Bacterial leaf stripe is a disease that can usually be found on wheat in the Red River Valley (RRV) later as crop growth stages progress.  The disease (caused by a Xanthomonas sp.) can develop and become severe rapidly after the crop reaches the heading growth stage.  Bacterial leaf stripe (BLS) can cause significant yield losses on some varieties.  Like other disease issues, development is dependent on weather conditions and the presence of susceptible plant hosts.  Epidemics of BLS occurred in the RRV during 2005 and again in 2008.

Symptoms.  Bacterial leaf stripe symptoms appear after the crop has reached the heading growth stage. Plant leaves show longitudinal striping, and/or blotchy yellow or brown lesions (see image below).  During periods with leaf wetness, lesions and plant tissues surrounding them, appear water-soaked and feel slimy if touched.  When plant tissues are dry and humidity is low, the same leaves will have a shiny appearance.  Leaves look glazed as if they were frosted with a thin sugary glazing, similar to the glazing on a donut.  In this case, however, the glazing consists of millions of dry bacterial cells that are awaiting transport to another leaf or plant.

If flag leaves are severely diseased, yield losses can result.  Maintaining the functioning photosynthetic area of flag leaves is important in preserving yield and test weight potentials. 

Spread.  Bacteria are transferred from one leaf to another during periods of leaf wetness.  Wind provides leaf movement which allows localized spread of bacteria from plant to plant.  Because the pathogen is spread through contact with diseased plants, fields may have initial “hot spots” or patterns of diseased plants that run parallel with wind direction.  Bacteria are also known to be spread by plant-visiting insects.  Bacteria can survive in soil organic matter for an undetermined period of time and on (or within) seed.

Management.  Application of fungicides is not recommended.  While fungicides are often applied to control diseases caused by fungi (e.g.: Fusarium head blight, tan spot), they have no activity against bacteria.  Identification of spring wheat varieties with BLS resistance is our best means of defense against loss.  Currently, little is known on this topic.

Image above: Bacterial leaf stripe symptoms on wheat

The hot windy weather this past weekend may have caused some wind burn damage to just emerging leaf tips.  The symptomology looks a lot like the damage of contact herbicide or fertilizer burn.  However, the damage is just limited to the newest, just emerging, leaf tips (Photo 1). Little if anything is reported in the literature whether this type of damage causes any yield losses.  Something analogous, however, may shed some light on the question whether yield losses should be anticipated – in studies where the whole or portions of the flag leaves were cut off with scissors, yield losses were limited to less than 15% when the whole flag leaf was removed.  This indicates that the lower canopy can partially, if not completely, compensate for the loss of photosynthetic activity of the flag leaf.

Photo 1 - Damaged leaf tips as a result of hot, windy weather

Usually, in some educational setting, soil temperature is listed when someone lists the various factors that affect nutrient uptake.  In most instances, however, there is no description of how this soil property impacts early crop growth.  Based on recent e-mails and telephone calls, soil temperature had a major impact on crop growth in some parts of Minnesota in May and early June.  In general, May and early June of this year can be characterized as cool, if not cool and dry. So, what effect did these cool/dry  conditions have on early crop growth?  Although the recent days of warm and humid weather have stimulated crop growth, this is a good time to review the impact and importance of soil temperature on nutrient availability, nutrient absorption and crop growth.

From the crop growth perspective, soil temperature has a direct impact on root growth and development. Corn, for example, can germinate at soil temperatures of 50 degrees F.  But, roots are slow to develop at this temperature and growth of the secondary root system is slowed.  To be absorbed by plants, the immobile nutrients (P,K, micronutrients) must either move to the root via a process called diffusion or the root must grow to and intercept the nutrient.  Thus uptake of the immobile nutrients is directly affected by soil temperature primarily because of the impact on root growth.  Except for the effect on the process called mineralization, availability is not affected by soil temperature.  The following table  illustrates the effect of soil temperature on phosphorus uptake.

