Tar Spot – A “New” Corn Disease

Adopted from: CPN-2012 Corn – Tar Spot, Crop Protection Network

Initial symptoms of tar spot are brownish lesions on the leaves. Black, spore-producing spots appear later, making the leaf feel rough or bumpy. (Purdue Botany and Plant Pathology photo/Kiersten Wise)

Tar spot is a foliar disease of corn that commonly occurs throughout Mexico, Central America, South America, and the Caribbean. The disease was identified in the United States for the first time in 2015 in northern Illinois and Indiana. As of 2018, it has been confirmed in Iowa, Michigan, Wisconsin, Ohio, and Florida.  During the 2018 growing season, the prevalence and severity of the disease increased dramatically, and in some areas tar spot caused substantial yield losses.

In the United States, tar spot of corn is caused by the fungus Phyllachora maydis. The fungus produces small (0.2-0.8 inch), round to semi-circular, raised black structures called stromata.  In severe cases, stromata may also be observed on leaf sheaths and husks.  Tar spot severity on ear leaves at growth stage R5 (dent stage) can exceed 50 percent in susceptible hybrids when conditions are favorable for the disease.

Corn at any developmental stage is susceptible to infec­tion by the tar spot fungus when conditions are favor­able. Disease symptoms have been observed as early as the third-leaf (V3) growth stage in the United States. P. maydis overwinters on infested corn residue on the soil surface, which serves as a source of inoculum for the subsequent growing season. It is not known if P. maydis overwinters on or infects any other plant hosts in the United States.

Conditions that Favor Disease   In Latin America, cool temperatures (60-70°F) and high relative humidity (greater than 75 percent) favor tar spot development. In addition, disease incidence increases when there is at least seven hours of free moisture on the leaves due to rain, fog, or high relative humidity. However, it is not currently known what conditions favor the disease in the United States. In both 2015 and 2018, warm weather and periods of persistent rain and high humidity during the growing season likely favored the development and spread of the disease.

Continuous corn cultivation with minimum tillage practices, and high application rates of nitrogen fertilizer are also positively correlated with increased disease in Latin America. Although corn lines have been identi­fied in Latin America that have resistance to tar spot complex, U.S. observations indicate that most hybrids grown in the North Central region are susceptible to P. maydis (although they differ in susceptibility).

Yield Losses and Impact   Preliminary data from the Midwest indicate that severe tar spot outbreaks can reduce yield by more than 30 bushels per acre. Yield losses are a function of reduced ear weight, poor kernel fill, loose kernels, and vivipary (a condition in which the seed germinates while still on the cob). Observations also suggest that stalk rot and lodging are increased when tar spot severity is high. Severe tar spot also reduces forage quality.

Diagnosis  You can diagnose corn tar spot in the field by examining corn leaves for the presence of black, tar-like spots. To date, tar spot has been observed most often during mid-to late grain fill (growth stages R3-R6) and usually on leaves below or near the ear leaf. You can observe stromata in green and senesced tissues. Occasionally, you may also observe necrotic brown tissue surrounding the black structures, which produces a fisheye appear­ance.

Management  Most of what we know about tar spot has originated from Mexico and Central America. However, differences in the environments, fungal populations, hybrid genetics, and cropping systems may influence disease development in different areas. Our understanding of this disease in the United States is limited because of its very recent history.

However, several management practices may help reduce tar spot development and severity.

  1. Manage residue. Tilling fields buries infected residue and encourages it to decompose, which may help reduce the amount of overwintering tar spot inoculum.
  2. Rotate to other crops. This will allow residue to decompose and reduce the primary It is not yet known how many years it may take to sufficiently reduce inoculum.
  3. Avoid highly susceptible hybrids.
  4. Investigate fungicides. Some fungicides may reduce tar spot, however, we have little data about application timing that will provide an effective and economical response.  Efforts are underway to understand the biology and epidemiology of this disease, which may help formulate fungicide application decisions in the future.

 

What effect will cold temperatures have on pests and pathogens?

Source: the Bulletin, University of Illinois

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 fungican 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.

Soybean Seed Quality Considerations for 2019

Source: Carl Bradley, University of Kentucky; Daren Mueller, Iowa State University; Damon Smith, University of Wisconsin-Madison; Shawn Conley, University of Wisconsin-Madison; and Kiersten Wise, University of Kentucky.

Seed quality from the 2018 soybean harvest was below average across the majority of the soybean-producing areas of the United States. Several factors led to poor-quality soybean seed, but some of the most important were wet conditions throughout the fall and subsequent delayed harvest.

The wet conditions delayed harvest and created the right conditions for several seed diseases, including:

  • Phomopsis seed decay (caused by the fungus Diaporthe longicolla, formerly known as Phomopsis longicolla)
  • Purple seed stain (caused by the fungi Cercospora kikuchii and Cercospora flagellaris)

Seeds affected by Phomopsis seed decay can be cracked, shriveled, and have a chalk-white color on the seed surface.

