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5/17/2010 - 5/23/2010

2010 Soybean Rust Update

By Daren Mueller, Department of Plant Pathology

This could be the world’s shortest ICM News article. It could simply read “soybean rust, meh”. But I will take some time to explain.

We have identified three critical steps that must happen for rust to get to Iowa. They are (1) to survive winters somewhere in the south, (2) build up inoculum (spores) where survival occurs and (3) movement of these spores to fields further north and successful infection of soybeans in those fields. These steps may need to reoccur several times for rust to get to Iowa.

In years past, soybean rust has survived in several states across the southern U.S. Droughts and other obstacles have prevented the overwintered spores from building up and moving north until late in the season. This past winter was a different story. While we are all too familiar with how much snow and cold weather we had here in Iowa. Freezing temperatures crept further south than normal. The main overwinter source of soybean rust, kudzu, was killed back in most places in the south. As a result, there have been NO known locations in the U.S. where soybean rust survived the winter. Kudzu has leafed out and soybeans are growing, but there still have been no known soybean rust finds in the U.S. to date.

 

Distribution of soybean rust in late May 2009 (top)  and May 2010 (bottom). Note no known sources of soybean rust in the U.S. in 2010.

 

During the 2010 growing season, we will have five sentinel plots scattered across Iowa. These will be coupled with our fungicide trials at the Northeast Farm (Nashua), Northeast Farm (Sutherland), Southeast Farm (Crawfordsville), Armstrong Farm (Lewis), and Curtiss Farm (Ames). We have a network in place to increase the number of mobile sentinel plots if the risk of soybean rust increases. We also will continue to use X.B. Yang’s predictive model to assess the risk of soybean rust getting to Iowa. According to early results from his predictive model, the risk of rust getting to Iowa is the lowest it has been since 2005, which was the first full year of soybean rust being in the U.S.

Last Section 18 fungicide now gone
Flutriafol (Topguard, Cheminova) was the last of the Section 18 soybean rust fungicides to expire. While the Section 18 label has expired, Topguard received a federal registration on soybeans in late April. Cheminova is working on the state registrations for this product and hopes to have product in the field by June. Topguard is in the triazole class of fungicides and has soybean rust, frogeye leaf spot, Cercospora leaf blight, brown spot and powdery mildew on its label.

Seasonal updates on soybean rust
As a reminder, you can view short, frequent updates on soybean rust on the ISU Soybean Rust webpage. You can also sign up to receive these updates through email. Information on how to sign up for these email updates also can be found on the webpage.

 

Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative. Mueller can be reached at (515) 460-8000 or by email at dsmuelle@iastate.edu.

Relationship of Nitrogen Conversion in Soil to N Loss

By John Sawyer, Department of Agronomy
 
Nitrogen processing in soil
If applied N or mineralized organic matter N (conversion from organic to ammonium) would stay in the ammonium (NH4+) form, then losses would not occur because ammonium attaches to soil and does not leach (move through the soil with water) or denitrify (microbial conversion to N gases when soils become saturated). Unfortunately, that isn’t the way it works. Ammonium is converted to nitrate (NO3-) via nitrification. Nitrate is the form that can be moved out of the soil profile by leaching or lost by denitrification. The conversion of ammonium to nitrate and the conversion of nitrate to N gases are both microbial processes. Hence, potential N loss is dependent upon factors that influence each – for nitrification soil temperature is very important (faster with warm soils, slower with cold soils); for denitrification soil temperature, soil moisture (only occurs when soils are saturated – anaerobic conditions) and readily available organic matter for an energy source. If fertilizer N is applied in the nitrate form, then that N is immediately subject to these loss pathways. Mineralization does occur when soils are saturated, so ammonium can accumulate in flooded soil and add to crop available N.

Potential for N losses
Greater losses occur when soils enter the spring season with recharged subsoil moisture, when more N is in the nitrate form, and when soils are warm. Deciding if losses are substantial enough to warrant supplemental N application, the following factors should be considered:

(1) amount of nitrate present, which is affected by time of N application, form of N applied, rate applied, and use of a nitrification inhibitor
(2) when and the length of time soils are saturated
(3) subsoil recharge, leaching rate, and drainage – water amount moved through the soil
(4) loss of crop yield potential from water damage

Leaching and denitrification are not uniform across the landscape. Thus, the potential for N loss is variable and difficult to predict. For example, with high intensity rains, runoff occurs and not all of the water soaks into the soil. Instead, water in excess of infiltration moves to the lower landscape where it may form ponds or spill over stream banks into floodplains.