Phosphorus uptake by corn as affected by soil temperature.

  lb. P2O5/acre                             —————–milligrams P/pot——————————

          35                                          3.5                             10.4                         18.0

The data were collected in a greenhouse experiment where temperature in the soil could be easily controlled.  Phosphorus uptake by corn was reported as milligrams per pot.  The actual numbers are not importrant. The relative differences are. The uptake measurements were taken 5 weeks after corn emergence.

At each rate of applied phosphate, phosphorus uptake increased as soil temperature increased.  With a higher probability of cool temperatures early in the growing season in Minnesota, it’s important to use management practices that maximize uptake of immobile nutrients. Use of banded fertilizer near the seed at planting is the major or most important management practices.

This spring, corn in some fields showed symptoms of phosphorus deficiency even though soil tests for P were medium to high.  At these soil test values, corn usually does not exhibit symptoms of phosphorus deficiency.  In most cases, the problem could be traced to cool soil temperatures.  There were peat soils and other other situations where corn was planted where there was a lot of residue from the preceeding crop. Soil temperature usually remains low in these situations early in the growing season.  Therefore P uptake would be reduced and there could be symptoms of P deficiency.

Soil temperature has a major effect on the breakdown or decomposition of soil organic matter. This process is usually referred to as mineralization.  This organic component of the soil system is a major reservoir for phosphorus, sulfur, and nitrogen.  For example,  approximately 90% of the total amount of sulfur in soil is found in the organic matter.  So, if decomposition (mineralization) is slowed, the ready availability of some nutrients necessary for crop growth can be restricted early in the growing season.

For example, there was one call this spring reporting that corn growing on soil with a high organic matter content was showing symptoms of sulfur deficiency.  These symptoms appeared when corn followed corn; but not when corn followed a soybean crop.  With a chisel plow used for the primary tillage more residue remained on the soil surface when corn followed corn this spring, therefore, delaying the decomposition of the organic matter.  Because of this, availability of sulfur may have been limited for a short period of time.  But, with the warmer temperatures, mineralization should increase, sulfur should become more available,and the sulfur deficiency symptoms should disappear without any subsequent reduction in corn yield.

Organic matter, of course, is a major source of nitrogen needed for crop production.  There is no question.  The release of nitrogen from organic matter is highly dependent on both soil moisture and soil temperature.  Researchers have worked for many years in an attempt to predict nitrogen release to the nitrate form if soil soil temperature and soil moisture content are known.  It is possible to predict this release in certain well defined and controlled conditions.  When looking at the big picture, however, there is no general agreement as to what this mathmatical relationship should be.  So, the research goes on.  If it were possible to measure soil moisture and soil temperature and predict the amount of nitrate-nitrogen released into the soil system, current N fertilizer guidelines could be fine tuned and there could be more precision in the use of N fertilizers.

Although it may not be possible to completely overcome the negative impacts of a cool season on crop growth and development, there are some management practices that can be used to keep the negative impacts of soil temperature to a minimum.

Removal of residue above the row and the use of a banded fertilizer at planting are two practices that can have positive effects when soil temperatures are low.  A residue free zone that is 10″ to 14″ wide over the row can stimulate germination and early growth.  This is easy to achieve when corn follows a soybean crop or sugarbeet crop and some primary tillage has been used.  Good residue removers on the planter are essential to maintain this zone if corn follows corn.

Use of a banded fertilizer placed near the seed at planting is the second practice that helps young plants to overcome low soil temperatures early in the growing season.  This fertilizer placement stimulates eartly root development as well as placing essential nutrients very near the seed. Use of these two practices does not guarantee that low soil temperatures will not have a negative effect on crop production.  These practices, however, improve the odds for improved production.

The decision of whether to spray at an early flower growth stage to manage Fusarium head blight (FHB, scab) is one that should’ve already been made for those farming south of the Red River Valley.  Knowing the (1) forecasted weather conditions and disease development before flowering, (2) type of crop residue present, and (3) disease resistance level of the variety grown are all key factors to consider when making an informed decision about applying a fungicide.