Seeds affected by Purple seed stain are covered in purple blotches, or the entire seed may be purple.

While some areas harvested high-quality soybean seed in 2018, many U.S. seed suppliers have reported that soybean seed for the 2019 crop is frequently testing positive for the Diaporthe fungus that causes Phomopsis seed decay. This is resulting in lower than normal seed germination rates, and could translate to lower than average germination rates in 2019.

While it is impossible to predict 2019 soybean planting conditions, if soil conditions are wet and cool during planting, then it is likely that both seedling survival and plant population will be diminished in fields planted with low-quality soybean seed. This means farmers need to decide now on how to manage low-quality soybean seed to minimize the impact of poor seed quality, low germination, risk for reduced stands, and lower yield in 2019.

Ways to Minimize the Impact of Low-Quality Soybean Seed Continue reading

American Society of Agronomy Webinar Series on Fusarium Head Blight

A national group of plant pathologists, including Pierce Paul from The Ohio State University, will be presenting a two-part webinar series to help U.S. wheat producers management Fusarium head blight (FHB), also known as head scab or scab. FHB affect wheat, barley and other small grain crops, reducing yield and contaminating grain with mycotoxins such as deoxynivalenol, AKA vomitoxin.

As part of this American Society of Agronomy series, Paul, Carl Bradley, plant pathologist at the University of Kentucky, and Christina Cowger, plant pathologist with the U.S. Department of Agriculture’s Agriculture Research Service, will present and discuss up-to-date research findings on cultural practices, variety resistance, and fungicides for effective management FHB and vomitoxin. The USDA-ARS U.S. Wheat and Barley Scab Initiative, which is sponsoring these webinars, funded much of the research the scientists will be presenting.

The webinars are at 11 a.m. CST on Monday Feb. 11 and Monday Feb. 18. Register for free at https://tinyurl.com/ycmvel4p.

Extensive Spread of Corn Toxin Could Affect 2019 Crop

A wetter than normal summer and fall in Ohio led to the worst spread of a toxin on corn in at least a decade, according to a grain disease expert with The Ohio State University.

And next year’s crop may be at risk as well. The fungus that produces the toxin can survive the winter, particularly if stalks or other plant material from the 2018 corn crop are left on the surface of the soil, said Pierce Paul, an Ohio State University Extension specialist in corn and small grain diseases. OSU Extension is the outreach arm of the College of Food, Agricultural, and Environmental Sciences (CFAES).

The extent of vomitoxin across Ohio and the rest of the Corn Belt led some farmers to receive a lower price for their crop, Paul said.

High moisture levels spur the spread of vomitoxin, which can cause people and animals to get sick. The rainy summer and fall in the state and across the Midwest not only left more moisture in fields, but also delayed some farmers from harvesting.

And any corn left standing in wet fields becomes more susceptible to vomitoxin, Paul said.

Gibberella ear rot, a fungal disease that produces vomitoxin, also sucks nutrients out of corn, leading to smaller and lighter kernels, which can reduce yields and what farmers earn for the grain.

“I know there were farmers who had problems with price discounts, and some had their grain completely rejected,” Paul said.

Vomitoxin can cause animals, particularly pigs, to vomit or simply refuse to eat the tainted corn. If contaminated grain or grain products are consumed, this toxin can also make people ill, which is why the U.S. Food and Drug Administration has set strict limits on the amount of vomitoxin allowed in grain for human and animal consumption.

Moldy corn still can be used to produce ethanol. But the byproduct of ethanol production, typically a rich source of nutrients for animals, cannot be given to them because it will have a high concentration of vomitoxin, Paul said.

Vomitoxin can also contaminate wheat and barley. However, in Ohio, both of these crops were harvested by the first few weeks of July and were out of the fields before the persistent rains came, Paul said.

Not every cornfield had a problem with vomitoxin, because rainfall amounts are never uniform across the state.

The fields that were tainted with vomitoxin could still be a problem next season if the same or another susceptible hybrid is planted, Paul said.

Gibberella ear rot can survive in a field through winter and potentially harm the new crop if wet weather occurs, and “there’s nothing you can do after the fact” to control the disease, Paul said.

As a result, it’s important for farmers to choose corn seed that’s resistant to the fungus, he said. No corn hybrid is totally immune to Gibberella ear rot.

So, buying a hybrid that resists the disease is akin to people getting a flu shot. The hybrid does not guarantee that the crop will not get the disease, but it reduces the odds of that happening. If the crop does get infected, the damage is less extensive.  

In a field contaminated with vomitoxin, burying the stalks and other plant material that remain will help reduce, but won’t eliminate, the spread of the fungus in next year’s crop, Paul said.

