Cool season viruses in wheat

Currently, most small grains are approaching jointing or just past jointing in many parts of the state.  Now is the time that you most likely will start to see early season viral diseases in some fields, specifically Wheat Soilborne Mosaic Virus and Wheat Spindle Streak Virus. These diseases are transmitted by soil borne microbes that thrive in cool, wet conditions. Infected plants typically are chlorotic and may be stunted. Often affected plants occur in low lying areas of the field or areas suffering from compaction. However, on some occasions entire fields can become symptomatic. Symptoms of spindle streak include necrotic dashes that run along the venation, giving the appearance of a spindle (Figure 1). Additional symptoms of soilborne mosaic virus are less conspicuous, but include mottling of lower foliage (Figure 2). Symptoms, including chlorosis and even stunting can look very similar to nutirent deficiencies.  Symptoms cease once temperatures are above 65°F and may be reduced after fertilization.  Even though symptoms may be reduced, it does not mean the viruses and their effects are removed from the crop.  I consider these diseases hidden yield robbers due to the fact that they often go misdiagnosed as nutrient issues and their effects often go unnoticed.    Confirmation can only be made through specialized testing methods such as ELISA and PCR.  The University of Illinois Clinic and Agdia are two options for having samples tested.

 

How should you send a virus sample?  The key for getting good virus results is to keep the sample cool and avoid excess moisture.  If the samples get warm or are kept too wet microbes can degrade the viruses, resulting in false negative results.    Place 15-20 leaves in a plastic bag with a dry paper towel and place immediately in a cooler on ice.  Make sure to ship samples overnight on ice early in the week so that they are processed immediately and not allowed to set at room temperature.  Our lab is conducting a wheat virus survey in 2019 and we are seeking people to help us sample fields.  If you are interested in helping, send me an email .

Figure 1. Symptoms of wheat spindle streak virus in wheat.

Figure 2. Symptoms of wheat soilborne mosiac virus.

What should you keep in mind? 1) Keep track of fields with these viruses. Once the viruses are established they will be present in those fields from here on out.  2) Try to harvest or work in these fields last to prevent spread to other fields. 3) Avoid compaction. 4) Plant tolerant varieties in fields with a history of these viruses. Unfortunately many varieties are screened for these viruses as a complex, so it may be difficult to determine if you are planting a spindle streak or soilborne mosaic tolerant variety in these cases.  However, the University of Illinois is one of the few places that does conduct screening for these viruses.  More information on wheat varieties for Illinois can be found by clicking here.


Early season diseases in soybeans

Now that the soils are warming, some producers are discussing planting soybeans in the ground.  When considering early planting of soybeans, there are two diseases that should be considered: 1) Sudden death syndrome (SDS)  and 2) Pythium root rot (PRR)

Both SDS and PRR are favored by cool, wet weather.  In the case of SDS, early season infections can reduce stands, and also result in colonization of root systems.  The SDS pathogen remains in the lower portion of the stem and roots until the the plant reaches the reproductive stages.  Heavy, alternating rains during reproduction can cause the fungus to more aggressively colonize the plants, as well as produce toxins, which can cause defoliation, wilting, and reduced yields.  If considering early planting into fields with a history of SDS, ensure that you select a cultivar with excellent SDS resistance and consider an SDS-seed treatment if it fits your production practices.

PRR is actually a complex of Pythium species that each have their own unique characteristics.  It is now understood that individual Pythium species and even isolates within species can differ significantly in their optimal temperature for infecting seedlings.  Regardless of temperature, if the growth of your soybeans is reduced due soil water saturation or cool conditions, you may see increased stand issues.  There are specific seed treatments that can be effective for suppressing Pythium.  However, it is important to realize that these treatments provide a window of protection that is intended to protect the emerging seedling and allow it to establish.  This window typically is 2-3 weeks.  Seed treatments will not protect a submerged seed from dying due to flooding, and will not provide protection after than window of protection is reached.  Remember- seed treatments are not fumigants- they are short-term, protective barriers.  Tile can be a great investment in fields prone to flooding and subsequent PRR issues.


Remember to check your hybrids for tar spot ratings, scout your fields

It is that time of year again.  Soon corn will be in the ground, and the 2019 field season will be taking off.  It is no surprise that I spent the majority of my time on the speaker circuit discussing tar spot in corn.  We have learned a fair amount since then, but there are many more things that need to be researched and learned before we have excellent tar spot IPM management programs.  However, there are a few points you should keep in mind this season that can help you determine your risk for tar spot and management practices that can help your bottom line.

The incidence of tar spot was fairly widespread last year.  Incidence is simply asking the question, “Do I have any tar spot in my field?”  Incidence does not incorporate the severity of infection.  One could have a field with a high incidence of tar spot, yet the severity (number of lesions on leaves of plants) could be low.  This link shows the tar spot incidence in 2018 :Tar Spot established in the United States-2018

If we were to estimate where the greatest severity of disease was last year, it likely tracked with the late season storms that pushed through the region in August and September.  In Illinois, severity was greatest in the region North of I-90, and most severe in the north central part of the state.  Increased severity likely means increased local inoculum for this season.  If you are planting corn in a region that was hit hard by tar spot last season, your risk for disease is elevated compared to areas where disease was sparse or absent.

The fungus that causes tar spot overwinters in residue, and spores are released from the stromata (raised black spots on foliage, stalks, husks) at night during periods of moderate, humid weather.  These spores spread locally and also can move at a minimum to nearby fields on rain and wind.  If you are planting into a field of corn residue from plants that were severely affected by tar spot, you may be at increased risk for disease compared to if you are following a field that was in soybean last year or is tilled.  That does not mean tar spot will not occur, as it can spread from nearby fields; however, planting after soybeans or tilled fields may reduce local inoculum levels, reducing disease onset and potentially severity.  The later the disease starts, the less impact it is likely to have on your crop.