“I applied a fungicide during flowering.”   If the crop was sprayed with a triazole-based active ingredient at early flower, plants will continue to maintain a level of protection against fungal infection for roughly ten days afterwards - depending on the product and rate used, as well as general plant health.  This protection arises from the fungicide’s residual activity within plant tissues, and is naturally greater the first day after application compared to the ninth.  Wheat fields receiving an application of fungicide at early heading have the added benefit of being protected against leaf rust (Fig 1) and other fungal leaf spotting diseases.  Rust can disrupt flag leaf tissue processes during the grain filling period and may result in reduced yields or smaller, lighter kernels. 

“The environment didn’t support scab, so I didn’t spray.”    Rust has recently been detected in Minnesota and North Dakota at trace levels.  While the disease has been slow to show up in our area, our northern-located crops are behind their average growth stages for this time of year.  This is a situation that could allow more severe disease to develop before plants mature.  If the crop wasn’t sprayed for FHB management and rust is already established in the lower plant canopy, disease severity could worsen during the grain development growth stages.  However, during 2009, winter wheat growers in southern states had fewer issues with rust than with FHB.  FHB has reduced wheat yields and quality in several winter wheat production states while leaf rust has been slower to establish and less widespread than in previous years. 

“I still have three weeks until harvest, should I spray after the field is done flowering?”   Depending on the triazole product used, fungicide application on wheat is off-label if harvest begins less than 30 days after the application and/or the crop’s growth stage is more advanced than 50% flower.  Last year, fungicide applications on wheat in Kansas triggered a widespread investigation and regulatory action.  Additional information about the incident can be found at http://www.extension.umn.edu/cropenews/2008/08MNCN16.html.  The moral of this story is to always read and follow pesticide label directions. 

In summary, the last major disease control decision comes before the early flower growth stage.  This is the time to evaluate weather conditions and disease forecasting information, and then to determine a sound economic managment strategy for protecting the crop, if needed.

Fig 1.  Leaf rust on a maturing wheat leaf.

I want to plug an event coming up next month in Des Moines: it’s the Drainage Water Management Training Workshop. It will be held on July 14 & 15, 2009 at the Airport Holiday Inn. If you already work with agricultural drainage this workshop will give you more information on the design considerations of drainage water management. You’ll have a chance to work with three very good instructors, Dr. Matt Helmers of Iowa State and Drs. Gary Sands and Jeff Strock from the University of Minnesota. Day One will focus on the elements of Drainage Water Management (DWM) and Day Two will be a field tour to the Iowa LICA Research Farm. See the attached PDF file for the complete registration information:

http://minnesotafarmguide.com/workshop.pdf

Drainage Water Management - What is it?

Subsurface drainage, also known as “tile” drainage,  is an essential water management practice on many highly productive soils in the Minnesota and throughout the Midwest.  Many of these soils are categorized as somewhat poorly drained, poorly drained or very poorly drained. The natural drainage classes refer to the frequency and duration of wet periods under conditions similar to those under which the soil developed. Characteristically the soil is wet at shallow depths periodically during the growing season. However, nitrate carried in drainage water can lead to water quality problems.  Strategies are needed to reduce the nitrate loads while maintaining adequate drainage for crop production.  Practices that can reduce nitrate loads on tile-drained soils include growing winter forage or cover crops, fine-tuning fertilizer application rates and timing, modifying drainage system design and operation, bioreactors, and treatment wetlands.

So what is drainage water management? Drainage water management is the practice of using a water control structure in the main or the submain drain to raise the drainage outlet to various depths. This allows farmers to have more control over drainage. Another way to say this is rather than let the system drain freely year round, the producer can control the potential height of the water table.

The outlet is: Raised after harvest until early spring to limit drainage outflow and reduce the delivery of nitrate to ditches and streams during the off-season.