If an N source was applied in the spring that has more rapid nitrification than anhydrous ammonia (urea, ammonium sulfate, ammonium in manure) or contains part of the N in the nitrate form (ammonium nitrate or UAN solutions), then conversion to nitrate would be faster. Conversely, if an ammonium-containing fertilizer (anhydrous ammonia, urea or ammonium sulfate) or manure was applied shortly before a wet period, then loss would be negligible because little nitrification to nitrate would have occurred because nitrification does not occur in saturated soils and will not resume until soils dry and become aerobic.

Conversion to nitrate does not equal loss; it just means the N is susceptible to loss. Rapid and large losses occur only with excess leaching (predominant concern with sandy/coarse-textured soils) or with saturated soils (predominant concern with heavier textured, poorly drained soils). For more on this topic see, Wet Soil Conditions and Nitrogen Loss.

 

John Sawyer is a professor of agronomy with research and extension responsibilities in soil fertility and nutrient management.

Wet Soil Conditions and Nitrogen Loss

By John Sawyer, Department of Agronomy

South and southeast Iowa have experienced much above normal precipitation and continued wet soils early  this spring. See the figures below for the precipitation and departure from normal for the April 18-May 17, 2010 period. What does this mean for nitrogen (N) loss, and is this a repeat of the last two years for that area of Iowa? While it has been wet, it has also been cooler than normal (see figure below). That is important because cool temperatures slow nitrification and denitrification, both biological processes important for potential N loss. While the wet period does not bode well for retaining N, it may not be a repeat of the last two years as those were continued wet late in the spring and into early summer. Time will tell if the wet conditions persist this year and promote N loss.

Many soils in south-southeast Iowa have poor internal drainage. This means denitrification is a major N loss pathway with wet conditions. There are also areas of coarse-textured sandy soils with high internal drainage and high percolation rates. Those soils have leaching as the predominant loss pathway. In those soils, N in the nitrate form is easily leached with high rainfall events and fertilizer applied in the nitrate form or converted to nitrate could be lost already this spring. This is why split spring preplant or at planting N and sidedress application is an important management practice for those soils. That management can be helpful for poorly drained soils as well.

Nitrogen applications most at risk from the early spring wet conditions are early fall manure containing high percentage as ammonium (liquid swine manure), early fall DAP/MAP, early fall anhydrous ammonia, spring ammonium nitrate and spring urea-ammonium nitrate (UAN 28 or 32 percent) solutions. Why these applications? With early fall application comes the conversion to nitrate by early spring and with the spring application the materials already contain nitrate and faster nitrification. With the late harvest last fall, there was less fall N application than usual, which will help.

If the corn crop is severely damaged by excess water, application of N will not overcome that damage or lost yield potential. So, additional N would not be warranted. If you are concerned about N loss, and since the corn is still small, make two or three strip applications of additional N across representative fields and watch the corn growth and color. If there is noticeable improvement, then additional N could be warranted. Give the corn plenty of time to recover and roots to grow into N that may be deeper in the rooting zone (due to nitrate movement downward). The strips and adjoining non-treated corn can be harvested and yield compared to see the yield response to the additional N application. The ISU Extension publication PM 2026, Sensing Nitrogen Stress in Corn  describes methods to implement these N strips and a method to compare the corn N status with a handheld SPAD chlorophyll meter. Canopy sensors are also available that can be used to determine corn N status at mid-vegetative growth stages. For use of these sensors, there needs to be reference areas – corn that does not exhibit N stress (non-limiting N); hence strips applied now with additional N can be used for reference areas across fields.

For additional information on N conversion in soil and its relationship to N loss, see Nitrogen Conversion in Soil Related to N Loss.

 

John Sawyer is a professor of agronomy with research and extension responsibilities in soil fertility and nutrient management.

Early Season Corn Nematode Scouting

By Greg Tylka, Department of Plant Pathology

Plant-parasitic nematodes can be a pest concern for corn. Current corn nematode management options include seed-treatment and soil-applied nematicides; both are implemented at the time of planting. There are no effective management strategies to use after the corn crop has been planted, however effective scouting is the foundation to a successful, integrated nematode management program. 

It is common to find low numbers of many species of plant-parasitic nematodes in soil of most any Iowa corn field. But damage only occurs when nematode numbers increase to high levels.

Symptoms of nematode damage to corn include stunting, yellowing of foliage and mid-day wilting; some nematode species also cause swollen and stubby roots. Symptoms will be apparent if nematode feeding is causing damage to the corn plants. But the symptoms of nematode damage are not unique and could be caused by many other factors. To determine if nematodes are damaging a corn crop, a soil and root sample should be collected from areas of a field where symptoms are observed.

Stubby root symptom caused by needle nematode feeding on corn near Muscatine, Iowa (Photo Tom Hillyer).

