has drowned out from standing water. In other cases, the crop has emerged poorly in fields or areas in fields due to crusting and some low temperatures.

It’s very late to plant or to replant corn, and for many with unplanted fields or with poor stands that may need to be replanted, the choice is whether it makes sense to plant corn this late, and if not, whether the best option is prevented-planting insurance or replacement with soybeans, as crop insurance provisions allow.

Table 2.3 in Chapter 2 of the Illinois Agronomy Handbook http://extension.cropsci.illinois.edu/handbook/  indicates that we can expect a corn crop to yield about two-thirds of its expected (early-planted) maximum yield if the crop is planted on June 8 and has a full stand. That is a projection, since most of our corn planting date studies include planting only through the end of May. Based on this, however, we have projected that corn reaches the point where we can expect 50 percent of maximum yield if it is planted sometime between June 15 and June 20.

I reanalyzed our more recent planting date data, and if – this is a big “if” – we can accept projections of yield that go well past the last planting date, we would move the planting date from which we’d expect half a crop a little later, closer to the end of June. But we know that corn planted during or after the middle of June will produce fair to good yields in some years and very little yield in other years, depending on unpredictable weather that follows. Hence it makes sense to consider June 15 to 20 to be that last “practical” date on which to plant corn if we want to produce grain.

If we do plant corn in mid- to late June, planting a very early hybrid, having the option of harvesting the crop as silage if grain production looks unlikely, and getting good rainfall throughout the rest of the season will all improve the chances of ending up with a profitable crop. The chances of having enough frost-free days to grow a crop are higher in central and southern Illinois than farther north, but higher water loss rates and lower water-holding capacity of soils can cancel this advantage. It may also be difficult to get seed of very early hybrids, and because early hybrids are not developed for the central and southern Corn Belt, there is no guarantee that they will do well under late planting.

If it’s too late to plant corn and we don’t expect enough yield to make a profit (or at least to make more than crop insurance would pay to plant nothing), does it make sense to plant soybeans instead? We have run our soybean planting date studies into the first or second week of June, but we still have to project expected yields past the last date we actually planted.

Going through the same exercise as we did for corn, we would expect soybeans planted at the end of June or early in July to yield half what they would if planted early. This is about two weeks later than the normal doublecrop planting date in southern Illinois. Doublecrop soybeans have averaged 72 percent of full-season soybean yields over the past 10 years at Brownstown, so using early July as the 50-percent-of-maximum-yield planting date seems reasonable.

We know that doublecrop (or very late-planted) soybean yields can range from zero to good, and there’s no way to predict when they are planted which end of this range they’ll be on. As many found out in 2012, planting into bone-dry soils is not usually conducive to high doublecrop soybean yields. And in northern and central Illinois, doublecrop soybeans or soybean planted (or replanted) in late June or early July have had a considerably lower rate of success than doublecrop soybeans in southern Illinois.

There is one important difference between doublecrop soybeans and soybeans that are planted late but that don’t follow wheat harvest. Wheat removes a substantial amount of water from the soil as it matures, and in years with average June rainfall, the soybean crop that follows wheat has much less soil water available to it than does the crop that follows only the crop from the previous year. As is always the case, good rainfall through the rest of the season can cancel out this advantage, but it won’t eliminate it averaged over years.

Soybean planted in mid- to late June needn’t be managed much differently than early-planted soybeans. Our recent research indicates that narrow rows tend to yield more regardless of planting date, and raising seeding rates seldom produces an advantage when planting late. Unless a late-maturing variety (for example, a Group 4 variety in central Illinois) was the first choice, there is no advantage to changing to an earlier variety for late planting.