In the spring the outlet is lowered a few weeks before planting and harvest to allow the field to drain more fully.

After the spring field operations are completer the outlet is raised to potentially store water for crops.

Much like 2008, the continued cool conditions across much of the State are near ideal for wheat, barley, and oats.  The cool weather favors tillering and the initiation of more spikelets per spike (Figure 2).  So despite the delay in planting the crop has the opportunity to develop two of the three yield components that ultimately determine grain yield under ideal temperatures.

The cool conditions also reduce the impact of the drought that has a firm grip on a portion of the State. Respiration is greatly reduced under cooler conditions, allowing wheat, barley, and oats to be water misers.  Between jointing and heading wheat will use less than 0.1 inch of water per day of temperatures stay below 60^oF while water usage will increase nearly 2.5 fold if daytime temperatures reach 80^oF. 

Figure 1 - The relationship between maximum daily temperature at the 4 to 5.5 leaf stage and the number if spikelets per spike that are initiated (courtesy of Terry Gregoire, NDSU).

The National Weather Service has issued a frost advisory for northern Minnesota, including portions of the Red River Valley. As the warning states, patchy frost is possible and sensitive outdoor plants may be killed if left uncovered.

Spring wheat is likely not going to be permanently damaged by this unusually cold weather. Some of the above ground growth may freeze and die. New leaves, however, should appear within a few days. Since even the earliest planted spring wheat has not reached the jointing stage, the growing point is still below the surface and well protected from any permanent damage.  Crown temperatures need to drop below 28°F  before we should worry about permanent damage.

Winter wheat that has reached jointing may sustain more damage. With the growing point already above ground, the crop is more vulnerable. Temperatures, however, will have to dip below 28°F before the growing point is damaged irreversibly and yield potential is impacted.

As a plant pathologist, I have an interest in all things microbial.  This time of year, when lawns green up and last year’s dead thatch is still visible; it’s easy to see symptoms of fungi called “fairy rings” (Fig. 1).  These circular patterns have dark grass perimeters that generally expand from year to year and can either be managed fairy easily or with considerable difficultly, depending on one’s approach to life.

 It’s all in a name.  The name of this turf grass issue is based on dancing, mythical creatures that can sometimes be imagined in the early dawn’s mist.  As legend has it, dancing fairies (aka pixies) sprinkle magical dust as they twirl and cavort on our lawns.  The dust not only causes the turf on which it falls to grow darker green, it also eventually produces mushrooms, or toadstools, to grow. 

 Myth to science.  Many soil-borne fungi can cause fairy rings.  Fungi expand in roughly circular shapes as they utilize food sources.  This is occurring in our lawns.  Saprophytic fungi are degrading organic matter, such as accumulated thatch, in a systematic fashion as they grow in an outward direction.  Fairy rings begin as small circles and over the years rings become increasingly larger.  If the leading edges of different fungal colonies contact each other, rings can become lopsided or scalloped.  Grass is greener at the leading edge of the colony where organic matter is being degraded by the fungi and previously unavailable nutrients are released to nearby plants.  The inside perimeter of the ring has a dense mat of white fungal mycelia below the soil surface which creates a less supportive growing environment for turf.  Plant roots in this zone generally don’t get enough water or nutrients due to the fungal barrier.  As a result, grass tends to be much less thrifty here, in direct contrast with the darker green grass at the leading edge.  Both symptoms work to enhance the visibility of the fairy ring.  Most fairy ring fungi produce mushrooms that can be poisonous.  Interested in additional information?  An in-depth article from Colorado State University can be found at http://www.coopext.colostate.edu/TRA/PLANTS/fairing.html

Different management practices for different folks.  Like many other things in life, fairy ring management can be approached in a number of ways. 