Symptoms of Gibberella ear rot include pinkish mold. But it can be easy to overlook if a growing crop has been tarnished by the fungus because the husk covers up where the damage occurs, on the ear of the corn.  

“A lot of farmers are caught off guard,” Paul said. “After you harvest the grain or when you take it into the grain elevator, that’s when you start seeing weird stuff and realize you have a problem.”

For more information on vomitoxin, see go.osu.edu/vomitoxinfacts

Dealing with the Weather and Unharvested Crops

Source: Penn State Extension (Edited)

WHAT A FALL!!!  According to the November 26 Crop Weather Report, approximately 14% of corn and 10% of beans still in the field.  The average moisture content of corn harvested last week was 17 percent and the average for soybeans was 16 percent, how big of a concern is this?

The weather continues to be unpredictable and give challenges to operators with grain and crops still in the field. Snow and ice over the last couple weeks have just been the latest in a long list of hurdles that growers have had to overcome this season. With some careful thought and planning you can still have a successfully harvest.

Having corn in the field now can be a double-edged sword. The longer it stays out, the dryer the corn will be when harvested, thus decreasing your drying costs. However, there is a higher risk of yield loss the longer the corn stays unharvested. Research on winter corn drydown showed that over a five-year span, corn grain would lose roughly 40% of its moisture between the months of October and December, when left in the field. The tradeoff is that we cannot anticipate the weather. The same study found that a single year yield decreased by 45% and another year decreased by only 5%.

Another concern of unharvested corn could be disease and mold. When discussing disease and mold, snow and ice pose no more danger to your crop than rain does. A positive of this situation is that the lower temperatures could have a limiting effect on pathogens’ ability to incubate or develop. A drawback of having laying snow is an increased opportunity for lodging. This year we have already seen a lot of lodging due to stem rots and adding snow to the mix may increase this risk. The risk of lodging is even further increased when coupled with winter winds and snow and ice to come. The takeaway is that disease and mold issues should not be your largest concern right now.

If you have a large amount of stock rot and lodging, harvesting as soon as possible will be best for a successful harvest. If your corn crop has lodged, one thing to remember is that this is not a usual harvest. Special consideration and care must be taken to get acceptable yields, which means slowing down and using caution. A few other options you have for getting a better harvestable yield are combining in the opposite direction, or “against the grain.” This will allow the head to get under the crop and lift it up. Another option is to use a corn reel. A corn reel is a specialized piece of equipment that mounts on the top of your corn head and uses rotating hooks to lift the corn and allow the head to get under the lodged crop.

The last concern is compaction and rutting of fields … Who Doesn’t Have Compaction Issues This Year??  Compaction will linger for years and will require attention to avoid problems with next year’s crop.

 

Stalk Quality Concerns

Source: Dr.’s Peter Thomison, Pierce Paul, OSU

Poor stalk quality is being observed and reported in Ohio corn fields. One of the primary causes of this problem is stalk rot. Corn stalk rot, and consequently, lodging, are the results of several different but interrelated factors. The actual disease, stalk rot, is caused by one or more of several fungi capable of colonizing and disintegrating of the inner tissues of the stalk. The most common members of the stalk rot complex are Gibberella zeaeColletotrichum graminicolaStenocarpella maydis and members of the genus Fusarium.

The extent to which these fungi infect and cause stalk rot depends on the health of the plant. In general, severely stressed plants (due to foliar diseases, insects, or weather) are more greatly affected by stalk rot than stress-free plants. The stalk rot fungi typically survive in corn residue on the soil surface and invade the base of the corn stalk either directly or through wounds made by corn borers, hail, or mechanical injury. Occasionally, fungal invasion occurs at nodes above ground or behind the leaf sheath. The plant tissue is usually resistant to fungal colonization up to silking, after which the fungus spreads from the roots to the stalks. When diseased stalks are split, the pith is usually discolored and shows signs of disintegration. As the pith disintegrates, it separates from the rind and the stalk becomes a hollow tube-like structure. Destruction of the internal stalk tissue by fungi predisposes the plant to lodging.

Nothing can be done about stalk rots at this stage; however, growers can minimize yield and quality losses associated with lodging by harvesting fields with stalk rot problems as early as possible. Scout fields early for visual symptoms of stalk rot and use the “squeeze test” to assess the potential for lodging. Since stalk rots affect stalk integrity, one or more of the inner nodes can easily be compressed when the stalk is squeezed between the thumb and the forefinger. The “push” test is another way to predict lodging. Push the stalks at the ear level, 6 to 8 inches from the vertical. If the stalk breaks between the ear and the lowest node, stalk rot is usually present. To minimize stalk rot damage, harvest promptly after physiological maturity. Harvest delays will increase the risk of stalk lodging and grain yield losses and slowdown the harvest operation. Since the level of stalk rot varies from field to field and hybrids vary in their stalk strength and susceptibility to stalk rot, each field should be scouted separately.