All commercially available hybrids are susceptible to tar spot, but some hybrids are more tolerant than others.  No particular brand is better than another.  Ask your seed dealer or check out Dr. Smith’s website    for information pertaining to specific hybrids and tar spot response.

Tar spot severity by brand. Numbers indicate individual hybrids within a brand. Data from DeKalb OVT, 2018. Rated at R5/6

 

Scouting is critical for this disease.  CCA’s and producers should ensure that fields are being scouted frequently and often, especially in the days/weeks approaching tasseling.  If you notice tar spot showing up prior to VT, a fungicide may help.  There are several products with a label or 2ee for tar spot suppression.  Like rusts, this is an obligate fungus, and you want to ensure that the ear leaf and leaves above are protected during the critical periods of grain fill.  You do not want to chase this disease-revenge sprays will not work.

Lastly, although tar spot is the hot topic, our most severe and widespread disease last year was, without a doubt, grey leaf spot.  Do not lose sight of this disease and other diseases that are observed and encountered more frequently and consistently in Illinois.  Tar spot is likely to be episodic, much like Fusarium head blight in wheat and white mold in soybeans.  It may be a while before we see significant disease as we did in 2018 (I hope this is the case).


Register now for Tar spot Webinar, March 1

3/1/19 at 9:00 am CST

 

 

Join Dr. Nathan Kleczewski from the University of Illinois Extension  for an update on Tar spot in corn.  This disease was first observed in the United States in 2015 in Northern Illinois and Indiana.  In 2018, the disease significantly affected corn production in the Midwest and Florida.  What is tar spot of corn?  How does it work?  What is our current understanding of this disease and its management?  These and other questions will be addressed through this free webinar.

 

Registration is free, but capped at 100 participants.

 

To register for this meeting, click the following link: https://web.extension.illinois.edu/registration/?RegistrationID=19924

 

We look forward to seeing you March first!


What effect will cold temperatures have on pests and pathogens?

Nathan Kleczewski Research assistant Professor and Extension Field Crop Pathologist

Nick Seiter- Research Assistant Professor and Extension Field Crop Entomologist

 

Many in the Illinois agricultural community are wondering what effects the recent extreme cold might have on pests and pathogens. While it would be nice if the cold temperatures we are experiencing could help to reduce our potential for pest damage, past experience tells us that the most serious pests we deal with are unlikely to be impacted much by these conditions.

Many of the pathogens and insect pests that commonly affect field crops in Illinois are well adapted to survive our winter conditions.  In many cases, pathogens produce recalcitrant survival structures (e.g. cysts in soybean cyst nematode, oospores in Phytophthora, sclerotia in white mold).  These structures allow the pathogen to survive extreme conditions including cold, drought, and flooding. Different species of insects overwinter in different life stages, including eggs (for example, western corn rootworm), larvae (Japanese beetles), pupae (corn earworm, though they do not survive the winter in most of Illinois), or adults (stink bugs). The overwintering stage has characteristics that help these insects to survive the winter, either by adjusting its physiology to better survive the cold, seeking out an overwintering site that protects it (such as soil, tree bark, or leaf litter), or both. The overwintering sites that insects find mean that they are not experiencing the same temperatures that we are when we venture outside. Wind chill has little effect for this reason (even though it has a major, unpleasant effect on us).

Extreme cold temperatures can impact some insects and plant pathogens, particularly those that may not overwinter as well (e.g. powdery mildew).  When cold weather pushes into the Southern regions of the country it can push certain diseases, such as rusts, further south, delaying disease onset in Illinois and other regions further north. The same is true of migratory insects, such as black cutworm and fall armyworm, which do not usually overwinter in Illinois; colder temperatures during winter often delay the arrival of these insects, and may ultimately lead to lower numbers. The opposite is also true – warmer than normal temperatures during the winter can allow these migratory insects to become a problem earlier in the season.

Although cold temperatures may not impact most of the diseases we encounter in Illinois field crops, fluctuation between conditions of cold and warm may have a negative impact on some diseases.  Dormancy by fungi can be broken by environmental conditions such as higher temperatures.  This is similar to what occurs in plants, where warm weather may result in trees flushing out buds and flowers.  Consequently, the wide swings in temperature that we have experienced during the 2018/19 winter may negatively impact some diseases. While some insects (such as stink bugs) can also break dormancy during brief warm periods, many of our most serious pests will stay “hunkered down” until the spring and avoid these fluctuations. Unfortunately, insects and plant diseases are unlikely to suffer as much from the recent cold as we have. The best way to reduce the impact of insects and pathogens on those cold days is to stay inside, grab a hot cup of coffee, and curl up to the latest UI Extension recommendations or UI applied research results guide.


New Tar Spot Publication Available

A new publication on Tar spot of corn is available through the Crop Protection Network.  In this publication we have summarized our current knowledge of the disease, it’s impacts, as well as presented management suggestions.  You can access the link to the library by clicking here.

In addition, I will be presenting a webinar on Tar spot at the end of February.  Pay attention to the Bulletin and my Field Crop Disease blog for additional details in the near future.

If you are not familiar with the Crop Protection Network, it is a great resource for disease and pest management information, plus it is free for use by the agricultural community.


Soybean Quality Issues in 2019

I have had several conversations with members of the agricultural community regarding seed quality resulting from issues derived from delayed harvest and persistent wet conditions encountered in many parts of Illinois in 2018.  For more information on what seed producers and soybean farmers should keep in mind going into this season, see my new article published on the Illinois Field Crop Disease blog, found by clicking here.