• For the serious turf grass manager with a strong back, all soil extending approximately two feet in front of and two feet behind each fairy ring can be dug to a depth of one foot and disposed of.  Non-infested soil should be placed into the hole and turf grass reestablished.
• For the less serious homeowner, ring symptoms can be hidden rather than removed.  Nitrogen fertilizer can be applied to promote the same dark green color across the entire lawn that is likely occurring at the leading edge of the fairy ring.  Lawn aeration behind the leading edge will temporarily open the fungal mat, allowing struggling plants in that zone to acquire sufficient water and nutrients.  This approach will promote turf growth and will result in more frequent mowing. 
• For the interested homeowner, this is a common and natural occurrence in most, if not all healthy lawn environments at one time or another.  Grass is fairly resilient and if your lawn is in good health it will recover on its own.  Consider the patterns as an interesting phenomenon and the work of hungry, efficient soil microbes that mean no harm to you, your family, or your pets (as long as you leave the mushrooms alone).  Pull up a lawn chair, put your feet up, and enjoy the scenery.  I think that shape looks like a cloud….

Crop producers are optimistic. Each spring when planting there is always the attitude that this year’s crop will be great and there will not be any problems. Yet, problems still seem to creep into the picture. In the past, solving problems sometimes was a complex process. There were weed, insect, disease and soil fertility issues.  Often, poor or stunted growth was a consequence of one or more of these factors.  With the use of genetic engineered hybrids and varities, poor growth caused by one or more of the factors just mentioned is usually eliminated from consideration.

This year is not different from others. The phone calls and e-mails tell me that there are still problems with crop production.  It’s difficult for me to diagnose a problem in either a telephone conversation or an exchange of e-mails.  Some data or information to work with is always helpful.

When looking at a problem in a field , it’s common logic to want to collect some soil and/or plant samples.  That’s fine.  That’s the correct thinking.  This collection, however, should not be a random process.  A systematic approach is likely to produce more useful information.  I believe,  that when considering a problem that might be related to soil fertility and fertilizer use, three plant and corresponding soil samples are important.  Collection of these samoles is described in the paragraphs that follow.

One set of samples should be collected from the area where the stunting or damage is most severe.  This area is usually quite obvious to anyone who has experience with crop production.  A second set of plant/soil samples should be collected from an area in the field where crop growth appears to be normal without stunting and discoloration.  The third sample is more of a challenge.  This set of samples should be collected from the part or parts of a field where stunting or discoloration is marginal.  Usually, there are places in a field the border the most problematic areas where there is some indication of stunting and/or discoloration.  A comparison of the results of the analysis of all three samples may yield a clue aas to the cause of the problem.

Unless leaching of N is suspected, collection of soil samples from a depth of 0 to 6 inches is usually satisfactory.  The part of the plant that is sampled varies with the crop and these suggestions are summarized in the paragraphs that follow.

CORN– Early in the season, collect whole plants cut off at the soil surface.  If the plant is between waist high and silking, collect the leaf at the top of the canopy.  This leaf is frequently called the most recently matured leaf.  At silking, collect the leaf opposite and below the emerging silk. Very little useful information is obtained from analysis of corn tissue collected after the silks have turned brown.

SOYBEAN– Throughout the season, collect the most recently matured leaflet.  A sample that consists of 50 leaflets is usually satisfactory for analysis.

SNALL GRAINS– Early in the growing season, collect whole plants. After flag leaf emergence, collect the flag leaf.  The sample should consist of 25 to 30 plants or leaves.

ALFALFA– Collect the top 6 inches of the alfalfa plants.  The sample should consist of 25 to 30 plants.

After the three samples have been analyzed, the challenge is to interpret the results.  If nutrient deficiency is the cause of the problem, the highest concentration of the nutrient of interest in the plant tissue should be in the normal plants with the lowest concentration in the stunted or off-color plants.  Concentration in the borderline plants should be somewhere between the highest and lowest values.