Fields with sandy soils can be infested with the two most damaging species of plant-parasitic nematodes that feed on corn – the needle nematode and the sting nematode. These two nematodes can exist only in soils with at least 70 percent sand content. The number of needle and sting nematodes needed to damage corn is as few as one nematode per half-cup of soil.

The needle and sting nematodes are found in soil samples in the spring and fall of each growing season, but are often not detected in samples collected in the middle of the growing season. Because these two nematodes are damaging at such low numbers, sandy fields may exhibit symptoms of needle and sting nematode damage in the first few weeks of the growing season and samples should be collected at that time.

Samples should consist of 10 to 20 12-inch-deep soil cores taken from around corn seedlings showing symptoms of damage. Also collect a few corn seedlings that are exhibiting symptoms. The soil and seedling samples can be sent to the Iowa State University Plant and Insect Diagnostic Clinic for processing; the test is called a complete nematode count. A past ICM News article has more information about collecting spring samples for needle and sting nematodes on corn.

For all other plant-parasitic nematodes that feed on corn, symptoms of damage are more likely to show up in the field through the middle part of the growing season. Soil and root samples should be collected when symptoms are observed in the middle of the growing season.

 

Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes.

Can Tank Mixing Fungicide with Post-Emergence Herbicide Increase Yield?

Alison Robertson, Department of Plant Pathology

The early application of foliar fungicides to corn (V4-V7) is currently being encouraged by chemical companies across the corn belt. The idea is to tank-mix the fungicide with the post-emergence herbicide application and save on application costs. Furthermore, for additional cost savings, half or lower rates of fungicides are being marketed. It is my understanding that this early application of fungicide will not necessarily replace the VT/R1 application of foliar fungicide, rather it is an addition to the tassel/silking application.

Since the primary action of a fungicide is to control disease, what diseases might be targeted with this early application? Usually the only foliar disease present at V4-V7 is anthracnose leaf blight.  Symptoms of this disease are more common in corn-following-corn fields and can be seen as early as V2-V3 on the lowest leaves of the corn plant. Thus, a V4-V7 application of a fungicide would be too late to completely control this disease. Once corn starts growing rapidly, it usually outgrows the disease. There have been suggestions that leaf blight could increase the risk of anthracnose stalk rot later in the season. Preliminary data from trials done in Iowa in the past three years indicate there is no relationship between anthracnose leaf blight and stalk rot.

Another early season foliar disease that can occur is eyespot. Eyespot is favored by cool, wet conditions, so in normal growing seasons this disease does not progress and become the problem it was in north and central Iowa in 2009.

Inoculum of other common foliar disease (gray leaf spot, common rust and northern corn leaf blight) may be present in corn fields at this time, and yes, an application of fungicide at V4-V7 would certainly prevent infection on the leaves exposed to the fungicide.  Remember, however, that new leaves, that have not been exposed to the fungicide, are emerging every 3 to 4 days (depending on the weather), and inoculum of foliar diseases is being produced 24/7.  Fungicides are not translocated from one leaf to another. Thus, early season infection of unprotected leaves is still possible, and may still lead to epidemic levels of disease later in the growing season.

Limited research has been done to support the practice of early applications of fungicides to corn. In 2009, we evaluated applications of Headline at early growth stages at two locations in Iowa.  At Crawfordsville, we looked at the effect of Headline alone or in combination with Headline AMP at various growth stages on disease severity, lodging, yield and moisture (Table 1). In terms of yield, we saw little justification for the use of an application of foliar fungicide at V6, either alone or in combination with a VT/R1 application.


 

At the second trial in Ames, the effect of Headline (6 oz/A) at multiple timings on disease severity, lodging yield and grain moisture was assessed (Table 2).  Similarly, there was little justification, in terms of yield, for early applications of fungicides on corn.   


 

Carl Bradley recently summarized the few replicated research trials done in five Midwestern states that evaluated the effect of early applications of foliar fungicide on corn yields. His summary includes data from the two Iowa sites.

This growing season we will, once more, be partnering with outlying ISU research farms to evaluate the effect of various foliar fungicide applications on corn. This year we will incorporate the V6 application timing in these trials.

 

 

Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Robertson may be reached at (515) 294-6708 or by email at alisonr@iastate.edu.

Crop Minute for Week Beginning May 17

Roger Elmore, ISU Extension corn agronomist, reports most corn is recovering well from frost  a week ago and the concern this week turns to southeast Iowa and fields with standing water. Once the water has gone down, he encourages producers to assess stands and take stand counts; he tells what to look for when making decisions about replanting.

Listen to an mp3 file of the  May 17 Crop Minute.



This article was published originally on 5/24/2010 The information contained within the article may or may not be up to date depending on when you are accessing the information.


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