Seed Quality Issues in Soybeans

Let’s face it – we’ve had historic rains in parts of Ohio during 2018 and we are now observing many late season issues that come with this.  Seed quality is one of them and the symptoms or warning signs that there could be issues are on the stems.  The stems in some fields are heavily colonized with a mix of disease pathogens that cause Anthracnose, Cercospora, and pod and stem blight (Figure 1).  The bottom line is that all of these diseases can be better managed with higher levels of resistance but ultimately during 2018 – we had a perfect storm, lower levels of resistance combined with higher than normal rainfall conditions and add in the presence of a new insect pest, stink bugs.  Below I’ve outlined the general conditions of the crop and for each disease, the distinguishing characteristics.

Figure 1

Continue reading

Tips for Harvest and Planning for the 2019 Field Season

Source: Dr. Anne Dorrance, OSU (edited)

Based on my trip across the state on Saturday, it is clear harvest has started.  A couple of things to keep track of as the combines run across the fields:

  • Make note of those low yield spots in soybeans to soil sample for soybean cyst nematode levels.
  • Did you leave unsprayed strips?  Harvest each of these first separately.  Yield is not even throughout a field so comparisons to the average of these unsprayed strips are a more accurate measure of what the baseline level of yield is within a field.  This is the number to compare yields for any treatments.
    • Note: the outside borders of the field are usually not comparable since these have additional secondary factors such as shade from trees, compaction, old fence rows etc. which can impact yield.
  • Fields with Sclerotinia should be harvested last. Yes, seed quality will continue to decline but this will avoid contaminating equipment with sclerotia which can then be introduced into more fields.  There are limited fields with this pathogen, and this approach will help keep it that way.
  • Fields with stink bug injury, generally moldy due to Phomopsis etc.: harvest those ASAP and get the seed dried down.  Phomopsis will continue to colonize pods from openings on those pods caused by insect feeding and then colonize the neighboring seeds.  This fungus that causes Phomopsis seed decay as well as other seed decay fungi tend to be a bit slow growing.  If the seed can be harvested, and dried down it will prevent further growth. It has also been noted that on a seed germination test in the fall, germs will be lower, but the seed where only the outside is colonized, not the germ, the fungus will die under out winter conditions (if the storage is dry) and then the germ will improve over the winter for Phomopsis seed decay.
  • While you are also harvesting make note of the varieties that did well on your farms. Not every soybean variety is meant for our wet poorly drained soils. We’ve had lots of reports and observed shallow root systems, extensive root rot, as well as Phytophthora stem rot and sudden death syndrome during 2018. In fields where diseases developed in 2018, pay attention to the resistance on these and other diseases for 2019. 
    • Remember, every company uses a different rating system, read the fine print to be sure that you understand what resistance actually means for those varieties. Is 1 dead or is it the best?

Early Yellowing Soybeans

Source: Dr Anne Dorrance, OSU Extension

Sudden Death Syndrome

Soybeans across the state range from ready to harvest to still flowering.  But in some fields, the yellowing was limited to pockets – some was sudden death syndrome or brown stem rot, charcoal rot, Phytophthora stem rot, and soybean cyst nematode.  There are some other early yellowing situations that we are still working on an accurate diagnosis, but yellowing in these cases may be linked to fertility issues and/or related to late flooding injury.  I think in 2018 we’ve observed just about everything, and it was all dependent on where in the state the soybeans were grown, how much rain occurred and when that rain fell, as well as the variety.  It did seem that we had calls on the same variety from multiple regions.

The heat this past weekend is also going to move the crop fairly fast. So if you haven’t driven by the earliest planted fields – this is the week to do so.  Sudden death syndrome is very widespread – but in most fields, it is limited to a scattering of plants throughout the wet areas.  The plants were not severely affected as most of the fields I visited were holding their leaves and not defoliating as quickly as I have observed for the most susceptible varieties. Late season Phytophthora stem rot is also present – in this disease, the plant wilts, holds its leaves and develops a brown canker that extends from the base of the plant up the stem.  Charcoal rot can also cause early yellowing or dying, and these symptoms were present last week in several areas of the state.  To distinguish this from other diseases, cut open the tap root and look for the black dots embedded in the tissue and lower stem.  When populations of soybean cyst nematode are high, plants will also mature earlier.  For cyst, you can dig up the plants, shake the soil off and see the small white pearls (females) on the roots.  Often we need a microscope as the cyst will turn tan to brown and becomes hard to see.

This round of late season scouting is important for variety selection, improving fertility applications for the fall, and prioritizing which fields to sample for soybean cyst nematode.  Let’s just hope the weather cools so we can get out of the trucks and walk into the fields!