Interpretation of the analysis can be tricky.  There can be situations where affected plants are so stunted that nutrient concentration in these plants is higher than the concentration of the nutrient of interest in the normal plants.  In these situations, attention should turn to the analysis of the soil samples.  As stated before, the collection of both soil and plant samples is a recommended practice.  Interpretation of the analysis of both soil and plant samples will likely help in diagnosis of a nutrient related problem in crop production.

Both plant and soil samples should be sent to the preferred laboratory as rapidly as possible.  If plant samples start to deteriorate, concentration of nutrients in the plant tissue can be seriously affected and analysis, then, becomes meaningless.  It is not a good idea to send plant samples through the mail.

Use of digital photographs can be a big help in solving mysteries.  Photos of normal and affected plants can be compared after they are sent electronically to someone who has experience and expertise in solving production problems. 

There is more than one way to approach the solving of a production mystery.  It’s important thostart when the problem first appears.  Very few problems are solved at the end of the growing season.

Tan spot symptoms are commonly seen on wheat when a susceptible variety is planted into a field with wheat residue.  The fungus that causes tan spot (Pyrenophora tritici-repentis) overwinters on wheat residue resulting in abundant spore production.  Young plants emerging through infested residue become infected during moist springs.  Unfortunately, disease can develop over a wide range of temperatures (68^o-82^oF) as long as adequate moisture is present for a sufficient period (12-24 hours).

Tan spot lesions are oval-shaped and tan, enlarging with age.  Lesions are often, but not always, surrounded by a yellow halo, and can coalesce until large areas of leaf tissues are diseased.  Yield and kernel quality can be affected if the disease is not controlled.

Spring wheat varieties have differing levels of disease resistance.  For example, Blade (2007 WestBred), Breaker (2008 Westbred), Faller (2007 NDSU), and Knudson (2001 AgriPro) have higher resistance levels while Alsen (2000 NDSU), Bigg Red (2004 WestBred), Marshall (1982 MN), Samson (2007 WestBred), and Vantage (2007 WestBred) are fairly susceptible.  More information on resistance levels of other commercially-available varieties can be found online at http://www.maes.umn.edu/09varietaltrials/redspringwheat.pdf

Rotating wheat after broadleaf crops (canola, sunflower, soybean, sugar beet, etc.) will help to prevent early-season disease development.  Planting any crop back into its residue is a tried-and-true production strategy for obtaining subtle, or sometimes not-so-subtle, yield and quality issues.  Whether it’s planting wheat into wheat residue, soybean into soybean residue, corn into corn residue, etc., the outcome relative to diseases is the same – inoculum levels of pathogens are increased with each successive year the crop is planted back into its residue.  Using an effective crop rotation is of paramount importance in managing disease issues in our crops.

Many producers interested in applying a fungicide for early-season disease control prefer to do so at approximately the 4-5 wheat leaf stage, when herbicides are applied.  Both pesticides can be tank-mixed for a one-pass application.  However, in some years phytotoxicity (plant burning or other deleterious crop injury) can occur by tank-mixed partners.  There were some reports of crop injury last year from certain herbicide/fungicide tank mixes.  Check with your local extension educator, agronomist, or product retailer if you’re uncertain whether you should tank-mix, or not. 

Fungicide application to control tan spot of wheat is recommended when the following conditions apply:

1.      A susceptible variety is grown

2.      The crop is planted into wheat residue

3.      Weather conditions remain moist for an extended period

4.      Substantial tan spot lesions are present in the lower canopy

Several fungicide products are labeled for early-season leaf spotting disease control.  Please see Table 1 published online in a similar article for those systemic fungicides that are labeled in Minnesota.  http://nwroc.umn.edu/Cropping_issues/2009/Issue2/05_26_09_no2.htm

Fungicide application should be a last resort disease control strategy since it is more costly than any other integrated pest management (IPM) approach (crop rotation and varietal resistance).  Producers who practice good IPM by using crop rotation and varietal resistance can eliminate much of the risk of a tan spot epidemic developing.